Download Mechanism of Evolution

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
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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.