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When we think about how evolutionary forces cause genetic change in
populations, we need to consider their effects at multiple levels. Let’s
do this with the evolutionary force of mutation.
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When a new mutation occurs in an individual, how does it impact
variation within that individual’s population (consider average
heterozygosity)? Explain.
When a mutation occurs in
one organism in a population, it is likely to produce a new allele for
the locus in which it occurs. This is because mutations are random and
thus it is not terribly likely that the exact change will already exist
in another individual. That means that a mutation occurring increases
genetic variation in the whole population, because a new allele has
arisen. Thus, we see a small increase in heterozygosity when a new
mutation occurs.
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When a new mutation occurs in an individual in a population, does it
make different populations more genetically alike or more genetically
divergent? Explain.
If we consider 2 populations (A
and B) and a new mutation happens in an individual in population A. From
the previous question, we realize that this mutation will not have
occurred in population B, since it’s extremely unlikely for the exact
same change to happen at random in two populations at once. Thus, we now
have a new way in which population A is different from population B, it
has this new unique mutation. So, we would say that the new mutation has
made the two populations differ more than before, they are now more
genetically divergent.
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When we think about the types of mutations that may occur and their
possible effects on the organism’s phenotype, we need to consider
mutations in light of what we know about gene expression.
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What types of mutations might be likely to be neutral, having no effect
on the organism’s phenotype?
Mutations that occur in
non-coding regions, with no regulatory function, won’t impact the
phenotype. Similarly, we think mutations that change the wobble base in
a codon (don’t change the amino acid, synonymous mutations) are also
unlikely to alter the organism’s phenotype.
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What types of mutations are likely to have an effect on phenotype even
when you only have one copy? What about those only likely to have an
effect on phenotype if you have two copies?
Mutations that are likely to
change phenotype when you have one copy are likely to be ones that alter
amino acids (maybe changing protein function) or that impact how much
mRNA is produced (without stopping transcription altogether), like
changes to enhancers or silencers. Mutations that will only show in
phenotype if you have two copies are most likely to be ones that prevent
transcription of a gene altogether or alter the protein so that it
doesn’t function at all anymore. As long as you have one working copy,
it’s possible that this kind of mutation would not affect phenotype.
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We think of gene flow as a force that keeps populations genetically
similar and preserves genetic variation within a population. Is this
always a good thing? Can you think of scenarios where gene flow might
not be the best thing for the average survival and reproduction of a
population?
Gene flow can be thought of
as the thing that keeps everyone in a species part of that species.
However, sometimes a species has many populations found in different
environments. In this case, it’s possible that the populations become
adapted to their specific conditions – for example the salinity of the
soil. If some populations experience high salinity and adapt so that
they have a high frequency of alleles that are helpful with high
salinity, then it could be harmful to survival if individuals arrive
from low salinity environments (and lack the high salinity alleles) and
then breed with the high salinity individuals. The offspring will not be
as well adapted to high salinity. Without that gene flow, the high
salinity population would have higher average survival.
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Why is the loss of genetic variation due to genetic drift in small
populations considered such a serious problem in conservation? Small
populations are still experiencing spontaneous mutations, so won’t they
be increasing genetic variation that way?
Mutation will increase
genetic variation only slowly, taking thousands of years to restore
genetic variation lost due to drift (which sometimes depletes genetic
variation very rapidly). Thus, mutuation is not sufficient to counter
drift in the short term.
In addition, we typically
think that maintaining genetic variation is beneficial in case the
environment should change, then there is some chance that some variants
will be more successful in the new environment. Of course, an
environmental change that favors only some variants will necessarily
reduce genetic variation too (but only for the parts of the genome
involved in the traits that are favored).
The bottom line is that we
think genetic variation can act as a buffer, enabling populations to
persist when change occurs. Having low genetic variation can be
a problem but we don’t actually have many examples of it causing a major
problem in natural populations (cheetahs lack variation but seem to be
doing ok, aside from problems caused by humans).
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What is a genetic bottleneck? What is the founder effect? How are these
phenomena different?
A genetic bottleneck is when
a sizable population suddenly decreases a lot in size, like in one
generation going from thousands to a few hundred (the population size
narrows like the neck of a bottle). So this occurs to a population that
is living in its usual environment but something happens that rapidly
kills many, many individuals at random (natural disaster like a
hurricane?). Generally, we might expect a population to grow rapidly
after this happens unless the cause of the sudden high mortality is
still present. This sudden loss of numbers can cause a random loss of
genetic variation (those who survived didn’t do so because they had
better genes, it was luck of the draw).
A founder effect is when a
small subset of a large population leaves and “founds” a new population
in a new location, previously uninhabited by this species. The group of
founders may be small and therefore at random may not be representative
of the genetic variation of the larger population (again, by chance).
The new population, being small and in a new area probably without much
competition, is expected to grow quickly and thus restore a larger
population size – but variation that was not present in the initial
founders will still be lacking.
As you can see, these
phenomena are different in that a genetic bottleneck occurs to an
existing population – which could mean that the variation that is lost
can not be recovered by other means. In some cases, bottlenecks might
impact entire species (if they are not widespread on the planet). In
contrast, the founder effect is when a new population arises from a few
individuals and they by chance start out with little variation. In this
case, there is some possibility that some variation could be recovered
IF migrants arrive from other populations.
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If a population does not experience a genetic bottleneck or a founder
effect, will it still change due to genetic drift? Explain.
Yes, even without a genetic
bottleneck or a founder effect, ALL POPULATIONS ALWAYS EXPERIENCE
GENETIC DRIFT. This is a result of the fact that populations are finite
in size and there will always be some amount of random change between
generations due to chance. However, larger populations will experience
smaller amounts of genetic drift (smaller chance changes in allele
frequency over generations) as compared to smaller populations.
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Genetic drift leads to the loss of genetic variation within populations,
increasing the difference from other populations. We often view drift as
a force that leads to problems for a population due to the loss of
variation. But we should think carefully. Will genetic drift always lead
to the loss of genetic variation? Since genetic drift causes populations
to become more different from each other, what might be the long-term
consequence of that process?
Genetic drift, given enough
time, will eventually lead to the loss of genetic variation within a
population. However, when there are multiple populations (which is most
of the time), that means that populations will become more different
from each other, thus there may be more genetic variation in the
species, even if not in individual populations. In the long-term, two
populations could become so different from each other, just by the
accumulation of chance changes over time, that they are no longer able
to inter-breed and become considered different species.