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5/24/2011 7:13:00 AM
IB Standard level Biology Dulwich College Shanghai
Topic 5: Ecology and Ecosystems
Evolution (pg. 184-195)
Define evolution (pg. 184)
Outline the evidence for evolution provided by the fossil record, selective
breeding of domestic animals and homologous structures. (pg. 185-186)
State that populations tend to produce more offspring than the environment
can support. (pg. 187)
Explain that the consequence of the potential overproduction of offspring is a
struggle for survival. (pg. 187)
State that the members of a species show variation. (pg. 187)
Explain how sexual reproduction promotes variation in a species. (pg. 187)
Explain how natural selection leads to evolution. (pg. 187)
Explain two examples of evolution in response to environmental change; one
must be antibiotic resistance in bacteria. (pg. 191 & 194)
5/24/2011 7:13:00 AM
IB Standard level Biology Dulwich College Shanghai
Topic 5: Ecology and Ecosystems
Evolution
5.4.1 Define evolution
Orange book  pg. 184
Green book  pg. 85
 Read the definition in your textbooks
 Do a brief search on the definition of ‘Evolution’.
 In your OWN words, try to define evolution.
5/24/2011 7:13:00 AM
IB Standard level Biology Dulwich College Shanghai
Topic 5: Ecology and Ecosystems
Evolution
Evidence
5.4.2 Outline the evidence for evolution provided by the fossil record,
selective breeding of domestic animals and homologous structures.
Orange book  pg. 185-186
Green book  pg. 85-86
1. Read the following information and then define these key words/terms:
Fossil
Fossilisation
Dating rocks and fossils
Relative dating
Absolute dating
Homologous structures
2. Outline how fossils (4 marks), homologous structures (6 marks) and
selective breeding (4 marks) provide evidence for evolution.
Fossil Record
*Much of the information I have included here is extra background material
to give you a better idea of fossils.
www.museum.vic.gov.au/dinosaurs/fossintro.stm
Most plants and animals that die are not fossilised. Their remains decay and
are broken up and recycled by natural processes. Occasionally
circumstances are right for fossilisation to occur.
What is a Fossil?
Fossils are remains moulds, or traces of organisms that died a long time ago
and were preserved in (usually) sedimentary rocks. About 250 000 different
fossil species have been identified.
How are fossils formed?
For millions of years, life was only found in the oceans. The oldest fossils
are therefore of marine creatures. When animals died, their remains
accumulated on the sea floor where they were buried by mud, sand or silt.
When land animals or plants died, the soft parts usually decomposed or were
eaten by scavengers. However, if the hard parts (bones, shells, wood) were
covered by a sudden flood, or sand, or even volcanic ash, they might be
preserved. Teeth are the hardest parts of an animal and were most likely to
be preserved.
Bone, wood and shell, although hard, have minute air spaces. When buried,
water containing dissolved minerals may seep into these spaces and deposit
minerals. Often, over millions of years, all of the original on or shell
dissolves away leaving a complete mineral replacement embedded in the
surrounding rock. The bones, wood and shell are then said to be petrified,
or turned to stone.
Fossils are found not only in rock:
Extinct insects have been found in fossil tree sap
Animals became trapped in natural tar pits and have been beautifully
preserved
Mammoths and other animals that lived during ice ages have been
incorporated in ice, or frozen ground, so that flesh, hair and even stomach
contents have been perfectly preserved.
Sometimes, the entire animal decayed away but left a ‘mould’ that was then
filled by sediments or minerals, making a natural ‘cast’. Similarly, footprints
made in soft ground created moulds that were later filled, making casts.
In some locations, impressions of skin have been discovered. Fossil
skeletons are sometimes found relatively intact, although they have usually
been scattered and only parts of the skeleton are recovered.
Dating Rocks and Fossils
The Earth is about 4600 million years old, the oldest fossils are about 3500
million years old, and the oldest fossils with hard parts are about 570 million
years old.
How do we know this?
There are two methods by which we can estimate the age of rocks and the
fossils contained in them.
Relative Dating
Rock dating is based on the study of rock strata and the fossils contained in
each stratum.
By examining quarries, cliffs, road cuttings and drill cores, a geologist can
plot the different layers of rock that make up Earth’s crust.
By assuming that any layer of rock is younger than the layer beneath it
(except when the Earth’s crust has folded) ancient geological happenings
were described sequentially using the names of the Geological Time Scale.
Although the order of geological events could often be interpreted, the age
in years was unknown.
Absolute Dating
Absolute dating is based on the rate of decay of radioactive elements in
rocks.
The discovery of radioactivity gave geologists a new ‘tool’ with which to
measure the age of the Earth.
Atoms of radioactive elements have unstable nuclei that progressively decay
to a more stable form. Uranium-238 goes through many changes before the
stable form, lead-206 is reached. The number refers to the number of
particles in the nucleus. Forms of the same element may have nuclei with
different numbers of neutrons and these are called isotopes.
Careful laboratory study has shown that each radioactive element decays
according to a distinct and measurable timetable. The number of decaying
parent atoms continually decreases while the number of daughter atoms
continually increases. The proportion, or percentage, of atoms that decay
during one unit of time is constant. It has become common practice to
designate that time unit in terms of the half life of the parent – meaning the
time required to reduce the parent atom by one half.
Palaeontologists have organised the results of their efforts in dating fossils
into a Geological Time Scale. Most of us have heard of the Jurassic Period,
but there are many other eras and periods.
Selective Breeding
Artificial selection (i.e. selective breeding) demonstrates the possibility that
a selection pressure can cause dramatic changes in the phenotypes of
animals/plants.
People have developed many new varieties of plants and animals by
selective breeding. Selection of specimens to breed based on particular
traits is, in effect, changing the environment for the population. Those
individuals lacking the desirable characteristics are not allowed to breed.
Therefore, the following generations more commonly have the desired traits.
Species that mature and reproduce large numbers in a short amount of time
have a potential for very fast evolutionary changes. Insects and
microorganisms often evolve at such rapid rates that our actions to combat
them quickly lose their effectiveness. We must constantly develop new
pesticides, antibiotics, and other measures in an ever escalating biological
arms race with these creatures. Unfortunately, there are a few kinds of
insects and microbes that are now significantly or completely resistant to our
counter measures, and some of these species are responsible for
devastating crop losses and deadly diseases.
If evolution has occurred, there should be many anatomical similarities
among varieties and species that have diverged from a common ancestor.
Those species with the most recent common ancestor should share the most
traits. For instance, the many anatomical similarities of wolves, dogs, and
other members of the genus Canis are due to the fact that they are
descended from the same ancient canine species. Wolves and dogs also
share similarities with foxes, indicating a slightly more distant ancestor with
them.
Homologous Structures
Homologous structures, like bones in the limbs of vertebrates, demonstrate
a commonality in underlying structure even as the actual appendages are
used for quite different things. The structure is called the pentadactyl limb
(five fingered limb), eg. Porpoise fins, bat wing, mole legs, horse legs,
human arms. This implies a common ancestry as, if each creature was
specially created, why would such structures, so divergent in use and outer
appearance, have the same bones, especially since we see analogous
structures (structures used for similar purposes and that may look
superficially similar, yet structurally are very different, eg. Wings of
butterflies, bats and birds) that they could look like instead.
5/24/2011 7:13:00 AM
IB Standard level Biology Dulwich College Shanghai
Topic 5: Ecology and Ecosystems
Evolution
Overproduction
State that populations tend to produce more offspring than the environment
can support.
Orange book  pg. 187
Green book  pg. 86
Read the following text then answer the questions at the end:
The process of evolution by means of natural selection depends on a number
of factors, including:
“Organisms produce more offspring than can be supported by the available
supply of food, light, space etc”.
Darwin appreciated that all species have the potential to increase their
number exponentially. To illustrate this point he wrote:
“Suppose there are eight pairs of birds, and that only four of them annually
…… rear only four young, and that these go on rearing their young at the
same rate, then at the end of seven years there will be 2048 birds instead of
the original 16”.
Darwin realised that, in nature, populations rarely, it ever, increased in size
at such a rate. He rightly concluded that the death rate of even the most
slow-breeding species must be extremely high. Why and how do organisms
reproduce so rapidly with little prospect of all but a tiny proportion of
offspring surviving? The reason why reproduction rates are high is because
a species cannot control the climate, rate of predation, availability of food
etc. Therefore to ensure a sufficiently large survives to breed and produce
the next generation, each species must produce vast numbers of offspring.
This is to compensate for considerable death rates from predation, lack of
food (including light in plants) and water, extremes of temperature, natural
disasters such as earthquakes and fire and disease.
How organism over-produce depends on the species in question and its
means of reproduction, some examples include:
A bacterium can divide by binary fission about every 20 minutes when
conditions are favourable. A single bacterium could therefore give rise to 4
x 1021 in just 24 hours.
Some fungi can produce over 500 000 spores each minute at the peak of
production. Each spore has the potential to form a new fungal mycelium.
Higher plants can spread rapidly by vegetative propagation, e.g. the
produce of bulbs, rhizomes and runners.
Flowering plants produce vast amounts of pollen from their anthers. These
can fetilise the many ovules in plants of the same species, leading to the
production, in some cases, of millions of seeds from a single parent.
Animals produce vast numbers of sperm, and sometimes large number of
eggs also. A female oyster, for example, can produce 100 million eggs in a
year and the male oyster produces many more times this number of sperm.
Many organisms, e.g. birds such as finches and mammals like the rabbit,
produce several clutches/ litters every year, each of which comprises several
offspring.
The importance of over-production to natural selection lies in the fact that,
where there are too many offspring for the available resources, there is
competition amongst individuals (intraspecific competition) for the limited
resources available. The greater the numbers, the greater this competition
and the more individuals will die in the struggle to survive. These death are,
however, not random. Those individuals best suited to prevailing conditions
(e.g. better able to hide from or escape predators, better able to obtain light
or catch prey, better able to resist disease or find a mate) will be more likely
to survive than those less well adapted. These individuals will be more likely
to breed and so pass on these favourable characteristics, via there alleles, to
the next generation, which will therefore be slightly different from the
previous one – i.e. the species will have evolved to be better adapted to the
prevailing conditions. The selection process, however, depends on
individuals of a species being genetically different from one another.
Questions:
1. According to Darwin’s theory of evolution, what causes the struggle for survival in
populations? (1 Mark)
A.
Overproduction of offspring
B.
Favourable heritable variations
C.
Natural selection
D.
Competition between the fittest individuals in the population
2. Why and how do organisms reproduce so rapidly with little prospect of all but a tiny
proportion of offspring surviving? (3 marks)
3. Where there are too many offspring for the available resources, there is competition
amongst individuals- Describe what is meant by competition giving at least 2 examples. (3
marks).
5/24/2011 7:13:00 AM
IB Standard level Biology Dulwich College Shanghai
Topic 5: Ecology and Ecosystems
Evolution
Struggle for Survival
Explain that the consequence of the potential overproduction of offspring is a
struggle for survival.
Orange book  pg. 187
Green book  pg. 86
Darwin's Theory of Evolution by Natural Selection
Darwin’s theory of natural solution is so simple that when Darwin’s close
friend, T. H. Huxley, read of it, he said ‘how stupid of me not to have
thought of it first’. The theory can be summarized by means of four
hypotheses which result in two conclusions:
Hypothesis 1:
Individuals within a species differ from each other - there
is variation.
Hypothesis 2:
inherited.
Offspring resemble their parents- characteristics are
Hypothesis 3: Far more offspring are generally produced than survive to
maturity - they suffer from predation, disease and competition.
(Overproduction)
Hypothesis 4: There is a struggle for survival; some individuals being
better adapted to their environment and therefore more successful than
others.
Conclusion 1: The better adapted individuals that survive and reproduce
pass on their beneficial characteristics to their offspring.
Conclusion 2: In time, the individuals in a species may give rise to a new
collection of individuals that are sufficiently distinct to be classified as a
separate species.
Darwin concluded that individuals that were better adapted to their
environment compete better than the others, survive longer and reproduce
more, so passing on more of their successful characteristics to the next
generation. Darwin used the memorable phrases survival of the fittest,
struggle for existence and natural selection.
Darwin explained the giraffe's long neck as follows. In a population of
horse-like animals there would be random genetic variation in neck length.
In an environment where there were trees and bushes, the longer-necked
animals were better adapted and so competed well compared to their
shorter-necked relatives. These animals lived longer, through more breeding
seasons, and so had more offspring. So in the next generation there were
more long-neck genes than short-neck genes in the population. If this
continued over very many generations, then in time the average neck length
would increase. Today it is thought more likely that the selection was for
long legs to run away from predators faster, and if you have long legs you
need a long neck to be able to drink. But the process of selection is just the
same.
Darwin wasn't the first to suggest evolution of species, but he was the first
to suggest a plausible mechanism for the evolution - natural selection, and
to provide a wealth of evidence for it.
Darwin used the analogy of selective breeding (or artificial selection) to
explain natural selection. In selective breeding, desirable characteristics are
chosen by humans, and only those individuals with the best characteristics
are used for breeding. In this way species can be changed over a long
period of time. All domesticated species of animal and plant have been
selectively bred like this, often for thousands of years, so that most of the
animals and plants we are most familiar with are not really natural and are
nothing like their wild relatives (if any exist). The analogy between artificial
and natural selection is a very good one, but there is one important different
- Humans have a goal in mind, nature does not.
5/24/2011 7:13:00 AM
IB Standard level Biology Dulwich College Shanghai
Topic 5: Ecology and Ecosystems
Evolution
Variation
State that the members of a species show variation.
Orange book  pg. 187
Green book  pg. 87
Read the text below then answer the questions that follow:
If there was no variation between the individuals within a species it is easy
to see that selection would not take place. Identical organisms would all
have the same characteristics that could be selected for or against and
hence distinguishing one organism from another as having an evolutionary
advantage would not be possible.
Since all individuals within a population show variation and a ‘struggle for
existence’, has been clearly established, it follows that some individuals
possessing particular variations will be more suited to survive and
reproduce. The key factor in determining survival is adaptation to the
environment. Any variation, however slight, be it physical, physiological or
behavioural, which gives one organism an advantage over another organism
will act as a selective advantage in the ‘struggle for existence’.
Favourable variations will be inherited by the next generation. Unfavourable
variations are ‘selected out’ or ‘selected against’, their presence conferring a
selective disadvantage on that organism.
If an organism can survive in the conditions in which it lives, you may
wonder why it doesn’t produce offspring that are identical to itself. These
will, after all, be equally capable of survival in these conditions, whereas
variation may produce individuals that are less suited. However, conditions
change over time and having a wide range of different individuals in the
population means that some will have the combination of genes needed to
survive in almost any set of new circumstances. Populations showing little
individual variation are vulnerable to new diseases and climate change. It is
also important that a species adapts to changes resulting from the evolution
of other species. If, for example, rabbits in a particular region adapt to run
faster, foxes and other predators will be less able to catch them and
therefore have less food, unless they in turn develop greater speed.
A species cannot predict future changes; it does not know whether the
climate will become wetter/ drier, warmer/ colder or how its prey or predator
will evolve or what new disease agent may arise. However, the larger the
population is, and the more genetically varied the organism within it, the
greater the chance that one or more individuals will have the genetic
characteristics that give it an advantage in the struggle for survival. These
individuals will therefore be more likely to breed and pass their more
suitable characteristics on to future generations. Variation therefore
provided the potential for a species to evolve and so adapt to new
circumstances.
Questions:
1. Define ‘Variation’ and ‘selection advantage’.
2. Describe how unfavourable variations/characteristics are selected against
within a population.
3. If an organism can survive in the conditions in which it lives, why doesn’t
nature allow it to produce offspring that are identical to itself?
5/24/2011 7:13:00 AM
IB Standard level Biology Dulwich College Shanghai
Topic 5: Ecology and Ecosystems
Evolution
Sexual Reproduction and Variation
Explain how sexual reproduction promotes variation in a species.
Orange book  pg. 187
Green book  pg. 87
Read the information below and define the following key terms:
Independent assortment
Crossing over in meiosis
Random fertilisation
Genetic Variation in Sexual Reproduction
The whole point of meiosis and sex is to introduce genetic variation, which
allows species to adapt to their environment and so to evolve. There are
three sources of genetic variation in sexual reproduction:
Independent assortment in meiosis
Crossing over in meiosis
Random fertilisation
Independent Assortment
This happens in meiosis, when the chromosomes line up on the equator.
They line up as two homologous chromosomes, which originally came from
two different parents (they’re often called maternal and paternal
chromosomes). Since they can line up in any orientation on the equator, the
maternal and paternal versions of the different chromosomes can be mixed
up in the final gametes.
In this simple example with 2 homologous chromosomes (n=2) there are 4
possible different gametes (22). In humans with n=23 there are over 8
million possible different gametes (223).
Crossing Over
This happens during meiosis when the homologous pairs line up together.
While the two homologous chromosomes are joined, bits of one chromosome
are swapped (crossed over) with the corresponding bits of the other
chromosome.
The points at which the chromosomes actually cross over are called
chiasmata (singular chiasma). There is always at least one chiasma in a
bivalent, but there are usually many. Crossing over means that maternal
and paternal alleles can be mixed, even though they are on the same
chromosome.
Random Fertilisation
This takes place when two gametes fuse to form a zygote. Each gamete has
a unique combination of genes, and any of the numerous male gametes can
fertilise any of the numerous female gametes. So every zygote is unique.
5/24/2011 7:13:00 AM
IB Standard level Biology Dulwich College Shanghai
Topic 5: Ecology and Ecosystems
Evolution
Natural Selection
Explain how natural selection leads to evolution.
Orange book  pg.187
Green book  pg. 87
Read the text and then answer the questions that follow:
*This is a recap on notes you have already read, the idea of natural selection
and how it brings about evolution should becoming clearer to you now.
Darwin's Theory of Evolution by Natural Selection
Darwin’s theory of natural solution is so simple that when Darwin’s close
friend, T. H. Huxley, read of it, he said ‘how stupid of me not to have
thought of it first’. The theory can be summarized by means of four
hypotheses which result in two conclusions:
Hypothesis 1:
is variation.
Individuals within a species differ from each other - there
Hypothesis 2:
inherited.
Offspring resemble their parents- characteristics are
Hypothesis 3: Far more offspring are generally produced than survive to
maturity - they suffer from predation, disease and competition.
Hypothesis 4: There is a struggle for existence; some individuals being
better adapted to their environment and therefore more successful than
others.
Conclusion 1: The better adapted individuals that survive and reproduce
pass on their beneficial characteristics to their offspring.
Conclusion 2: In time, the individuals in a species may give rise to a new
collection of individuals that are sufficiently distinct to be classified as a
separate species.
Darwin concluded that individuals that were better adapted to their
environment compete better than the others, survive longer and reproduce
more, so passing on more of their successful characteristics to the next
generation. Darwin used the memorable phrases survival of the fittest,
struggle for existence and natural selection.
Darwin explained the giraffe's long neck as follows. In a population of
horse-like animals there would be random genetic variation in neck length.
In an environment where there were trees and bushes, the longer-necked
animals were better adapted and so competed well compared to their
shorter-necked relatives. These animals lived longer, through more breeding
seasons, and so had more offspring. So in the next generation there were
more long-neck genes than short-neck genes in the population. If this
continued over very many generations, then in time the average neck length
would increase. Today it is thought more likely that the selection was for
long legs to run away from predators faster, and if you have long legs you
need a long neck to be able to drink. But the process of selection is just the
same.
Darwin wasn't the first to suggest evolution of species, but he was the first
to suggest a plausible mechanism for the evolution - natural selection, and
to provide a wealth of evidence for it.
Darwin used the analogy of selective breeding (or artificial selection) to
explain natural selection. In selective breeding, desirable characteristics are
chosen by humans, and only those individuals with the best characteristics
are used for breeding. In this way species can be changed over a long
period of time. All domesticated species of animal and plant have been
selectively bred like this, often for thousands of years, so that most of the
animals and plants we are most familiar with are not really natural and are
nothing like their wild relatives (if any exist). The analogy between artificial
and natural selection is a very good one, but there is one important different
- Humans have a goal in mind, nature does not.
Questions:
1.
Which process has the greatest effect in determining which members of a population
are most likely to survive until reproductive age?
A.
Evolution
B.
Natural selection
C.
Meiosis
D.
Hybridization
2. Which factors could be important for a species to evolve by natural selection?
I.
Environmental change
II.
Inbreeding
III.
Variation
A.
I only
B.
I and II only
C.
I and III only
D.
I, II and III
3. What is natural selection?
A.
The mechanism that increases the chance of certain individuals reproducing.
B.
The mechanism that leads to increasing variation within a population.
C.
The cumulative change in the heritable characteristics of a population.
D.
The mechanism that explains why populations produce more offspring than the
environment can support.
5/24/2011 7:13:00 AM
IB Standard level Biology Dulwich College Shanghai
Topic 5: Ecology and Ecosystems
Evolution
Examples of Evolution
Explain two examples of evolution in response to environmental change; one
must be antibiotic resistance in bacteria.
Orange book  pg. 191 & 194
Green book  pg. 88
Read the example below and then summarise how antibiotic
resistance in bacteria is an example of evolution in response to
environmental change (Use your class notes on DDT resisitance in
mosquitos/peppered moth melanism as a template for structuring
your answer).
Development of Antibiotic Resistance
How do bacteria become resistant to antibiotics?
Some species may be resistant because they don't posses the antibiotic
target. E.g. penicillin targets the cell wall and weakens it by inhibiting the
enzymes which make peptidoglycan for the cell wall. The bacteria which
causes Chlamydia has a cell wall that does not contain with no peptidoglycan
and hence penicillin will not act on it.
In general, resistance first develops due to a mutation. Bacteria reproduce
asexually, so all the offspring should be the same, but sometimes, at
random, mutations occur when DNA is replicated. These mutations may
have any effect (and most will be fatal), but just occasionally a mutation
occurs that makes that bacterium resistant to an antibiotic.
For example a mutation could slightly alter a protein so that an antibiotic can
no longer bind, or a mutation could slightly alter an enzyme, changing its
substrate specificity so that its active site will now bind penicillin. Such
mutations are very rare, but bacteria reproduce so rapidly, and there are so
many bacterial cells, that new resistance mutations do crop up at a
significant rate (a few times per year somewhere on the planet). Remember
that development of antibiotic resistance is a random event, and is not
caused by the presence of the antibiotic. It is certainly not an adaptation
that bacteria acquire.
As an example, imagine a community of different bacterial species living in
your gut, and one particular cell has just mutated to become resistant to
penicillin.
What happens next?
It will reproduce by binary fission and pass on its resistance gene to all its
offspring, forming a new strain of bacteria in your gut. If there is no
antibiotic present in your gut (most likely) this mutated strain may well die
out due to competition with all the other bacteria, and the mutation will be
lost again. However, if you are taking penicillin, then penicillin will be
present in the bacteria's environment, and these mutated cells are now at a
selective advantage: the antibiotic kills all the normal bacterial cells, leaving
only the mutant cells alive. These cells can then reproduce rapidly without
competition and will colonise the whole environment. This a good example of
natural selection at work.
Spread of Antibiotic Resistance
How do these resistant bacteria spread to other people?
In the example above the bacteria will contaminate faeces and may then
infect other individuals through poor hygiene. In fact the resistant bacteria
can spread by any of the normal methods of spreading an infection: through
water, food, sneezing, infected instruments, etc. Some bacteria can form
spores to aid their dispersal, and so a mutated strain can survive long
journeys and long periods of time. In most new environments the mutated
strain will die out through competition, but whenever it encounters penicillin
it will thrive, out-competing all other bacteria.
This is how resistance of one bacterial strain to one antibiotic can spread,
but unfortunately resistance can also spread between species. Bacteria have
a trick that no other organisms can do: they can transfer genes between
each other; even between different species. In this way a resistance gene
can spread from the bacterium in which it arose to other, perhaps more
dangerous, species. There are three methods of gene transfer in bacteria:
Conjugation
This is the transfer of DNA between bacterial cells via a cytoplasmic bridge.
From time to time two bacterial cells can join together (conjugate), and DNA
passes from one (the donor) to the other (the recipient). The transferred
DNA can be one or more plasmids, or can be all or part of the whole
bacterial chromosome (in which case the donor cell dies). Conjugation is
sometimes referred to as bacterial sex or mating, but it is quite distinct from
sexual reproduction, because the gene exchange is not equal, it can take
place between different species, and bacteria do not use conjugation for
reproduction. It is better thought of as an alternative to sex, where these
asexual organisms gain some of the advantages of genetic exchange.
Evolution essay
5/24/2011 7:13:00 AM
Essay title:
Explain evolution of a species by natural selection using a named example
(18 marks)
Your essay should be hand written.