Download Speciation and Barriers between Gene Pools

Species and Speciation
Assessment Statement
D.2.1
D.2.2
D.2.3
D.2.4
D.2.5
D.2.6
D.2.7
D.2.8
D.2.9
D.2.10
D.2.11
Define allele frequency and gene pool
State that evolution involves a change in allele frequency in a
population’s gene pool over a number of generations
Discuss the definition of the term species
Describe three examples of barriers between gene pools
Explain how polyploidy can contribute to speciation
Compare allopatric and sympatric speciation
Outline the process of adaptive radiation
Compare convergent and divergent evolution
Discuss ideas on the pace of evolution, including gradualism and
punctuated equilibrium
Describe one example of transient polymorphism
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
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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.
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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:
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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.