What is Genetic Drift?
A general definition is a change in the allele frequency of
a population resulting from sampling error in taking gametes from the gene pool
to make zygotes and from chance variation in the survival/reproductive success
of individuals. If a population is small, there will inevitably be sampling
error.
It is extremely important in small populations. Most species
originate from small populations because drift can cause rapid genetic
divergence in a relatively short period of time.
The frequencies of alleles in a population will drift up and
down with each generation and eventually alleles will become fixed. For
example, take the "A" gene. It has 2 alleles, "a" and
"A". By chance, one form will bexome fixed at 1 (the only form
present) and the other will become fixed at 0 (not present). This results in
the population eventually becoming homozygous at either the "a" or
"A" locus.
Why aren't all of our genes homozygous then?
Variation is maintained in a normal population by balancing
the rate at which alleles disappear due to random drift with the rate at which
new alleles appear due to mutation.
What are the effects of Genetic Drift?
• random loss of alleles
• random fixation of existing and new alleles
• nonrandom sampling of the ancestral population
• rapid divergenece in isolated populations
What is a Bottleneck?
A bottleneck is a large scale, but short term decrease in
population size followed by an increase in population size.
Rare alleles tend to be lost due to genetic drift so it does
not generally reduce the genetic diversity substantially. For the genetic
diversity to be greatly effected, the founding population must be very very
small!
What is Founder Effect?
Founder Effect is the establishment of a new population by a
small number of individuals.
It is a good example of sampling error because just by
chance the frequencies of alleles in a new population are likely to be
different than those of the source population
Founder Effect can have a substantial impact on an isolated
population, if one of the original alleles is responsible for a genetic
disease. This allele can resultantly become fixed in this small population at a
much higher rate than found in the source population.
What is Natural Selection?
Natural selection is a difference between the survival or
production of offspring by individuals with certain phenotypes compared to
individuals with other phenotypes.
Why is natural selection important?
It is an important catalyst in encouraging the divergence of
isolated populations. This in turn can lead to speciation events &
evolution!
Natural selection can also cause populations to diverge even
when there is a small amount of gene flow (therefore it is a powerful force).
References
• Cooke, F. and Buckley, P.A. 1987. Avian Genetics A
Population and Ecological Approach. Academic Press. Chapter 6.
• Freeman, S. and Herron, J. C. 2001. Evolutionary Analysis
Second Edition. Prentice-Hall, Inc. New Jersey.
• Griffiths, A. J. F., Miller, J. H., Suzuki, D. T., Lewontin,
R. C., and Gelbart, W. M. 1996. An Introduction to Genetic Analysis Sixth
Edition. W.H. Freeman and company. New York.