Download Genetic Drift (1.A.3.a) Genetic drift affects the genetic makeup of the

Genetic Drift (1.A.3.a)
Genetic drift affects the genetic makeup of the population but unlike natural selection, through an entirely
random process. So although genetic drift is a mechanism of evolution, it doesn’t work to produce
adaptations.
Decreasing variation: Imagine that our random draws from the marble bag produced the following
pattern: 5:5, 6:4, 7:3, 4:6, 8:2, 10:0, 10:0, 10:0, 10:0, 10:0... Why did we keep drawing 10:0? Because if
the green marbles fail to be represented in just one draw, we can’t get them back—we are “stuck” with
only brown marbles. The cartoon below illustrates this process, beginning with the fourth draw.
The same thing can happen to populations. If the gene for green coloration drifts out of the population, the
gene is gone for good—unless, of course, a mutation or gene flow reintroduces the green gene.
The 10:0 situation illustrates one of the most important effects of genetic drift: it reduces the amount of
genetic variation in a population. And with less genetic variation, there is less for natural selection to
work with. If the green gene drifts out of the population, and the population ends up in a situation where it
would be advantageous to be green, the population is out of luck. Selection cannot increase the frequency
of the green gene, because it’s not there for selection to act on. Selection can only act on what variation is
already in a population; it cannot create variation.
The impact on small populations: The marble-drawing scenario also illustrates why drift affects small
populations more. Imagine that your bag is only big enough for 20 marbles (a tiny bag!) and that you can
only draw four marbles to represent gene frequencies in the next generation. Something like this might
happen:
Notice how quickly and drastically the marble ratio changed: 1:1, 1:3, 0:1.
The same process operates in small populations. All populations experience drift, but the smaller the
population is, the sooner drift will have a drastic effect. This may be a big problem for endangered species
that have low population sizes.
http://evolution.berkeley.edu/evosite/evo101
Bottleneck Effect
Genetic drift can cause big losses of genetic variation for small populations.
Population bottlenecks occur when a population’s size is reduced for at least one generation. Because
genetic drift acts more quickly to reduce genetic variation in small populations, undergoing a bottleneck
can reduce a population’s genetic variation by a lot, even if the bottleneck doesn’t last for very many
generations. This is illustrated by the bags of marbles shown below, where, in generation 2, an unusually
small draw creates a bottleneck.
Reduced genetic variation means that the population may not be able to adapt to new selection pressures,
such as climatic change or a shift in available resources, because the genetic variation that selection would
act on may have already drifted out of the population.
Founder effect: The loss of genetic variation when a new colony is formed by a very small number of
individuals from a larger population. Only the alleles carried by these organisms will be present in the
new population
Eastern Pennsylvania is home to beautiful farmlands and countryside, but it's also a gold mine of
information for geneticists, who have studied the region's Amish culture for decades. Because of their
closed population stemming from a small number of German immigrants -- about 200 individuals -- the
Amish carry unusual concentrations of gene mutations that cause a number of otherwise rare inherited
disorders, including forms of dwarfism.
One form of dwarfism, Ellis-van Creveld syndrome, involves not only short stature but polydactyly (extra
fingers or toes), abnormalities of the nails and teeth, and, in about half of individuals, a hole between the
two upper chambers of the heart. The syndrome is common in the Amish because of the founder effect.
When a small part of a population moves to a new locale, or when the population is reduced to a small
size because of some environmental change, the genes of the "founders" of the new society are
disproportionately frequent in the resulting population. If individuals in the group tend to marry within it, there's a greater likelihood that the recessive genes of
the founders will come together in the cells that produce offspring. Thus diseases of recessive genes,
which require two copies of the gene to cause the disease, will show up more frequently than they would
if the population married outside the group. In the Amish, in fact, Ellis-van Creveld syndrome has been traced back to one couple, Samuel King and
his wife, who came to the area in 1744. The mutated gene that causes the syndrome was passed along
from the Kings and their offspring, and today it is many times more common in the Amish population
than in the American population at large. www.pbs.org