Download Genetic Drift

http://evolution.berkeley.edu/evosite/evo101/IIID2Genesdriftp2.shtml
Genetic Drift
Genetic drift—along with natural selection, mutation, and migration—is one of the basic mechanisms of evolution.
In each generation, some individuals may, just by chance, leave behind a few more descendents (and genes, of
course!) than other individuals. The genes of the next generation will be the genes of the “lucky” individuals, not
necessarily the healthier or “better” individuals. That, in a nutshell, is genetic drift. It happens to ALL
populations—there’s no avoiding the vagaries of chance.
Effects of Genetic Drift
Through sampling error, genetic drift can cause populations to lose genetic variation.
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. (It is important to realize that this is all happening by chance- one color is not at an
advantage over the other).
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.
Bottlenecks and Founder Effects
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.
In humans, founder effects can arise from cultural isolation, and inevitably, endogamy. For example, the Amish
populations in the United States exhibit founder effects. This is because they have grown from a very few founders,
have not recruited newcomers, and tend to marry within the community. Though still rare, phenomena such as
polydactyly (extra fingers and toes) are more common in Amish communities than in the American population at
large.
In 1814, 15 British colonists founded a settlement on Tristan da Cunha, a group of small islands in the Atlantic
Ocean, midway between Africa and South America. One of the early colonists apparently carried a recessive allele for
retinitis pigmentosa, a progressive form of blindness that afflicts homozygous individuals. Of the founding colonists'
240 descendants on the island in the late 1960s, 4 had retinitis pigmentosa. The frequency of the allele that causes
this disease is ten times higher on Tristan da Cunha than in the populations from which the founders came.
_________________________________________
Speciation
Speciation is a lineage-splitting event that produces two or more separate species. Imagine that you are looking at a
tip of the tree of life that constitutes a species of fruit fly. Move down the phylogeny to where your fruit fly twig is
connected to the rest of the tree. That branching point, and every other branching point on the tree, is a speciation
event. At that point genetic changes resulted in two separate fruit fly lineages, where previously there had just been
one lineage. But why and how did it happen?
The branching points on this partial Drosophila phylogeny represent long past speciation events. Here is one scenario
that exemplifies how speciation can happen:

The scene: a population of wild fruit flies minding its own business on several bunches of rotting bananas,

Disaster strikes: A hurricane washes the bananas and the immature fruit flies they contain out to sea. The

The populations diverge: Ecological conditions are slightly different on the island, and the island

So we meet again: When another storm reintroduces the island flies to the mainland, they will not readily
cheerfully laying their eggs in the mushy fruit...
banana bunch eventually washes up on an island off the coast of the mainland. The fruit flies mature and
emerge from their slimy nursery onto the lonely island. The two portions of the population, mainland and
island, are now too far apart for gene flow to unite them. At this point, speciation has not occurred—any fruit
flies that got back to the mainland could mate and produce healthy offspring with the mainland flies.
population evolves under different selective pressures and experiences different random events than the
mainland population does. Morphology, food preferences, and courtship displays change over the course of
many generations of natural selection.
mate with the mainland flies since they’ve evolved different courtship behaviors. The few that do mate with
the mainland flies, produce inviable eggs because of other genetic differences between the two populations.
The lineage has split now that genes cannot flow between the populations.
This is a simplified model of speciation by geographic isolation, but it gives an idea of some of the processes that
might be at work in speciation. In most real-life cases, we can only put together part of the story from the available
evidence. However, the evidence that this sort of process does happen is strong.
Allopatric Speciation: The Great Divide
Allopatric speciation is just a fancy name for speciation by geographic isolation,
discussed earlier. In this mode of speciation, something extrinsic to the organisms
prevents two or more groups from mating with each other regularly, eventually
causing that lineage to speciate. Isolation might occur because of great distance or a
physical barrier, such as a desert or river, as shown below.
Allopatric speciation can occur even if the barrier is a little “porous,” that is, even if a few individuals can cross the
barrier to mate with members of the other group. In order for a speciation even to be considered “allopatric,” gene
flow between the soon-to-be species must be greatly reduced—but it doesn’t have to be reduced completely to zero.
_____________________________________
Gene flow
In population genetics, gene flow (also known as gene migration) is the transfer of alleles or genes from one
population to another.
Migration into or out of a population may be responsible for a marked change in allele frequencies (the proportion of
members carrying a particular variant of a gene). Immigration may also result in the addition of new genetic variants
to the established gene pool of a particular species or population.
There are a number of factors that affect the rate of gene flow between different populations. One of the most
significant factors is mobility, as greater mobility of an individual tends to give it greater migratory potential. Animals
tend to be more mobile than plants, although pollen and seeds may be carried great distances by animals or wind.
Maintained gene flow between two populations can also lead to a combination of the two gene pools, reducing the
genetic variation between the two groups. It is for this reason that gene flow strongly acts against speciation, by
recombining the gene pools of the groups, and thus, repairing the developing differences in genetic variation that
would have led to full speciation and creation of daughter species.
For example, if a species of grass grows on both sides of a highway, pollen is likely to be transported from one side
to the other and vice versa. If this pollen is able to fertilize the plant where it ends up and produce viable offspring,
then the alleles in the pollen have effectively been able to move from the population on one side of the highway to
the other.
Barriers to gene flow
Physical barriers to gene flow are usually, but not always, natural. They may include impassable mountain ranges,
oceans, or vast deserts. In some cases, they can be artificial, man-made barriers, such as the Great Wall of China,
which has hindered the gene flow of native plant populations.[1] One of these native plants, Ulmus pumila,
demonstrated a lower prevalence of genetic differentiation than the plants Vitex negundo, Ziziphus jujuba,
Heteropappus hispidus, and Prunus armeniaca whose habitat is located on the opposite side of the Great Wall of
China where Ulmus pumila grows.[1] This is because Ulmus pumila has wind-pollination as its primary means of
propagation and the latter-plants carry out pollination through insects.[1] Samples of the same species which grow on
either side have been shown to have developed genetic differences, because there is little to no gene flow to provide
recombination of the gene pools.
_________________________________________
Natural Selection
Natural selection is one of the basic mechanisms of evolution, along with mutation, migration, and genetic drift.
Darwin’s grand idea of evolution by natural selection is relatively simple but often misunderstood. To find out how
it works, imagine a population of beetles:
1. There is variation in traits.
For example, some beetles are green and some are
brown.
2. There is differential reproduction.
Since the environment can’t support unlimited population growth, not all
individuals get to reproduce to their full potential. In this example, green
beetles tend to get eaten by birds and survive to reproduce less often than
brown beetles do.
3. There is heredity.
The surviving brown beetles have brown baby beetles because this trait has a genetic
basis.
4. End result:
The more advantageous trait, brown coloration, which allows the beetle to have more
offspring, becomes more common in the population. If this process continues,
eventually, all individuals in the population will be brown.
If you have variation, differential reproduction, and heredity, you will have evolution by natural selection as an
outcome. It is as simple as that.
Misconceptions about
Natural Selection
Because natural selection can produce amazing adaptations, it’s tempting to think of it as an all-powerful force,
urging organisms on, constantly pushing them in the direction of progress—but this is not what natural selection is
like at all.
First, natural selection is not all-powerful; it does not produce perfection. If your genes are “good enough,” you’ll get
some offspring into the next generation—you don’t have to be perfect. This should be pretty clear just by looking at
the populations around us: people may have genes for genetic diseases, plants may not have the genes to survive
a drought, a predator may not be quite fast enough to catch her prey every time she is hungry. No population or
organism is perfectly adapted.
Second, it’s more accurate to think of natural selection as a process rather than as a guiding hand. Natural selection
is the simple result of variation, differential reproduction, and heredity—it is mindless and mechanistic. It has no
goals; it’s not striving to produce “progress” or a balanced ecosystem.
This is why “need,” “try,” and “want” are not very accurate words when it comes to explaining evolution. The
population or individual does not “want” or “try” to evolve, and natural selection cannot try to supply what an
organism “needs.” Natural selection just selects among whatever variations exist in the population. The result is
evolution.
At the opposite end scale, natural selection is sometimes interpreted as a random process. This is also a
misconception. The genetic variation that occurs in a population because of mutation is random-but selection acts on
that variation in a very non-random way: genetic variants that aid survival and reproduction are much more likely to
become common than variants that don't. Natural selection is NOT random!