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Evolution
SL Unit 5 Ecology – Paper
1 and 2
HL Option D – Paper 3
Introduction to Evolution
Assessment Statement
5.4.1
5.4.2
5.4.3
5.4.4
5.4.5
5.4.6
5.4.7
5.4.8
Define evolution
Outline the evidence for evolution provided by the fossil record, selective
breeding of domesticated animals and homologous structures
State that populations tend to produce more offspring than the
environment can support
Explain that the consequence of the potential overproduction of offspring is
a struggle for survival
State that the members of a species show variation
Explain how sexual reproduction promotes variation in species
Explain how natural selection leads to evolution
Explain two examples of evolution in response to environmental change;
one must be antibiotic resistant bacteria
Theories of Evolution
For many centuries, people accepted the species they saw around them have always been
there. However, in the 18th century, the finding of many different strange species in other
parts of the world, did people start to question. Fossils were also discovered, and
inquiring minds wanted to know.
One was Jean Baptiste de Lamarck (1744-1829). He suggested that all species were
created by a higher power, but they undergo change over time. His summary of how this
occurred was, “inheritance of acquired characters”. What this means is that the
behaviour of the individual determines the character that its offspring inherit. (ex. a
giraffe)
The problem with this is that we have been making physical changes to animals (ie.
cropping ears in dogs) but the changes do not carry over to their offspring.
Next – Darwin – Wallace – Natural Selection
Russel Wallace (1823-1913) and Charles Darwin (1809-1882) both suggested the
alternative idea of natural selection, or a ‘struggle for existence’, as a mechanism for
change over a period of time.
Darwin and Wallace had studied the works of others, and both had travelled to far corners
of the world. Wallace went to South America and Indonesia. Darwin, who we have all
heard of, took the HMS Beagle to South America and the Galapagos Islands. Both
published works, but the better known and controversial ‘On the Origin of Species’, was
published by Darwin in 1859.
Natural Selection could be explained using Lamarck’s example of the giraffe. The
giraffe is always reaching for leaves, but the giraffe with the long neck gets more food
than the one with the short neck. The long necked giraffe will be more successful in
reproduction and the genes for the long neck are passed on. The mutation for the longer
neck is random, just as the shorter neck, but as competition for leaves increases, the
longer necked giraffes will have more accessibility to food resources. As a result, the
short necked giraffe will probably die of starvation and not pass on its genes.
Other evidence for selection, is in the breeding of dogs and the evolution of the horse.
With the dog, humans have created many different breeds in a relatively short time. This
is called artificial selection. (ex. agriculture)
Fossils have been found showing that 53 million years ago, the ancestor of the horse was
a small herbivore probably living in the forest. It had four toes on its front feet and three
toes on its hind feet. Over time, the animal started to live on a grassy plains, grew bigger
and number of toes reduced until the horse just has one (nail is the hoof). This allowed
the horse to run faster, which is important if you are on an open plain with predators.
Fossil records also show this. (Ex. Peppered Moth, Biston betularia)
Other Theories of the Origin of Life
Special Creation or Creationism
Panspermia – life from elsewhere came to Earth
Evidence for Evolution
Evolution describes the changes in the gene pool of a species over time. These changes
are the result of mutations, natural selection and genetic drift.
Evolution – the process of cumulative change in the heritable characteristics of a
population
Over time, if enough changes occur in a population, a new species can arise. The
members of the new population will be different enough from the pre-existing one they
came from that they will no longer be able to interbreed. Such a process is rarely
observable during a human lifetime.
The three areas we will look at to provide evidence for the theory of evolution by natural
selection are:
1. Fossil Records
2. Artificial Selection
3. Homologous Anatomical Structures
1. Fossil Records
Fossils can tell us a lot about the past.
Fossil – any form of preserved remains from a living organism.
Some examples are:
 Mammoths frozen in Siberia
 Mummies in acidic swamps in Scandinavia
 Insects in amber
 Bones in rock
Fossils are only formed in some circumstances. Most individuals do not leave a fossil
after death.
A fossil has to be formed when an organism dies and gets buried in sedimentary silt. It
will decay slowly and leave a space in the silt. The gap becomes solid and is filled the
exactly the same as the organism left behind. The silt may solidify, becoming
sedimentary rock and in it is the fossil.
To see how old fossils are and their forms, carbon dating is used, usually Carbon 14 and
potassium 40, which are isotopes. (More on this later)
Palaeontologists have discovered the following:
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Overall, life, which existed more than 500 million years ago, was very different
from life today.
Although the planet Earth has had extensive oceans for most of its existence, fish
fossils have only been found in rocks 500 million years old or younger (less than
15 % of the history of life)
Although most of the top predators today are mammals such as bears, orca
whales, big cats wolves and the like, none of them existed at the time of the
dinosaurs or before
Apart from organisms such as certain types of sharks, cockroaches or ferns, many
living organisms today have no identical form in the fossil record.
One conclusion that can be drawn from observing fossils is that life on Earth is constantly
changing. For example, in some cases, as for the example of the horse, we see
macroevolution. The first fossils of the ancestors of the modern horse are 53 million
years old. They had 4 toes on the front foot and 3 toes on the back. Their eyes were
halfway up their head, between the nose and ears and the teeth structure showed it ate
leaves, not grass. This early horse was known as Eohippus, which means dawn horse.
Fossils in the upper strata of sedimentary rocks (younger fossils) show the horse grew
larger, one of its toes grew bigger and the others reduced. We also see that the vegetation
changes from thick forests to grasslands, due to fossils of early vegetation. The eyes
grew closer to the back of the head, closer to its ears, to improve its peripheral vision to
be able to watch for predators while they grazed. The teeth also became bigger and
stronger to promote grazing.
Many fossils of horses that do not have these features have been found, but the evidence
is that they became extinct (ie. they could not outrun predators, attain food, etc.). They
were eventually replaced by species that were better suited to the environment. The only
line that continued into our time, was Equas, the modern horse.
2. Artificial Selection
The fossil record is not complete, but breeding domesticated animals provides a good
record of recent changes in heritable characteristics.
By watching mating of males and females, and the offspring, breeders select the desirable
traits they want. After practicing selective breeding for hundreds of dozens of years,
certain varieties of animals had unique combinations of traits not seen before. The
evidence is that small changes are occurring over time, which is driven by humans or is
artificial. If evolution can be controlled artificially, then it could also be natural.
3. Homologous Structures
Comparative Anatomy concentrates on studying homologous structures. Two structures
are homologous if they come form the same origin though they may look different now
and have different functions.
Analogous structures are those that have the same functions, but come from differ
origins. For example, the wing of a bird and a wing of an insect are both used for flying,
but the wing of the bird used to be a limb and the wing of the insect comes from a fold in
the skin. This tells us that there is not a common ancestor here.
Examples of homologous structures are the arm of a human, the wing of a bat and flipper
of a seal. They all have the same pentadactyl limb. This means they have the same basic
patterns of bones, including five digits. The pentadactyl limb is used differently in
different mammals, but the common structure could lead to the conclusion that there is a
common ancestor.
We can also look at physiological evidence, by looking at the functions of parts of
organisms or rudimentary structures (ie. pelvis in a whale).
We have seen that the wastes from birds and reptiles have the same chemical makeup and
the hormones from sheep and pigs are also present in humans!!
We can also look at Embryology, which is the study of organisms in early stages of
development.
Scientists have discovered a similarity between the embryos of different species and
it is theorized that this similarity is due to their evolution from a common ancestor.
There is also a theory that every organism repeats its own evolutionary development
as the embryo develops.
Mechanism of Evolution - The Idea behind Evolution and Neo Darwinism
Darwin and Wallace suggested a process. This process is known as natural selection. It
works by over production of offspring and the presence of natural variation.
Too many offspring
Populations tend to produce more offspring than the environment can support. The
production of offspring involves the expenditure of energy and resources. This over
production of offspring leads to intra-species competition and survival of the individuals
best suited to that particular environment. Example, trees have active compounds that
ward off insects. Competition can also lead to adaptive behaviours.
Natural Variation within a Population
Sexual Reproduction promotes variation in species. Darwin knew nothing of Mendel’s
work, like most scientists of the time. They believed in blended inheritance, which
would lead to less variation. Neo-Darwinism restates the essential concepts of
evolution in terms of Mendelian and Post-Mendelian Genetics.
Creating gametes by meiosis involves the separation of the homologous pairs of
chromosomes. Since this process is random, a gamete has a mixture of paternal and
maternal chromosomes. Two gametes from different individuals fuse to create a new
organism. Since gametes from one individual differ, this mixing will lead to further
variation.
To summarize, variation arise via:
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random assortment of chromosomes
crossing over of segments of chromosomes result in new combinations of genes,
different than the parental combinations
random fusion of gametes in sexual reproduction
additional variations arise due to mutations, either chromosomal or gene
As a result of all of these, the individual offspring of parents are not identical and show
variations in their characteristics. If the variations are successful, the organism will be
successful.
Variety may be caused by:
 Mutations
 Sexual Reproduction – random splitting of cells during meiosis will determine the
genetic variety.
Natural Selection and Favourable Heritable Variations
Sexual reproduction and/or mutations, leads to variations of a species. Variation is nondirectional or random. The selection process is dictated by the environment and leads to
differential survival.
The result is that the individuals best adapted to a particular environment will survive.
They will be able to get the most food, find the best shelter, find a mate, reproduce and
care for their offspring as well as not be eaten by other species.
Since most environments are different, the “best adapted” may be different too. Also,
environments may change. This can happen gradually or suddenly, due to a natural
disaster, for example. As a result, the criteria for the “best adapted will also change.
This process of natural selection can lead to changes in the species. It can also lead to
speciation. When two groups of a species are in different environments and they cannot
interbreed, selection pressure will be different and eventually they will become different
species (adaptation) due to their natural environment. This is what Darwin noticed in the
many species of finches in the Galapagos Islands. Sometimes the idea of natural
selection is summarized in the phrase “survival of the fittest”, and not “the strong
survive”, although these words were not used by Charles Darwin.
Natural Selection Summarized
1.
2.
The favourable characteristics are expressed in the phenotypes of some of the
offspring
These offspring may be better able to survive and reproduce in a particular
environment; others will be less able to compete successfully to survive and
reproduce.
Examples of Evolution in response to Environmental Change
If a species cannot adapt to the changing environment, then the species will die out. As
the dinosaurs did not find a way to deal with the climate becoming colder, they did not
survive. Their place was taken by the homeothermic, or warm-blooded mammals.
1.
Multiple Antibiotic Resistance in Bacteria
Penicillin is not effective over the entire field of micro-organisms pathogenic to humans.
During the 1950’s, the search for antibiotics to fill this gap resulted in a steady stream of
them, some with a much wider antibacterial range than penicillin (broad spectrum
antibiotics). Some were capable of coping with those micro-organisms that are
inherently resistant to penicillin or that have developed resistance through exposure to
penicillin.
Many diseases caused by bacteria have been successfully treated with penicillin and other
antibiotics. However, since WWII, when the use of antibiotics became widespread, many
disease-causing bacteria have developed resistance against antibiotics. There are strains
of bacteria causing tuberculosis, which are resistant to all known antibiotics. The same
applies for cholera, as there is only one effective antibiotic available. This means that is
you become infected with these bacteria, treatment with antibiotics will not cure you and
the disease may become fatal.
Staphylococcus aureus is a common bacterium found living on the skin. This species is
usually harmless but, in certain circumstances, can invade your blood stream, infect
tissues in the kidneys or bones and could become fatal. These days, strains of S. aureus
exist which are resistant to all known antibiotics. These MRSA bacteria (methycillinresistant Staphylococcus aureus) are of grave concern to hospitals all over the world.
The resistance to antibiotics is probably caused by spontaneous mutation. As a result, the
bacterium produces penicillinase, for example, an enzyme, which breaks down penicillin.
If the bacteria are exposed t penicillin, the one without resistance will be killed.
However, those with resistance will survive and, due to lack to competition, grow
rapidly.
The genetic information for antibiotic resistance is often found on plasmids, which can be
spread rapidly over a population and can even cross into other species of bacteria. This is
likely to occur when a small dose of antibiotics is used for a short time. It will kill some
of the bacteria, but not all and may lead to the creation of some bacteria that have some
resistance. The next time antibiotics are used, these bacteria are less vulnerable and some
more may survive. Repeated use of small doses of antibiotics can produce very resistant
strains. This explains why doctors always insist on patients finishing the course of
antibiotics even if they are feeling better.
Overuse of antibiotics in medicine, the cattle industry and antibiotic soaps have led to a
rise in antibiotic strains.
2.
The Peppered Moth (biston betularia)
This moth is found in England, near Manchester (boo United!!!). Before 1848, trees on
which they rested were covered with off-white lichen. The moths were white, and
therefore camouflaged from predation by birds. Occasionally a black moth would
appear, and due to its high visibility, would have a high possibility of falling prey.
Due to coal base industry, the trees became covered with soot and the white moths were
easily spotted and eaten. The dark (melanic) form now had an advantage and became
predominant (95%) in certain areas in 1950. Reduce use of coal has now made the trees
green (covered in algae) and both forms are common. This is called balance
polymorphism. This is a short termed example of evolution.
3.
Heavy Metal tolerance in plants
This is a phenomenon associated with those plants able to survive and even flourish on
the bare waste tips and spoil heaps found at mining sites.
Heavy metals, such as, copper, zinc, lead and nickel may be present as ions dissolved in
soil moisture at concentration that generate toxic conditions for plants normally present
on the surrounding unpolluted soil.
Some heavy metal ions are essential for normal plant growth when present in trace
amounts, but in mining spoils, the levels are exceeded. For many years, the areas around
mines were largely bare of all plant life, even when surrounding, unpolluted soils have
dense vegetation cover. Seeds from these plants regularly fall on spoil heap soil, but
plants fail to establish themselves.
However, careful observations of spoil heaps have shown some plant species have
evolved tolerance. One example is the grass Agrostis tenuis (Bent Grass), populations of
which are tolerant of toxic concentrations of copper.
A variety of biochemical and physiological mechanisms have evolved in tolerant species,
including:
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the selective ability to avoid uptake of heavy metal ions
the accumulation of ions that enter in insoluble compounds in cell walls by
formation of stable complexes with wall polysaccharides
transport of toxic ions into the vacuoles of cells, the membranes of which are
unable to pump them out again, so avoiding interactions with cell enzymes.
The evolution of this form of tolerance has been demonstrated in several species of
terrestrial plants, and also in species of seaweeds, now tolerant of copper-based
antifouling paints frequently applied to the hulls of ships. This is also a fear associated
with the genetically modified Round-up resistant wheat.
Evolution – The Origin of Life
Assessment Statement
D.1.1
Describe four processes needed for the spontaneous origin of life on Earth
D.1.2
Outline the experiments of Miller and Urey into the origin of organic
compounds
State that comets may have delivered organic compounds to Earth
Discuss possible locations where conditions would have allowed the
synthesis of organic compounds
Outline two properties of RNA that would have allowed it t play a role in
the origin of life
State that living cell may have been preceded by protobionts, with an
internal chemical environment different from their surroundings
Outline the contribution of prokaryotes to the creation of an oxygen rich
atmosphere
Discuss the endosymbiotic theory for the origin of eukaryotes
D.1.3
D.1.4
D.1.5
D.1.6
D.1.7
D.1.8
Biologists believe that organic evolution by natural selection accounts for the major
steps in evolution.
These are macroevolution – major developments such as the origin of the eukaryotic
cell, the origin of multicellular organisms, and the origin of vertebrates from nonvertebrates; and microevolution – the relatively minor
changes that arise and lead to the appearance of new, but closely related species.
There are several theories as to how life originated on our planet. None have been
proven, but there is one that is accepted as the “hypothesis of evolution” or theory of
evolution. This is called the Big Bang.
The Big Bang Theory, Experiments and Theories
The Earth is one of the smallest planets grouped in the Solar System around a central
star, the Sun. The fact that the planets all revolve in the same plane supports the theory
that the Sun and planets were all formed from the condensation of a single revolving disc
of matter. It is likely the Earth originated from masses of molten rock that collided and
coalesced. With cooling, a crust formed but the restless surface was initially
continuously disturbed as other matter collided. Heat from impacts and from the decay of
radioactive elements such as uranium was probably sufficient to melt matter and keep it
molten.
In the liquid state, the bulk of heavy elements, particularly iron, formed the Earth’s liquid
core of dense matter. Radioactive elements, though present in small amounts, have had
enormous effects on the Earth’s geological evolution and they continue to keep the
interior hot.
The surface of the Earth eventually cooled to 100 °C and below, and an atmosphere
developed. The gravitational field on Earth was strong enough to retain this atmosphere,
unlike that of the Moon. The major constituents of the atmosphere would have been:
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nitrogen, water vapour and carbon dioxide;
smaller amounts of methane, ammonia, carbon monoxide, sulphur dioxide,
hydrogen sulphide and hydrogen cyanide.
These are all products of the effects of heat on the lighter chemical elements of the crust,
and of lightning and ultra-violet radiation. (The arrival of comets was a possible
alternative source for some of the gases, particularly water vapour.) The atmosphere
was virtually without oxygen – in fact, any trace of free oxygen would have immediately
reacted with the large quantity of iron present.
The rock of the Earth’s crust is a relatively thin layer. It is divided into huge plates that
move about on the surface, and where they meet, one or both turn under and become part
of the mantle layer below.
As the Earth continued to cool, the water vapour in the atmosphere condensed and
returned to the surface as rain, forming rivers and lakes. Seas formed. Now the process of
erosion began to mould the landscape, and the eroded debris became the first
sedimentary rocks.
Question – Why do those fossils found in the lowest strata in sedimentary rocks,
bear the least resemblance to present day forms?
At this point there was still no life on Earth. It was speculated that the atmosphere was
probably a reducing atmosphere (gaining electrons), because metals in old rocks are
found in their oxidized forms (ie. Fe+2 and Fe+3). It is possible to form organic
molecules in a reducing atmosphere but is difficult to do it in an atmosphere that contains
oxygen, because oxygen wants to lose electrons.
Life in the form of living cells may have developed spontaneously in evolving conditions
similar to those described above. If so, the following steps would have been involved:
 the non-living synthesis of simple organic molecules, such as sugars and amino
acids;
 the assembly of these molecules into polymers;
 the development of self-replicating molecules, such as nucleic acids;
 the packaging of these molecules within membranous sacs, so that an internal
chemistry can develop, different from the surrounding environment.
Other people have speculated there must have been oxygen in the atmosphere as without
ozone, the Earth would have been bombarded with UV radiation, killing all life. Also,
living organisms synthesize the proteins and nucleic acids needed inside cells using
enzymes. There had to be a way to make organic molecules outside a cell.
Hypothesis and Experiment A – Stanley Miller and Harold Urey
Experimental evidence of how simple organic molecules might have arisen from the
ingredients thought to be present at the time before there was life on Earth was produced
by S. L. Miller and H. C. Urey in 1953.
They set up a reaction vessel in which particular environmental conditions could be
reproduced. Here, strong electric sparks (simulating lightning) were passed through
mixtures of methane, ammonia, hydrogen and water vapour for a period of time. They
discovered that amino acids were naturally formed (some of them known to
be components of cell proteins, such as adenine and ribose) as well as other compounds.
This approach confirmed that organic molecules can be synthesised outside cells, in the
absence of oxygen. The experiment has subsequently been repeated, sometimes using
different gaseous mixtures and other sources of energy (UV light, in particular), in
similar apparatus.
The products have included amino acids, fatty acids, and sugars such as glucose. In
addition, nucleotide bases have been formed, and in some cases, simple polymers of all
these molecules have been found.
To summarise, we can see how it is possible that a wide range of organic compounds
could have formed on the pre-biotic Earth, including some of the building blocks of the
cells of modern organisms.
What environments could organic compounds formed?
As will be mentioned later, comets, are chunks of ice wandering through space. As they
travel, they could have carried organic molecules to Earth. If this was the case, for which
we have no conclusive proof, what environments would be favourable to form or sustain
early organic molecules?
In space
By studying the spectral lines of distant clouds of cosmic dust particles, astronomers
claim to have revealed the presence of glycine, which is the simplest amino acid.
Organic molecules could form in space and be carried by comets, as the above
observation suggests. Lab experiments, which recreate the low pressure, low temperature
environment in space, have been able to synthesize amino acids. Francis Crick, the codiscoverer of the structure of DNA, was a modern supporter of a suggestion that organic
molecules, the essential precursors of living cells, may have emerged on another planet or
moon and ‘hitched a ride’ to Earth on a comet. The idea that life did not originate on
Earth but arrived in some form from an extraterrestrial source is known as panspermia
(Greek for‘all seeds’ – it was a Greek philosopher who 2500 years ago proposed that all
life originated from combinations of tiny seeds pervading the cosmos).
Currently, this idea is being researched by astrobiologists and planetary geologists in
America. NASA scientists have confirmed that early in the history of our Solar System,
conditions essential for life were present elsewhere. For example, on Mars, water flowed
intermittently, and life may have existed there. Also, Europa, the fourth-largest moon of
Jupiter, appears to possess liquid water under an icy surface. Titan, the largest satellite of
Saturn, is rich in organic compounds. The expanse of interplanetary space has been
crossed in ways that may have transportedorganic matter. For example, about 30
meteorites found on Earth originated from Mars.
Biological matter is more likely to survive travel in the interior of meteorites, either in the
form of RNA alone or assembled with ribosomes in ‘protein factories’. As yet, though,
there is no evidence it happened.
In alternating wet and dry environments
So how did DNA come about, if it didn’t hitch a ride to Earth? Currently DNA can
replicate, but it needs enzymes to do this. The DNA is transcribed into RNA and the
RNA makes the proteins needed to make the DNA. Which came first – the chicken or
the egg?
One suggestion is the Catalytic Action of Clay assisted in the formation of polypeptides
from amino acids, as made by Katchalsky, Cairns – Smith and Bernal
The basis for this idea is as follows:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Some clays can grow by attracting molecules to themselves. They will then
repeat a lattice-like organization over and over again.
Amino acids may have stick to the clay lattice and have been incorporated into it.
They may have been attached to each other as well.
Some clay particles may have become a template for a protein.
If the protein product was a weak enzyme it may have speeded up the process of
protein synthesis with clay as a template.
Then the clay template for this particular enzyme would make more protein than
another template whose product was not an enzyme.
Then nucleotides could have been attracted by the clay template, or the template
with the attached proteins, and could have polymerized (into RNA) and come to
act as a co-enzyme.
The more successful template is the one where the enzyme and co-enzyme work
together to produce more of themselves.
Eventually, the co-enzyme (nucleotide polymer, RNA) could become the
template for protein synthesis.
However, no one has yet been able to synthesise DNA and globular proteins in
any of the reported experiments repeating Miller and Urey’s demonstration of
how biological important molecules could be synthesised in the pre-biotic world.
Near volcanoes
A third possibility is when a volcano erupts, the force can be destructive, but it spews out
water vapour, other gases and various minerals which could be used to form organic
mater. The rich sources of raw materials plus the warmth of the vocanic activity could
have provided conditions favourable to the formation of amino acids and sugars.
In deep oceans
Organic molecules could have been formed around hydrothermal vents – places where
hot water comes out of the ocean floor, like an under water geyser. Some times the vents
are called “black smokers” because the water coming out of them contains so many dark
minerals that is looks like smoke. It has been observed that entire communities live
around these vents, such as meter long white and red tube worms which absorb the
minerals and pas them on to symbiotic bacteria. The bacteria make food from the
minerals and nourish the tube worms. Even though there is no sunlight, life flourishes
here, making the hypothesis of life originating her plausible.
The Role of RNA in early life
So what may have filled the roles of DNA and enzymes in the origin of life?
A possible answer was found in the unexpected by-product of genetic engineering
experiments involving in vitro investigation of the enzymes required to patch and join
short lengths of RNA (a process that genetic engineers call splicing). These experiments
showed, to everyone’s surprise, that when the naturally occurring protein enzymes that
catalyse RNA patching (obtained from cells) were omitted from the reaction mixtures,
the RNA fragments still spliced on their own. It had been assumed that the RNA-patching
enzyme (a protein) was the essential catalyst.
This was the first demonstration that short lengths of RNA, as well as being ‘information
molecules’, also function as enzymes. These catalytic RNA molecules have been named
ribozymes.
Perhaps short lengths of RNA filled the dual roles of information molecules and enzymes
in the evolution of life.
Now we have experimental evidence that short lengths of RNA can also function as
enzymes, although they may rarely do so in modern cells.
In present day eukaryotic cells, messenger RNA (mRNA) carries the genetic code
between nucleus and the site of protein synthesis, the ribosomes (themselves another
form of RNA).
Other RNA, known as transfer RNA (tRNA) brings the amino acids to the ribosome for
the building of the protein. However, the enzymes that catalyse the chemical reactions
involved throughout are proteins.
Further investigations show ribozymes to be fairly inefficient enzymes – slow and
unpredictable at times, but that they work satisfactorily with polynucleotide substrates.
They can catalyse simple replications, although they do this in an error-prone way, on
occasions. Thus, ribozymes may catalyse the formation of DNA, for example.
The discovery of ribozymes completes the story of a possible and credible route from the
prebiotic soup to living things, simply because this form of RNA is an information
molecule that both replicates and may function as enzymes.
Keep in mind, all the series of events described above are not entirely random. The
conditions on Earth made some processes more likely than others to occur.
Hypothesis – Membrane Formation from the Primordial DNA – Fox and Oparin
The first cells were prokaryotes. This we know from the fossil record. Were they
preceded by a ‘lower’ or lesser level of organisation – some form of protobiont?
A limited number of lipid molecules, once formed, arrange into a monolayer on the
surface of water. When more lipids become available, the whole re-forms into lipid
bilayers – the basis of plasma membranes today. If such bilayers formed and linked up
into microspheres that surrounded a small amount of the pre-biotic soup of polymers and
monomers, perhaps these were the fore-runners of cells?
Fox did an experiment in which he heated amino acids without water and produced long
protein chains. When the water was added and the mixture cooled, stable microspheres
formed. Microspheres were able to accumulate certain compounds inside them so that
they became more concentrated inside than outside. They also attracted lipids, forming a
lipid protein layer around them.
Microspheres might be dubbed ‘membrane systems with a distinctive internal chemistry’,
for the contents have the potential to develop a chemical environment different from the
surroundings. Also observed are structures called coacervates. These are formed from
dilute solutions of two substances each having large polymer molecules carrying opposite
charges. The two most commonly studied are gelatine and gum arabic. At certain
concentration, these separate into sol (liquid) and gel (solid) phases. Each phase contains
both polymers but at different concentrations. However, both contain large amounts of
other molecules in solution.
Complex coacervates have been observed, one gel droplet within another. If such droplets
came into existence and contained enzymes, they would form a model for the
biochemistry of the cell. The Russian biochemist Oparin (1894–1980), who pioneered
the chemical approach to the origin of life, attached great importance to coacervates in
the evolution of life.
A prokaryote cell differs from these models in a number of ways. For example, attached
to the plasma membrane in the prokaryote cell is a single circular chromosome of DNA,
known as a nucleoid. Also, a cell wall of complex chemistry is secreted outside the
membrane barrier to the cell contents. However, both protobionts and the first
prokaryotes could have survived nutritionally on the organic molecules of the pre-biotic
soup. In this early life environment, with a wealth of simple organic molecules
surrounding simple cells, digestion and respiration would have demanded only limited
enzymic machinery. Biochemical sophistications would have to evolve with time – if life
originated in this manner.
Hypothesis – Prokaryotes and Endosymbiotic Theory and where the Oxygen could have
come from
Did they contribute to our atmosphere?
Some of the earliest prokaryote fossils contain cells very similar to modern cyanobacteria
(modern prokaryotes that are photosynthetic). They are present in large mounds known as
stromatolites, fossilized examples of which are common, and of which there are also still
living examples.
Stromatolites are formed in shallow waters, the mounds built of layer upon layer of
bacterial mats. The earliest fossil stromatolites date from some 3500 million years ago.
In stromatolite mounds, the outer layer is of filamentous cyanobacteria – photosynthetic
bacteria that absorb light, produce carbohydrates, and release oxygen. Below is a layer
of purple bacteria that also absorb light and also manufacture carbohydrate, but do so
without releasing oxygen. Further below is a layer of other bacteria that are saprotrophic.
Some are able to fix atmospheric nitrogen into combined nitrogen of amino acids, for
example. The combined components of stromatolites are a biochemically able
assortment.
In the beginning the theory is that the bacteria were anaerobic. As the food stores
became scarce, the bacteria that produced their own food, has an advantage.
Photosynthetic prokaryotes began the process by which free oxygen accumulated in the
Earth’s atmosphere. With free oxygen in the atmosphere, the formation of an ozone
layer in the upper atmosphere commenced. Once formed, the ozone layer began to
reduce the incidence of UV light reaching the Earth’s surface. Terrestrial existence
(rather than life restricted to below the water surface) became a possibility. The oxygen,
which was possibly toxic to the anaerobes, killed off a large population.
Meanwhile other prokaryotes, more akin to modern aerobic bacteria, simply ‘fed’ on the
organic molecules available in their environment. However, these bacteria had evolved
aerobic respiration (only possible as a result of the free oxygen from photosynthetic
cyanobacteria, now present in the atmosphere) and so had the enzymes not only of
glycolysis, but also of the Krebs cycle and terminal oxidation.
Eukaryotic cells are only 1.5 billion years old. We know that the first cells were
prokaryotes. It is likely that some larger prokaryote cells came to contain their
chromosome (whether of RNA or DNA) in a sac of infolded plasma membrane. If so, a
distinct nucleus was now present. But how might the other organelles have originated?
Remember, membranous organelles are a feature of eukaryotes, in addition to their
discrete nucleus.
Is it possible the early cells acted as a primordial habitat – a possible origin for
mitochondria and chloroplasts?
Lynn Margulis suggested independent prokaryotes developed a symbiotic relationship
with another prokaryote (mitochondria, chloroplasts) Both mitochondria and chloroplasts
contain a ring of DNA double helix, just like that contained by a prokaryote. They also
contain the small ribosomes, like those of prokaryotes. These features have caused some
evolutionary biologists to suggest that some organelles are descendants of freeliving
prokaryotic organisms that came to inhabit larger cells. It seems a fanciful idea, but not
an impossible one.
Present day prokaryotes are similar to fossil prokaryotes, some of which are 3500 million
years old. By comparison, the earliest eukaryote cells date back only 1000 million years.
Thus eukaryotes must have evolved, surrounded by prokaryotes that were longestablished organisms.
It is possible that, in the evolution of the eukaryotic cell, prokaryotic cells (which at one
stage were taken up into food vacuoles for digestion) came to survive as organelles
instead. If so, with time they would have become integrated into the biochemistry of their
host cell. This concept is known as the endosymbiotic origin of eukaryotes.
Species and Speciation
Assessment Statement
D.2.1 Define allele frequency and gene pool
D.2.2
State that evolution involves a change in allele frequency in a
population’s gene pool over a number of generations
D.2.3
Discuss the definition of the term species
D.2.4
Describe three examples of barriers between gene pools
D.2.5
Explain how polyploidy can contribute to speciation
D.2.6
Compare allopatric and sympatric speciation
D.2.7
Outline the process of adaptive radiation
D.2.8
Compare convergent and divergent evolution
D.2.9
Discuss ideas on the pace of evolution, including gradualism and
punctuated equilibrium
D.2.10
Describe one example of transient polymorphism
D.2.11
Describe sickle-cell anemia as an example of balanced polymorphism
As was stated earlier, “New” or Neo-Darwinism is a restatement of the concepts of
evolution by natural selection in terms of Mendelian and post-Mendelian genetics.
Neo-Darwinism looks at:
1. Mutations as changes that are due to chance, but occur with predictable
frequency.
2. Variations in populations are due to recombination of alleles.
3. Adaptations (or micro-evolutionary steps) may occur as a result of an allele
frequency in a population’s gene pool.
a. Evolution of one species into another species involves the accumulation of
the advantageous alleles in a gene pool.
b. The process of speciation
4. Polyploidy
5. Allopatric and Sympatric Speciation
6. Adaptive Radiation
7. Convergent and Divergent Evolution
8. The pace of evolution is controlled by gradualism and punctuated equilibrium
9. Transient Polymorphism
10. Balanced Polymorphism
Allele Frequency and Gene Pools
Gene Pool – all of the genetic information present in the reproducing members of a
population at a given time. It can be thought of as a reservoir of genes from which the
population can get its various traits.
Allele Frequency – is a measure of the proportion of a specific variation of a gene in a
population. The allele frequency is expressed as a proportion or a percent, and can be
calculated by the Hardy-Weinberg equation (more later). For example, it is possible that
a certain allele if present in 25% of the chromosomes studied in a population. One
quarter of the loci for that gene are occupied by that allele. Keep in mind it is not the
same as the number of people who show a particular trait.
Evolution and alleles
Gene pools are generally relatively stable over time but not always. Mutations are
changes to genes or chromosomes due to chance, but with predictable frequencies.
Because they happen, it is believed they play a role in evolution. Old alleles disappear
and the last organism carrying the allele dies. Some alleles are disadvantageous and are
not as frequent and those that are advantageous tend to be more frequent. We know from
our Genetics unit that homologous chromosomes pair up during meiosis and then cross
over. With 3 chromosomes, the possible gametes are 8 (23 = 8). This allows for
variation.
The changes caused by variation are said to be non-directional, because every change has
an equal chance of occurring. If and when the change is made, the environment
determines if the change is beneficial or not. If it is beneficial, the individual will live to
pass on its genes, thus increasing the percentage of the allele in the population or gene
pool.
NO CHANGE IN ALLELE FREQUENCY = NO EVOLUTION
Some examples we have already mentioned, such as Down’s Syndrome and Klinefelter
Syndrome. As a result, we get some variation, due to the mutation.
Another example is the Peppered Moth, which was mentioned earlier. The colour is
determined by the alleles present for one gene. Originally there was a balanced
polymorphism, or having multiple alleles for a gene in a population, which usually
expresses different genotypes. In the case of the moth, the dark allele was rarely present,
as it was selected against. As was mentioned previously, the lichens were growing in
fewer numbers, and as a result the bark on trees became darker. The selection then
favoured the dark species, and the dark allele in the species increased (transient
polymorphism). Since the air pollution decreased, the light coloured morph and the allele
for the light colour now increases in the population.
Another example is PKU or phenylketouria. It is a genetic disease caused by the
presence of a homozygous recessive allele. A PKU individual cannot produce a certain
enzyme to break down phenylalanine to tyrosine. Phenylalanine levels build up which
are harmful to the brain. This can lead to brain damage. Once they eat a diet with little
phenylalanine, they can eat normally.
The above examples show that, at some point in time, the normal allele mutated and a
new allele was created. The new allele was not favourable but some individuals passed
on the allele.
The moth example can be summarized below:
Defining Species
Present-day flora and fauna have arisen by change from pre-existing forms of life. Most
biologists believe this. This process has been variously called ‘descent with
modification’, ‘organic evolution’, and ‘microevolution’, but perhaps speciation is
appropriate here because it emphasizes that species change.
A species is a group of organisms:
 of common ancestry (PHYLOGENY) that closely resemble each other
structurally and biochemically and are distinct from other species
 which are members of natural populations that are actually or potentially
capable of breeding with each other to produce fertile offspring, and which
do not interbreed with members of other species.
There are challenges to the definition




The last part of this definition cannot be applied to self-fertilizing populations or
to organisms that reproduce only asexually. Such groups are species because they
look very similar (morphologically similar). They behave and respond in similar
ways, with bodies that function similarly (they are physiologically similar).
Sometimes members of separate by similar species reproduce and succeed in
producing offspring. A horse and a zebra form a zebroid as the parents are
equines. They do not have the same number of chromosomes, which is one of the
reasons the offspring are infertile.
Does being infertile mean they are not part of the species?
What about two populations which could potentially interbreed, but do not
because they are living in different niches or are separate by long distances?
But however we define the term, since species may change with time (mostly a slow
process), there is a time when the differences between members of a species become
significant enough to identify separate varieties or subspecies. Eventually these may
become new species. All these points are a matter of judgment.
So a population of garden snails might occupy a small part of a garden, say around a
compost heap. A population of thrushes (snail-eating birds) might occupy several gardens
and surrounding fields. In other words, the area occupied by a population depends on the
size of the organism and on how mobile it is, for example, as well as on environmental
factors (e.g. food supply, predation, etc.).
The boundaries of a population may be hard to define. Some populations are fully open,
with individuals moving in or out, from nearby populations. Alternatively, some
populations are more or less closed – that is, isolated communities almost completely cut
off from neighbours of the same species. Obviously, the fish found in small lakes are a
good example of the latter.
Speciation and Barriers between Gene Pools
Speciation, the evolution of new species, requires that allele frequencies change with
time in populations.
Some of the processes known to bring about significant change, leading to the eventual
appearance of a local population of organisms that are a new species, unable to breed
successfully with members of the population from which they originated are due to
isolation.
Speciation by isolation
A step towards speciation may be when a local population becomes isolated from the
main bulk of the population, so the local gene pool is completely cut off and permanently
isolated. The result is reproductive isolation within the original population. Even when
reproductive isolation has occurred, many generations may elapse before the composition
of the gene pool has changed sufficiently to allow us to call the new individuals a
different species.
However it does happen, and isolation that is effective in leading to genetic change can
occur in space (geographical isolation), time (temporal isolation) and as a product of
behaviour (behavioural isolation).
A. Geographical isolation
This is the consequence of the development of a barrier within a local population. Today,
both natural and human-imposed barriers can occur abruptly, sharply restricting
movement of individuals (or their spores and gametes, in the case of plants) between
divided populations.
Before separation, individuals shared a common gene pool, but after isolation, ‘disturbing
processes’ like natural selection, mutation and random genetic drift may trigger change.
Genetic drift is random change in gene frequency in small isolated populations.
For example, a new population may form from a tiny sample that became isolated and
separated from a much larger population. While numbers in the new population may
rapidly increase, the gene pool from which they formed might have been totally
unrepresentative of the original, with many alleles lost altogether.
The outcome of these processes may be marked divergence between populations, leading
to their having distinctly different characteristics.
Geographic isolation also arises when motile or mobile species are dispersed to isolated
habitats – as, for example, when organisms are accidentally rafted from mainland
territories to distant islands. The 2004 tsunami generated examples of this in Southeast
Asia. Violent events of this type have surprisingly frequently punctuated world
geological history.
Another example would be in the Galapagos Islands. The iguana lizard here had no
mammal competition when it arrived on the Galapagos. It became the dominant form of
vertebrate life, and was extremely abundant when Darwin visited.
By then two species were present, one terrestrial and the other fully adapted to marine
life. The latter is assumed to have evolved locally as a result of pressure from
overcrowding and competition for food on the islands (both species are vegetarian)
driving some members of the population out of the terrestrial habitat.
B. Temporal isolation
This is illustrated when two very closely related species occupy the same habitat and
differ only in the time of year that they complete their life cycles. Reproductive isolation
may develop in this situation within a local population so that some members produce
gametes at distinctly different times of the year from others; thus, two distinctive gene
pools start to evolve.
Examples of the outcome of temporal isolation include two members of the genus Pinus
found in Californian forests.
C. Behavioural isolation
This type of isolation results when members of a population acquire distinctive behaviour
routines in their growth and development, courtship or mating process that are not
matched by all individuals of the same species.
An example occurs in the imprinting behaviour of the young of geese, swans and other
birds. When chicks of these species hatch out of the egg, the adult birds are in the
vicinity, caring for them. The young imprint the image of their parents as they relate to
and learn from them. They associate socially only with their own species (or variety), and
as adults, they will eventually only bond with and breed with their own species.
Imprinting became apparent when a goose chick, on hatching, was placed with swan
adults as parents. The goose, when an adult, bred with a swan, and the offspring was an
infertile ‘Gwan’. Clearly, the swan and goose are related species that have evolved apart
for long enough for their progeny to be infertile, but not long enough to exclude the
formation of a hybrid. (Konrad Lorenz)
Other examples of behavioural isolation are demonstrated by closely related species of
fish, including in guppies (Poecilia spp.) with different, distinctive body markings by
which pairs select their mates, and in four species of gull of the Canadian arctic (Larus
spp.) with distinctive plumage by which they are identified during breeding periods.
D. Hybrids
There are many challenges for hybrids. The majority of hybrids are infertile. Eventually
one generation will not come to be.
In summary, species do not evolve in a simple or rapid way. The process is usually
gradual, taking place over a long period of time. In fact, in many cases speciation may
occur over several thousand years. Complex though it is, we can recognize that all cases
of speciation require ‘isolation’.
Allopatric and Sympatric Speciation
A deme is the name we give to a small, isolated population.
The individuals of a deme are not exactly alike, but they resemble one another more
closely than they resemble members of other demes. This similarity is to be expected,
partly because the members are closely related genetically (similar genotypes), and partly
because they experience the same environmental conditions (which affect their
phenotype).
The ways demes become isolated have been discussed already. Reviewing these, we see
they fall into two groups, depending on the way isolation is brought about.


Isolating mechanisms that involve special separation are known as allopatric
speciation (literally ‘different country’). An example might occur in a land
dwelling species when sea level rise. The populations could be cut off from one
another. As sea levels dropped again many years later, each could have evolved
so differently that they could no longer interbreed.
Isolating mechanisms involving demes in the same location are known as
sympatric speciation (literally ‘same country’). An example could be moths
which, produce pheromones to attract a mate. If there is a mutation in the
pheromone and it is slightly different, it might attract new mates. The
interbreeding would breed a new type of moth that produces that specific
pheromone. Within a certain number of generations, the new combinations of
alleles would produce a new species of moth.
So, isolation may result from a deme becoming spatially separated from the rest of the
local population, or it may occur within a local population. Either way, natural selection
may come to act differently on the demes and, if this continues over a large number of
generations, complete divergence may be the final outcome.
Polyploidy
Haploid cells contain one set of chromosomes (n). Diploid cells are 2n. Polyploidy
refers to the situation in which a cell contains three or more sets of chromosomes (3n,
etc.)
This arises when cell division does not completely separate the copies of chromosomes.
In plants, this is more common, the extra sets of chromosomes lead to more vigorous
plants which produce bigger fruit or food storage organs that are more resistant to
disease. The consequence is the replication errors become more common.
If one population is triploid and the other is tetraploid, the evolution of each will be
different and then they will eventually become separate species.
Adaptive Radiation
Adaptive radiation occurs when many similar but distinctive species evolve relatively
rapidly from a single species or from a small number of species. This happens as
variations in the population allow certain members to exploit a slightly different niche in
a more successful way. By natural selection and the presence of one or more of the
barriers described above, new species evolve.
Lemurs are an example. Without competition from apes or monkeys, on the islands, the
species was able to proliferate. Large numbers of offspring meant a greater chance of
phenotypic diversity. They are not found in areas with other primates and have adapted
and show variations in their behaviours (some are nocturnal, dinural, live in trees or on
the ground). Fossils have been found in other areas, but not the lemur. Why? Because
they were not successful competing with their cousins.
This would explain why you see prosimians (lemurs) or anthropoids (apes and monkeys).
Some lemurs are endangered as they come in contact with recently evolved anthropoids,
humans.
Another example is the Darwin Finches. (see handout)
Convergent and Divergent Evolution (VERY IMPORTANT)
One species can have various splits over time creating a greater diversity between
species. In some cases, the branches of the phylogenic tree can become so far apart that
the species that were once closely related do not physically resemble each other. This is
called Divergent Evolution.
In other cases, it is possible to have two organisms with very different phylogenies but
look quite similar. This is called Convergent Evolution.
Each type of evolution of organisms is to allow it to fill a niche, or a place in an
ecosystem. If the environment is favorable for a certain form or behavior, the successful
organisms will change to fill that niche, and be able to survive and continue the gene
pool.
Diagram
Examples
Divergent
1. Adaptive radiation is one example of divergent evolution. The red fox and the kit
fox provide and example of two species that have undergone divergent evolution.
The red fox lives in mixed farmlands and forests, where its red color helps it
blend in with surrounding trees. The kit fox lives on the plains and in the deserts,
where its sandy color helps conceal it from prey and predators. The ears of the kit
fox are larger than those of the red fox. The kit fox's large ears are an adaptation
to its desert environment. The enlarged surface area of its ears helps the fox get
rid of excess body heat. Similarities in structure indicate that the red fox and the
kit fox had a common ancestor. As they adapted to different environments, the
appearance of the two species diverged.
2. Darwin’s finches – as the finches were cut off from one another due to
geographical isolation (allopatric speciation), the finches changed over time to
survive in their environments.
3. Marsupials had one common ancestor. A possum, kangaroo, koala and a wombat
are all marsupials, but they look very different from each other. As they adapted
to their environment, they changed over time.
Convergent
1. Fish, sharks and whales, all swim, but they do this in order to survive in their
environments. A fish and a whale are bony, while a shark is cartilaginous. Fish
and sharks have gills, while a whale has lungs. They are only similar due to their
environments. We have more in common with whales than fish do, even though
they look similar.
2. Types of plants have adapted to desert environments. The resemblance of the
cactus, which grows in the American desert, to the euphorbia, which grows in the
African deserts is very similar. Both have fleshy stems armed with spines. These
adaptations help the plants store water and ward off predators, but they are two
totally different species
3. This can also refer to how some animals use certain molecules. The use of
bioluminescence by marine organisms, bacteria and fungi is an example of
convergent evolution. The use of haemoglobin is another.
Coevolution is the joint change of two or more species in close interaction. Predators and
their prey sometimes coevolve; parasites and their hosts often coevolve; plant-eating
animals and the plants upon which they feed also coevolve. One example of coevolution
is between plants and the animals that pollinate them.
In tropical regions bats visiting flowers to eat nectar. The fur on the bat's face and neck
picks up pollen, which the bat transfers to the next flower it visits. Bats that feed at
flowers have a slender muzzle and a long tongue with a brushed tip. These adaptations
aid the bat in feeding. Flowers that have coevolved with bats are light in color. Therefore,
bats, which are active at night, can easily locate them. The flowers also have a fruity odor
attractive to bats.
In all types of evolution, it is the process of natural selection that allowed the
organisms to adapt to their environment in the ways in which they did.
Divergent and convergent evolution and coevolution are different ways organisms adapt
to the environment. These are examples of how the diversity of life on earth is due to the
ever-changing interaction between a species and its environment.
Pace of evolution: gradualism versus punctuated equilibria
Since geologists estimate the age of the Earth as being 4500 million years, and that life
originated about 3500 million years ago (mya), the timescale over which evolution has
occurred has seemed almost unimaginably long.
The fossil record provides evidence of the long evolutionary history of most major
groups. This observation of evolution by natural selection as being an exceedingly
gradual process is known as gradualism.
From the theory of evolution by natural selection we might expect species to only
gradually disappear, and be replaced by new species at a similar slow rate. Instead, this
may not have always been the case. Some new species have appeared in the fossil record
relatively quickly (in terms of geological time), and then have tended apparently to
remain unchanged or little changed, for millions of years.
Sometimes, periods of stability were followed by periodic mass extinctions, all evidenced
by the fossil record. Some say the fossil record looks like this because we have a partial
(distorted) fossil record, when compared to the numbers of organisms that have lived.
This is quite possible; we have no way of being certain the fossil record is fully
representative of life in earlier times. This is a possible explanation.
However, two evolutionary biologists, Niles Eldredge and Stephen Gould, proposed an
alternative explanation. They argue that the fossil record for some groups is not
significantly incomplete, but rather, accords with their hypothesis of the origins of new
species, which they called punctuated equilibria. This hypothesis holds that:





When environments become unfavourable, populations attempt to migrate to
more favourable situations.
If the switch to adverse conditions is very sudden or very violent, then a mass
extinction occurs. Major volcanic eruptions or major meteor impacts can throw so
much detritus into the atmosphere that the Earth’s surface is darkened for many
months, cooling the Earth and killing off much plant life. Populations at the fringe
of a massive disturbance may be sheltered or protected from the worst effects of
extreme conditions, and survive.
Members of these populations may become small, isolated reproductive
communities, from which repopulation eventually occurs.
The surviving group(s) may have an unrepresentative selection of alleles of the
original genepool. If one becomes the basis of a repopulation event and adapts to
the new conditions quickly, then abrupt genetic changes may occur. This
phenomenon is known as the founder effect.
The successful organisms will fill the niche that was left void by the sudden
movement or extinction of the original species.
So there are alternative proposals for the ways natural selection has operated in practice
in the establishment of life in geological time. In fact, gradualism and punctuated
equilibria may not be alternatives; both may have contributed to the pattern of life on
Earth in geological time.
The only way to say which is the preferred pattern is to look at fossil records.
Proponents of gradualism point to the fact that some species have lived for millions of
years with little or no change, like a shark or a cockroach. Critics point to fossils that
show massive extinctions and are incomplete. Critics of punctuated equilibrium argue
that the jumpy effect of this theory could simply be and artifact of the incompleteness of
the fossil record.
One difficulty of supporting either claim is the only evidence used is fossil evidence.
Pigmentation, behavior or mating calls cannot be fossilized. Another argument is just
because a fossil looks like a modern day organism, does not indicate the latter is a direct
descendent of the former or that the two species would have been able to reproduce
together.
Transient polymorphism and Balanced Polymorphism
Transient polymorphism
Within a population, there is often more than one common form. Different versions of a
species are referred to as polymorphisms (many shapes) and can be the result of a
mutation. When one form changes to another due to the environment, and then changes
back over time, this is called Transient polymorphism. An example is the Peppered
Moth, or Biston betularia.
Balanced Polymorphism
When two or more alleles are stabilized by natural selection, this is called balanced
polymorphism. Sickle Cell anaemia is an example.
As mentioned before, the dominant allele, if in the homozygous form means the person
has normal blood cells. This means that the person is susceptible to malaria. If the
person has both the recessive alleles, the person will have anaemia, but are very resistant
to malaria. To be heterozygous means you have anaemia but are more resistant to
malaria.
Because of this paradox, the allele frequency for the sickle cell trait is relatively stable
and therefore shows balanced polymorphism. Two pressures of selection maintain this
balance. On the one hand, the sickle cell trait should be selected against because it can be
debilitating or lethal. On the other hand, there is a selection for it because having it gives
people more resistance to malaria. The balance is reached in heterozygous individuals
who tend to be more fit for survival in zones plagued by malaria but do not suffer severe
anaemia.
Human Evolution
Assessment Statement
D.3.1
Outline the method for dating rocks and fossils using radioisotopes, with
reference to 14C and 40K
D.3.2
D.3.3
Define half-life
Deduce the approximate age of materials based on a simple decay curve
for a radioisotope
Describe the major anatomical features that define humans as primates
Outline the trends illustrated by the fossils of Ardipithecus ramidus,
Australopithecus, including A. afarensis, and A. africanus, and Homo,
including H. erectus, H. neanderthalensis and H. sapiens
State that, at various stages in hominid evolution, several species may
have coexisted
Discuss the incompleteness of the fossil record ant the resulting
uncertainties about human evolution
Discuss the correlation between the change in diet and increase in brain
size during hominid evolution
Distinguish between genetic and cultural evolution
Discuss the relative importance of genetic and cultural evolution in the
recent evolution of humans
D.3.4
D.3.5
D.3.6
D.3.7
D.3.8
D.3.9
D.3.10
Humans are known as Homo sapiens (modern man). The full classification is:
Kingdom:
Phylum:
Subphylum:
Class:
Subclass:
Order:
Suborder:
Family:
Genus:
Species:
Animalia
Chordata
Vertebrata
Mammalia
Eutheria
Primates
Anthropoids
Hominidae
Homo
Sapiens
The fossil record, allows us to look at common morphology and deduce common
ancestry. The relatedness of organisms is investigated by comparative biochemical
studies, particularly of mitochondrial DNA, which in each generation is passed from
mother to offspring unchanged. This type of DNA undergoes a steady rate of mutation –
it changes as a function of time alone. The degree of difference between mitochondrial
DNA samples discloses how recently groups of organisms shared a common ancestor.
Dating Rocks and Fossils
Fossils can tell us a lot about the past.
Fossil – any form of preserved remains from a living organism.
Some examples are:
 Mammoths frozen in Siberia
 Mummies in acidic swamps in Scandinavia
 Insects in amber
 Bones in rock
Fossils are only formed in some circumstances. Most individuals do not leave a fossil
after death.
A fossil has to be formed when an organism dies and gets buried in sedimentary silt. It
will decay slowly and leave a space in the silt. The gap becomes solid and is filled the
exactly the same as the organism left behind. The silt may solidify, becoming
sedimentary rock and in it is the fossil.
To see how old fossils are and their forms, carbon dating is used, usually Carbon 14 and
potassium 40, which are isotopes.
Isotopes are atoms of the element that have different numbers of neutrons. Therefore,
they are unstable and will spontaneously change into one or more atoms to other
elements, often emitting some radiation. The time taken for this change is determined by
the kind of isotope. After a period of time, at a fixed interval, the radioactive decay
will be half of what it was before. This is called half-life. For C14, the half-life is
5730 years.
Using C14
Most carbon is C12, but due to cosmic radiation C14 is formed at a low, steady rate. While
alive, organisms absorb carbon in the ratio of C12 / C14 present in the environment around
them. After death, accumulation of radioactive (and other) atoms stops. Meanwhile, C14
steadily breaks down:
half-life of 5.6 X 103 years
C
14
N14
So the ratio of C14: C12 in a fossil decreases with age; the less C14, the older the fossil.
This technique gives good dates for fossils of the last
60 000 years.
Using the ratio of K40:Ar40
Rocks do not eat or photosynthesize. Some contain no carbon at all. Instead we use
potassium – 40. The pyroclastic rocks flowing out of volcanoes may contain
radioactive isotopes such as potassium-40, which decays to argon-40, as shown:
K40
half-life of 1.3 X 109 yrs
Ar40 (gas)
In hot lava, argon gas boils away into the atmosphere. Once lava has solidified by
cooling, which occurs quickly after volcanic eruptions, the argon gas that is then formed
by radioactive decay is trapped in the rock.
By measuring the ratio of K40: Ar40 in lava deposits, the exact ages of the lava and the
approximate age of the sedimentary rocks (and their fossils) below and above lava layers
are estimated.
This technique spans the whole of geological time back to the Cambrian period (580 million years ago), but it is too slow to give
reliable results over the most recent half million years.
Determining the age of a fossil or rock.
You look at the percentage of Carbon – 14 or potassium – 40 left in the fossil or rock. If
there is 50% left, that means there has been 5730 years past for C14. For 25%, that would
be double the half-life or 11 460 years. Another way, is to look at a decay curve and see
where the amount of remaining C14 falls on the curve to estimate the time.
Humans as Primates
Humans belong to the mammalian order Primates. This order contains three distinctive
groups of animals, namely the apes (which includes the genus Homo), the monkeys, and
the prosimians (a name meaning ‘before the monkeys’). These are mostly tree-dwelling
species with grasping hands and feet. The range of animals that constitute the Primates
and how they are related are summarized below.
Apart from humans, who have achieved worldwide distribution, most primates live in
tropical and sub-tropical regions. An interesting feature of primates is their relatively
unspecialized body structure, combined with some highly sophisticated behavior patterns.
Why are humans defined as primates?
To the biologist, humans are primate mammals. By this we mean that humans show
many of the characteristics of other mammals, the general characteristics common
to other primates, and many of the features shown by the great apes to which we are
most closely related.
Major features, which describe humans, as primates are adaptations to tree life. They are
the opposable thumb, acute vision (stereoscopic vision), mobile arms and shoulder
girdle and a skull modified for upright posture.
Having an opposable thumb means you can manipulate objects and be able to grasp.
Mobile arms, allows movement in three planes and transfer weight via the arms. This is
very important for tree dwellers and for movements above the head. Also, living in trees
means you can see further. As a result, the eyes are places more forward, on a flat
front. This gives a smaller field of vision, but more acute and the overlap of vision
allows for good depth perception and judgment of distance. Along with this is color
vision. The positioning of the magnum foramen (mentioned later in these notes),
allows for the spinal column to insert into the skull in a more upright position, lessening
the curvature of the spine and allowing primates to walk upright.
Origins of Humans – Trends in hominid and human fossils
The earliest fossils which we confidently identify as anthropoids (apes) have been found
at many sites in Africa. They date from about 35 million years ago (mya). Humans
clearly demonstrate one form of anthropoid body organization, so we can say the human
story has taken about 35 million years to unfold.
There are some similarities and some major differences between all the hominid skulls
that have been unearthed. The details to pay close attention to are summarized below.
One key area is the foramen magnum. It is the hole where the skull is attached to the
spinal column. In modern humans the hole is in the center of the base of the skull, giving
rise to the theory of walking upright. In apes, the hole is further back to accommodate
the spinal column in an animal that walks on four legs.
There is some discussion as to what a hominid is. It can only refer to those bipedal
primates, which are direct ancestors of modern humans.
The chronological order of some of the species of hominid for which have been found is
below.
Fossils of Hominidae have lead to several speculations about evolution.
1. Ardipithicus ramidus
a. Lived approximately 5.8 – 4.4 mya in Ethiopia. This species is believed to
be very close to the split between the line of organisms, which became
more human-like and the line, which became more chimpanzee-like. Most
of the fossils are teeth and therefore, it is difficult to be sure. From what
has been found, the Ardipithicus ramidus was very similar to a
chimpanzee with a few hominid features. The molars show more ape-like
characteristics, as the length is greater than the breadth. The canines are
more hominid, as they are shorter and not as sharp as ape canines.
2. Australopithecines (southern ape) lived about 4 mya. They had 500 cm3 brains
and walked upright.
a. The first species was A. afarensis from the Afar desert (4-2.8 mya) found
in Ethiopia and Tanzania. (Lucy skeleton) It had a tall lower jaw, fairly
large molar teeth and a projecting face. The cranial capacity was 380 –
430 cm3.
b. Later came africanus (3-2 mya) found in South Africa. It is thought to be
the same species as afarensis as features are similar and walked upright. It
had a tall, thick lower jaw, large molars and a projecting face. The cranial
capacity was 435 – 530 cm3.
c. Later was A. robustus (2-1.4 mya) in South Africa. They were larger and
heavily built.
3. Then came the Homo genus. They were from around 2 mya and had larger
brains (600 cm3) and walked upright.
a. First was H. habilis (handy man). It is thought he arose from A.
afarensis 2 mya in East Africa and used simple tools. It had a flatter face,
larger molars but the cranial capacity was still only about 600 cm3.
b. Homo erectus was from Africa. It is thought it migrated to other parts of
the world and had a larger brain than H. habilis. H. erectus spread to Asia
and Europe. However, it is believed that H. sapiens evolved at one place
in Africa and from their spread out over the world. It has a smaller jaw, a
receding forehead, large brow ridges and smaller molars. Its cranial
capacity was 1000 cm3.
c. H. neanderthalensis, which lived in Eurasia from 200 000 to 30 000 years
ago. The species survived several ice ages. It has a smaller jaw, a lower
forehead, smaller brow ridges and smaller molars than the previous
species. They had larger brains than modern humans, with a cranial
capacity of up to 1600 cm3.
d. Next was H. sapiens, which came to Europe. The first subspecies was
Cro-Magnon man, who looked a lot like modern humans and though to
have used the first language. H. sapiens lived around 140 000 to 70 000
years ago in Africa and Asia as well. They had a high forehead, no brow
ridges, a flat face, small molars and a very small jaw. This species
developed cave paintings, tools and weapons. The cranial capacity was
similar to today’s humans of 1300 cm3.
Based upon where the skulls were found and dating, we can see that many species may
have coexisted. As the Homo sapiens developed, so were the homo neaderthalenis.
The incompleteness of the Fossil Record
Anthropologists disagree about the origin of modern humans from time to time. They use
evidence from fossil remains, from artifacts like stone tools that can be associated with
particular hominids, and the record in animal bones that surrounded their habitations and
which indicate diet. Fresh evidence of these types is frequently discovered, and existing
data are sometimes reinterpreted.
Re-interpretation occurs in the light of new biochemical evidence or the development of
new analytical techniques. For example, until quite recently, another theory about the
origin of modern humans vied with the current ‘out of Africa’ theory.
The alternative was a multiregional model, in which H. sapiens emerged wherever
populations of H. erectus had become established, in Africa, Europe and Asia. This made
H. neanderthalensis only one example of an archaic hominid form, intermediate between
H. erectus and modern humans. According to this model, there was ongoing genetic
exchange between populations of various archaic forms until H. sapiens emerged and
replaced all others. Currently, the body of evidence is increasingly against this theory.
Controversy will continue because of the inevitable incompleteness of the fossil record.
Fossilization is an extremely rare, chance event. This is because predators, scavengers
and bacterial action normally break down dead plant and animal structures long before
they can be fossilized. Of the relatively few fossils formed, most remain buried, or if they
do become exposed, are often overlooked or may be accidentally destroyed before
discovery.
Nevertheless, numerous fossils have been found, and as more hominid fossils are
discovered, so our knowledge may change and our understanding of our past be
advanced. This is yet one more branch of science where the frontier of knowledge is
entirely open. You can follow the debate from now on.
Brain Size
So why did the brain develop? Some think because the environment was so diverse, a
larger brain was needed to deal with the challenges, and therefore, larger brains were
selected as an advantage.
Habilines were the first hominids to be associated with tools – they used large pebbles,
chipped in at least two directions, as sharpened implements to crush, break and cut. Their
additional brain capacity had resulted in advanced manual dexterity. It was applied
to the making and using of simple tools (selected strong stones) to chip pebbles, for a
purpose. Using tools to make tools (i.e. the development of a tool industry) is what
distinguishes hominid toolmakers from all other tool-users in the living world.
Skull endocasts (casts of the inside of the brain case of the skull) show that the areas
of the brain associated with speech and language are significantly developed, so we
can assume that cultural evolution was also under way. This was also the first
hominid to use fire consistently, which will have aided the colonization of areas so far
north of equatorial Africa, and also with its habit of eating meat.
By modern human standards, H. erectus had a marked brow-ridge and protruding jaws,
but the pronounced sexual dimorphism of earlier hominids was reduced – adult males
were now only about 20–30% larger than females.
The brain size of Neanderthals was larger than that of modern humans. This may reflect
the requirements of controlling the large musculature, because they were heavier and
more muscular than H. sapiens. The latter are more slightly built, but taller and longerlimbed.
Humans were hunter–gatherers but compared with many of the competing wild animals,
not especially strong or fast. Scavenging would have been a major source of nutrients,
at least initially. Only with the development of agriculture and other advances in
technology (e.g. brewing, cheese making) did humans move into circumstances in which
population sizes grew significantly, and they could start to dominate their environment
and become secure.
The issue of the actual diet at each stage may have been a critical factor. This is because
brains are metabolically expensive. Our brains make up 2% of our body mass but respire
about 20% of our energy budget. The human brain is about three times the size of that of
an equivalently sized ape. We may conclude that the expansion in the brains that we
have noted in the succeeding species of Homo will have demanded enhanced energy
supplies. This means that human evolution must have been increasingly dependent on a
reliable supply of protein and fat. However, it is not dependent on advanced hunting
skills, at least not from the outset. Hominids will have discovered that the long bones of
herbivorous mammals (discarded by the large carnivorous mammal hunters around them)
were a rich source of bone marrow. Bone marrow is rich in protein and fat and could be
accessed by nothing more sophisticated than a heavy blow from a rock onto the bone
shaft, held against a hard place.
Bipedalism
Parts of skulls of this genus have been uncovered in various locations (including at
Taung, in 1924), prior to the discovery of a new hominid fossil, first known as Lucy, at
Hadar in Ethiopia in 1974.
Lucy is identified as Australopithecus afarensis. She was ape-like in that she had the
same limited brain capacity as ape species of the period, but hominid-like in that she was
a powerful, upright walker (the pelvis was of characteristically human form) and had no
long muzzle.
We now recognize that upright walking (known as bipedalism) was an early stage in the
evolution of the hominids. The Lucy fossil was laid down 3 mya.
We are confident about bipedalism at this time, because of the discovery of the footsteps
at Laetoli, imprinted in volcanic ash, 3.6 mya. The soft ash was presumably moistened by
rain (no additional prints added), immediately baked into hard rock, and then buried by
soil blown in. The footsteps were discovered in 1976. Two adults had walked in line, in a
northerly direction, with a youngster who later ran off to one side. Being volcanic ash,
this trace fossil can be dated precisely by the potassium:argon ratio method.
One advantage of bipedalism (perhaps the chief advantage, initially) is as a mechanism
to prevent the head region of the body overheating at the high midday temperatures
of equatorial latitudes. Being upright does not expose as much surface area to the sun.
Australopithecines lived in mosaic environments: part tropical rainforest, part woodland
and tree-savannah, part scrub. Wherever they lived, no doubt they preferred to shelter at
times of greatest temperature. But they may have often needed to travel to new venues,
visit water holes, or scavenge and collect food at times when faster and stronger
predatory animals were most likely to be resting. If so, being bipeds gave them an
advantage.
Another critical advantage of bipedalism is that hands are freed for obtaining and
carrying food. Apes breed slowly, producing few offspring at a time. A male ape that
had mastered bipedalism could improve his mate’s reproductive capacity by feeding her,
thus freeing her to concentrate on the production and rearing of young. The genes of apes
with a tendency for bipedalism will have had a better chance of replication in future
generations. This would have been particularly effective in male–female pairs, rather than
in troops of primates where males invested time and energy maintaining dominance over
the females. On this account, hominids would have tended to be monogamous apes with
lessened sexual dimorphism (males the same size as females).
Genetic vs. Cultural Evolution
Genetic evolution refers to the changes in allele frequencies that result in changes in
individuals and therefore in populations, brought about by natural selection.
In outline, these are due to:
 Genetic variations, which arise via mutations, random assortment of paternal and
maternal chromosomes in meiosis, recombination of segments of maternal and
paternal homologous chromosomes during crossing over that occurs in meiosis in
gamete formation, and the random fusion of male and female gametes in sexual
reproduction.
 When genetic variation has arisen in organisms, it is expressed in their
phenotypes. Some phenotypes are better able to survive and reproduce in a
particular environment, and natural selection operates to determine the survivors
and the genes that are perpetuated in a population. In time, this process may lead
to new varieties and new species.
By cultural evolution we refer to the development of the customs, civilization and
achievements of people. The development and transmission of human culture has a
biological basis.
Key to this was the extension of the period of parental care, delayed onset of puberty,
and the resulting long period of childhood when the next generation of a population are
trained and schooled as they develop essential survival skills – all features of the
evolution of the genus Homo.
The development of language is the most important human characteristic central to the
evolution of culture. Endocasts give a slight impression of the areas of the brain that
developed and were enlarged (the chief neural machinery for speech in most modern
humans is found in the left hemisphere). Also critical is the position of the vocal folds in
the neck. On both counts it seems likely that only Neanderthals and H. sapiens achieved
the structures necessary for elaborate vocal communication. In particular, the high palate
and high larynx found in H. sapiens allowed a greater range of resonance for complex
word sounds.
Once established, verbal communication allowed advantageous developments (for
example, in the form of new ideas) to be passed on rapidly. The potential speed of
development of this form of cultural evolution contrasts markedly with change brought
about by slow inherited accumulation of advantages by genetic evolution. Today’s latest
cultural-sharing breakthroughs – the Internet and the human genome project – are cases
in point.
The developments in tool technology were also dependent on the development of a
large brain.
Compared to the achievements of the Habilines in this, from about 35 000 years ago,
modern humans made spectacular advances. Bone and antler were added to the list of raw
materials, and advances in the skills of fashioning stone flakes and blades into finely
worked scrapers, chisels, drills, arrowheads and barbs were spectacular. Tool-kits
comprised items for engraving and sculpture. Functional implements like spears became
decorated with life-like animal carvings.
The latter point relates to human use of the brain, powers of detailed observation, and
manual dexterity, all of which underpin cultural development. Homo sapiens as observers
and artists achieved incredible feats at the earliest phase of their development.
We have a remarkable record of the artistic skills of our first human ancestors in the cave
paintings from this period that have been discovered. The drawings, produced by human
communities from 25 000 to 10 000 years ago, show contemporary animals in scientific
detail. The pictures demonstrate perspective representation.
The relative importance of genetic and cultural evolution is quite obvious.
Genetic evolution has given rise to the diversity of living things, including human beings.
However, this is a process that has taken thousands of millions of years. The special
features that humans have developed, mostly unique to them, have been the basis of
cultural evolution. For example, with the development of agriculture and other
technologies, humans have changed their immediate environment with the creation of
settlements and then gone on to evolve communal living. Enlarged populations have been
both necessary to the new way of life, and sustained by it. Rules and laws have succeeded
basic customs, and individuals have acquired rights and responsibilities. Consequently,
the conditions for genetic evolution have been progressively sidelined as the processes of
cultural evolution have taken over.
Brain Size and Evolution
Taxonomy
The Science of Classification
Assessment Statement
5.5.1
5.5.2
5.5.3
5.5.4
5.5.5
Outline the binomial system of nomenclature
List seven levels in the hierarchy of taxa – kingdom, phylum, class, order,
family, genus and species, using an example from two different kingdoms
for each level
Distinguish between the following phyla of plants, using simple external
recognition features; bryophyta, filicinophyta, coniferophyta and
angiospermophyta
Distinguish between the following phyla of animals, using simple external
recognition features; porifera, cnidaria, platyhelminthes, annelida,
mollusca, and arthropoda
Apply and design a key for a group of up to eight organisms
Classification is an essential tool in Biology as there are many organisms to name.
The process of classification involves giving every organism an agreed name and the
arranging of organisms into groupings of apparently related organisms. Overall, we see a
scheme of the overall diversity of living things. Classification also attempts to reflect any
evolutionary links.
The Binomial System
The binomial system of nomenclature was invented by Carolus Linnaeus in the 18th
Century. It is still used today and is based on the idea that every species has a Latin
name, made up of two parts.
The first part of the name is the genus or the generic name based upon a noun. The
second name is the species, or the specific name, based upon an adjective.
For example:
The Scheme of Classification
The science of classification is taxonomy. “Taxa” is the general word for groups or
categories. Biological classification is the invention of biologists, based upon the best
evidence at the time.
There are 7 categories for naming:







Kingdom – largest and most inclusive grouping
Phylum / division – organisms constructed on a similar plan
Class – a grouping of orders within a phylum
Order – a group of apparently related families
Family- a group of apparently related genera
Genus - a group of similar and closely related species
Species – a group of organisms capable of interbreeding to produce fertile
offspring
There are 5 kingdoms to classify organisms. They are:
1.
Prokaryotes – unicellular organisms lacking nuclei and other membrane bound
organelles. DNA is mainly circular and is not organized in chromosomes.
Examples are bacteria and cyanobacteria
2.
Protista – unicellular and multicellular eukaryotic organisms that may be
autotrophic or heterotrophic and may live in salt or fresh water. Examples are
Euglena and Paramecium
3.
Fungi – Eukaryotic filamentous or unicellular. Filamentous fungi grow a
mycelium from which mushrooms or toadstools grow. They are heterotrophic
and they feed by absorption of nutrients. Their cells have chitin in the cells
walls, as opposed to cellulose. Examples are yeasts and mushrooms.
4.
Plantae – Eukaryotic, multicellular, phosynthetic organisms. The cells walls
contain cellulose, and most cells contain chlorophyll. Examples are mosses,
ferns, flowering plants.
5.
Animalia - Eukaryotic, multicellular, heterotrophic organisms that are often
motile, and feed by ingestion. Examples are humans and jellyfish.
Below are some examples.
Distinguishing Between the Phyla
1. Plantae phyla
To distinguish between the four phyla, two categories can be used:
Vegetative characteristics such as leaves and stems
Reproductive characteristics
Bryophytes (mosses, liverworts)
 are non-vascular as the have no true xylem or phloem
 do not produce seeds or flowers, but spores which are transported by water (which
is the reason they are found in moist environments
Filicinophyta (ferns and horsetails)
 are vascular but reproduce by spores
Coniferophyta (cedars, junipers, fir, pine trees)
 Vascular, woody stems and leaves are in the form of needles or scales
 All species of conifer use wind to help them reproduce by pollination
 Produce seed cones with seed scales
Angiospermophyta
 Vascular stems
 Produce seeds that are not all pollinated by wind, and use insects, birds and other
animals
 Use flowers to reproduce and fruit hold the seeds.
2. Animalia Phyla
All of the six phyla are invertebrates (they have no backbone)
Porifera (sponges)
 Simple marine animals that are sessile (stuck in place)
 No mouth or digestive tract
 Feed by pumping water though their tissues to filter out food
 Have no muscle of nerve tissue and no distinct internal organs
Cnidaria (corals, sea anemones, jellyfish, sea jellies, hydra)
 Some are sessile, others are free swimming
 To digest food, they catch it in their tentacles and have a gastric pouch with only
one opening.
 ALL have stinging cells call nematocysts
Platyhelminthes (flatworms)
 Only one body cavity a gut with an opening for food to enter and waste to exit
 No heart or lungs
 Flat shape to have cells close to surface for gas exchange by diffusion
Annelida (earthworms, leeches and polychaetes)
 Segmented worms, as their bodies are divided up into sections, separated by rings
 Bristles on their body
 Gastric tract with a mouth at one end and the intestines have an opening at the
other end where wastes are released
Mollusca (snails, clams, and octopi)
 Many produce a shell and are not segmented
 Gastric tract with a mouth at one end and the intestines have an opening at the
other end where wastes are released
THEY WILL NOT SHARE!!!
Arthropoda (insects, spiders, scorpions, and crustaceans (crabs, shrimp))
 Have a hard exoskeleton, made of chitin
 Segmented bodies
The Dichotomous Key
The process of naming unknown organisms in ecological fieldwork is time consuming.
Often comparisons are made using books with illustrations and information that provide
us with clues about habitat and habits, which we can use to identify organisms.
Alternatively, the use of keys may assist in the identification of unknown organisms. The
advantage is that it requires careful observation. The structural features of organisms,
allows us to understand how different organisms may be related.
Steps in Key Construction
A dichotomous key is a method for determining the identity of something (like the name
of a butterfly, a plant, a lichen, or a rock) by going through a series of choices that leads
the user to the correct name of the item. Dichotomous means “divided in two parts”.
At each step of the process of using the key, the user is given two choices; each
alternative leads to another question until the item is identified. It is like playing “Guess
Who” or 20 Questions.
For example, a question in a dichotomous key for trees might be something like, “Are the
leaves flat or needle-like?” If the answer was “needle like”, then the next question might
be something like, “Are the needles in a bunch or spread along the branch?” Eventually,
when enough questions have been answered, the identity of the tree is revealed.
Below are several leaves of several trees. Also there is a Spider Key and a Couplet
Key. Each is acceptable.
You will construct a key with the above fictional animals. When you construct a key,
keep the following in mind:
1.
2.
3.
4.
5.
6.
7.
Use constant characteristics rather than variable ones.
Use measurements rather than terms like “large” and “small”.
Use characteristics that are generally available to the user of the key, rather than
seasonal characteristics or those only in the field.
Make the choice a positive one – something “is” instead of “is not”.
If possible, start both choices of a pair with the same word.
If possible, start different pairs of choices with different words.
Precede the descriptive terms with the name of the part to which they apply.
When you are done, move on to the assignment on Constructing a Dichotomous Key.
Mathematics of Population at Equilibrium
Hardy-Weinberg Principle
Assessment Statement
D.4.1
Explain how the Hardy-Weinberg equation is derived
D.4.2
Calculate allele, genotype and phenotype frequencies for two alleles of a
gene, using the Hardy-Weinberg equation
State the assumptions made when the HardyWeinberg equation is used
D.4.3
We have noted that in any population, the total of the alleles of the genes located in the
reproductive cells of the individuals make up a gene pool. A sample of the alleles of the
gene pool will contribute to form the genomes (gene sets of individuals) of the next
generation, and so on, from generation to generation.
When the gene pool of a population remains more or less unchanged, then we know that
population is not evolving. However, if the gene pool of a population is changing (i.e. the
proportions of particular alleles are altered – we say ‘disturbed’ in some way), then
evolution may be going on.
How can we detect change or constancy in gene pools?
The answer is, by a mathematical formula called the Hardy–Weinberg formula.
Independently, this principle was discovered by two people in the process of explaining
why dominant characteristics don’t take over in populations, driving out the recessive
form of that characteristic. For example, at the time, people thought (wrongly) that
human eye colour was controlled by a single gene, and that an allele for blue eyes was
dominant to the allele for brown eyes. They wanted to answer the question, “Why doesn’t
the population become blue-eyed?”.
Hardy and Weinberg came up with an idea that is all factors remain the constant in a
population, the gene pools composition will remain the same as well.
To test this, field studies need to be done to determine the relative percentage of
phenotypes in population.
An example was the Peppered Moth. Studying the amounts of white vs. dark began in
1959. Keep in mind this was the time when the pollution was greatest. The results were
as follows:
-
1959 – 94% of moths were dark
1969 – 90% of moths were dark
1979 – 79% of moths were dark
1989 – 40% of moths were dark
1994 – 19% of moths were dark
This could also be calculated using the Hardy-Weinberg Principle. It is a mathematical
model for calculating allele frequency for a gene with two (or three) alleles.
The formula
For 2 alleles of a gene:
- Use B for dominant, and its frequency in the population is p (a number between 0
–1)
- Use b for recessive, and its frequency in the population is q (a number between 0
–1)
- A gene must have an allele, with the options either B or b. No other options are
available, so if B is present, it frequency is 1, and b is 0, therefore p + q = 1 (1+0)
- Each gene has two alleles, so if the frequency of B is p, then BB is p2
- If the frequency of b is q, then bb is q2
- If you have Bb, the frequency is 2pq
- Since genotypes must be one of the three, the percentage in a population will be:
p2
+
2pq
+
q2 = 1
This is the Hardy-Weinberg equation. In order to be used, the following conditions
need to be observed.
-
Large population
Random mating occurs
No directional selection (no advantage)
No allele specific mortality
No mutations
No immigration or emigration
Example 1
In a certain population of Drosophila, 64 individuals are found to have red eyes (wild
type) and 36 are found to have white eyes. Find the allele frequency for each allele and
the genotype and phenotype frequency.
Questions using the Hardy-Weinberg equation
1.
Suppose a recessive genetic disorder occurs in 9% of the population. Determine
what percentage of the population is heterozygous for this disorder.
2.
For a hypothetical moth population, suppose that 60% of the moths are white
coloured and 40% are dark coloured, with white being dominant. Three years
later, the percentages are 65% white and 35% black. What does this shift say
about the dark phenotype?
3.
In a population of mosquitoes, the frequency of the recessive allele for vestigial
wings is 30%. Predict how many flies would be expected to have normal wings
in a population of 125.
Phylogeny and Systematics
Assessment Statement
D.5.1
D.5.2
D.5.3
D.5.4
D.5.5
D.5.6
D.5.7
D.5.8
D.5.9
D.5.10
Outline the value of classifying organisms
Explain the biochemical evidence provided by the universality of DNA
and protein structures for the common ancestry of living organisms
Explain how variations in specific molecules can indicate phylogeny
Discuss how biochemical variations can be used as an evolutionary clock
Define clade and cladistics
Distinguish, with examples, between analogous and homologous
characteristics
Outline the methods used to construct cladograms and the conclusions
that can be drawn from them
Construct a simple cladogram
Analyse cladograms in terms of phylogenetic relationships
Discuss the relationship between cladograms and the classification of
living organisms
One of the objectives of classification is to represent how living and extinct organisms
are connected, which means natural classification. Phylogeny is the study of the
evolutionary past of a species. Species which are the most similar are most likely to be
closely related, whereas those which show a higher degree of difference are considered
less likely to be closely related.
There are several values to classifying this way.
1. We can identify unknown organisms, as other similar organisms are grouped
together using a key.
2. We can see how organisms are related in and evolutionary way. By looking at
organisms, which have similar anatomical features, it is possible to see
relationships on their phylogenetic tree. DNA evidence confirms the anatomical
evidence for placing organisms in the same group.
3. It allows for the prediction of characteristics shared by members of a group.
Biochemical Evidence for common ancestry
Biochemical evidence, including DNA and other protein structures, has brought new
validity and confirmation to the ideas of a common ancestor.
The fact that every known living organism on Earth uses DNA as its main source of
genetic information is compelling evidence that all life came from a common ancestor.
All the proteins found in living organisms use the same 20 amino acids to forms their
polypeptide chains. Genetic engineering has provided some evidence of this.
Amino acids can have two possible orientations: left-handed and right-handed,
depending on how the atoms are attached. All the living organisms on Earth have lefthanded amino acids and none are right-handed, leading to the belief that there is a
common ancestor.
Traditionally, looking for similarities has been done using morphology. More attention
recently to molecular differences is now the area of study.
Although the same components are used to make DNA and protein in all organisms, the
sequence of these components may be different. If we compare the amino acid sequences
of haemoglobin in humans, cats and earthworms, we see that cats and humans have
greater similarities that humans and earthworms.
This shows two trends:
1. The more similar the biochemical evidence, the more interrelated the species are
2. The more similar the evidence, there is less time since the two species had a
common ancestor (ie. The ancestor of earthworms lived a longer time ago than
the ancestor of cats and human.
3. Changes in the DNA sequences of genes from one generation to another are
partly due to mutations and the more differences there are between two species,
the les closely related they are.
Here is an imaginary example of DNA sequence from four different species.
1.
2.
3.
4.
AAAATTTTCCCCGGGG
AAAATTTACCCCGGGG
AAAATTTACCCGCGGG
AACATCTTCCACGCTG
It is clear that species 1 and 2 have the fewest differences between them and we can
conclude that they are more closely related.
Since this evidence is not conclusive on its own, it is often used together with other data,
such as palaeontological data.
The evolutionary clock
The principle is you study similar molecules in different species and determine how
much difference there is between the molecules. The more difference there is, the longer
the time span since the two species had a common ancestor. Differences in polypeptide
sequences accumulate steadily and gradually over time, as mutations occur from
generation to generation in a species. The changes can be used as a kind of clock to
estimate how far back in time two related species split from a common ancestor. This is
called the evolutionary clock.
Commonly used proteins are haemoglobin, cytochrome c (a respiratory protein which is
part of the electron transport chain) and nucleic acids. We count up the number of base
pairs, which do not match.
Using haemoglobin show that humans are more closely related to chimps rather than
gorillas or gibbons. Using cytochrome c, we see that humans have identical molecules,
while rhesus monkeys only differ by one amino acid. Humans and rhodospirillium
(bacteria) or yeast (fungi) have identical amino acid sequences in part of the cytochrome
c molecule!!!
Number of differences in the Beta Haemoglobin Chain compared to Human
Haemoglobin.
Imagine comparing certain DNA sequences form three species A, B and C. Between the
DNA samples from A and C there are 83 differences. Between A and B, there are only
26 differences. We can conclude that A is more closely related to B than C. There has
been more time for DNA mutations to occur since the split between A and C than since
the split of A and B.
One technique, which has been successful in measuring differences in biochemical
studies, is DNA hybridization. We take one strand of DNA from species A and a
homologous strand from B and fuse them together. Where the base pairs connect, there
is a match; where they are repelled and do not connect, there is a difference in the DNA
sequence.
This can be taken further. If we see that 83 differences is approximately three times more
than 26 differences, we can conclude that the split between species A and C happened
about three times further in the past that the split between species A and B. We can
express this in a cladogram. There are two forms, which we will look at in a little bit.
Percentage difference in DNA
A
B
C
C
B
A
Time in mya
Keep in mind this clock is not a consistent “tick-tock” like the clock on the wall.
Mutations happen at varying rates. The above is an estimation of the events. Again this
is all compared to morphological data and radioisotope dating.
Clades and Cladistics
Cladistics – a system of classification, which groups taxa together according to the
characteristics, which have most recently evolved. It is the concept of common
descent that decides into which group an organism belongs. It is therefore an example
of natural classification, where primitive and derived traits are looked at as to how
many are shared.
Clade – a monophyletic group. This means it is a group composed of the most recent
common ancestor of the group and all its descendents. It could be made up of
several species. Comes from the Greek work ‘klados’ meaning ‘branch’.
To decide how close a common ancestor is, researchers look at how many primitive and
derived characteristics the organisms share. Primitive traits (plesiomorphic traits) are
characteristics which have the same structure and function and which evolved early on in
the organism’s development. Derived traits (apomorphic traits) are characteristics which
have the same structure and function, but which evolved more recently as modifications
of a previous trait. A primitive trait would be plants with vascular tissue in leaves but a
derived trait are the flowers, which developed after the leaves in angiosperms.
Analogous and homologous characteristics
To put organisms in the appropriate clades, two types of characteristics considered are
analogous and homologous characteristics.
Homologous – are characteristics from the same part of the common ancestor.
Pentyldactal limbs are examples. Eyes are another example.
Analogous – are characteristics which may have the same function but do not have the
same structure. All animals with wings fly, but they are not in the same
clade due to the structural differences between a fly wing, and a bird’s
wing.
How cladograms are made
To represent the findings of cladistics in a visual way, a cladogram is used. It is a
diagram, in which nodes are used to separate species and organisms, which have diverged
from the common ancestor and form a clade. The cladogram below takes into account
skeletal structures and that bats and dolphins are placental mammals.
The way to construct a cladogram is to look at biochemical differences or morphological
differences.
1. Make a list of the organisms involved
2. Make a list of as many possible characteristics, which each organism possesses.
3. From the list many traits will clearly be derived characteristics
a. Examples are:
i. Eukaryotic
ii. Backbone
iii. Amniote egg
iv. Limbs
v. Hair
vi. Opposable thumbs
vii. Multicellular
viii. Segmented body
ix. Jaws
x. Placenta
xi. Mammary glands
4. Once the list has been established, there will be one, which is common to all the
organisms being studied. The ancestral trait is considered the primitive
characteristic. Morphologically, would be eukaryotic or multicellular. In
biochemical data, it might be a certain sequence to base pairs.
5. You make a table like below, showing the derived characteristics.
6. You make the cladogram with the first branch form the bottom belonging to the
organism with the fewest derived traits. The organism with the most derived
characteristics goes to the top of the last branch.
Why are cladograms constructed?
To show the evolutionary relationships between organisms. It can be concluded that
organisms whose branches start at the bottom of the cladogram are the earliest ones to
have evolved and the ones at the top are the ones, which have evolved most recently
among the organisms considered in the cladogram.
Each time there is a point where the branch forks into two, a split occurred between
species to develop into two lineages. This splitting point is called a node and it shows
where a new species and a new clade, was founded. This makes the assumption that only
one branching off can happen at any one time, generating two species where there was
previously one.
One of the basic ideas behind cladistics is the concept of parsimony. This refers to the
preference for the least complicated explanation for a phenomenon. It would be unlikely
that a species would take two steps to evolve, if one step is possible.
To confirm the common ancestry from a cladogram, which is based on morphological
evidence, another should be made using biochemical data for the same organisms. The
two cladograms should be identical.
Construct a Cladogram
The organisms are paramecium, flatworm, shark, hawk, koala, camel, human
Characteristics are eukaryotic, multicellular, have a vertebral column, produce an
amniote egg, have hair, have a placenta, have one opposable thumb on each forelimb.
Construct your cladogram
Analyze
What was the primitive characteristic?
For each node, list the characteristic to put the organism in each clade.
Cladograms and classification
Cladistics attempts to find the most logical and most natural connections between
organisms to reveal their evolutionary past. Every cladogram drawn is a working
hypothesis. It is open for testing and falsification. This makes cladistics scientific but
changes are new evidence arises.
Each time a derived characteristic is added to the list shared by organisms in a clade, the
effect is similar to going up one level in the traditional hierarchy of the Linnaean
classification scheme. Hair is what defines a mammal, so any species with hair is a
mammal.
What about feathers? If an organism has feathers, is it automatically a bird? In
traditional classification, birds occupy a class of their own, but this is where cladistics
comes up with a surprise. When preparing a cladogram, it becomes clear that birds share
a significant number of derived characteristics with a group of dinosaurs called the
theropods. This suggests that birds are an offshoot of dinosaurs rather than a separate
class of their own.
Since birds are one of the most well documented classes of organisms on Earth, this idea
was controversial. Some derived characteristics are:
 Fused clavicle (wishbone)
 Flexible wrists
 Hollow bones
 Characteristic egg shell
 Hip and leg structure, notably with backward pointed knees
Following parsimony, it would be more likely that birds evolved from dinosaurs that they
evolved from another common ancestor. This is where cladistics is clearer than the
Linnaean system. In cladistics, the rules are always the same concerning shared derived
characteristics and parsimony. In the Linnaean system, apart from the definition of
species, which we have already seen can be challenged, the other hierarchical groupings
are not always clearly defined: what makes a class a class and a phylum a phylum?
Biologists now increasingly adopt cladistics as a useful tool for determining natural
classification and evolutionary connections.