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Migration, Genetic Drift and Nonrandom Mating
I. Introduction –Greater Prairie Chicken Example (Tympanuchus cupido pinnatus)
A. Classic example of a species that uses a lek in mating.
1. A lek represents a polygynous mating system where males aggregate in a particular area
and exhibit their courtship displays to females that visit the group. One male may mate
with more than one female.
2. The decline in Greater Prairie Chicken populations can be attributed to the following 3
violations of H-W. They are 1) migration, 2) genetic drift and 3) nonrandom mating.
II. Migration – the movement of alleles between populations.
A. The one-island model of migration as an example.
B. Example of the impact of alleles entering a small population.
C. An example with water snakes (Nerodia sipedon)
1. Individuals vary in appearance ranging from strongly banded to undbanded.
a. Banding pattern is controlled (more or less) by a single locus with two alleles.
b. Banding is dominant over no bands.
2. On the mainland, virtually all of the snakes are banded, but on the island, the unbanded
variant predominates.
a. Unbanded pattern are more cryptic and presumably less vulnerable to predators.
3. If being unbanded should confer greater fitness, then why isn’t there a greater frequency
of unbanded snakes on islands?
a. Migration of individuals from the mainland.
b. The natural selection against banded individuals on the islands would be expected to
fix the allele frequency of being unbanded to 1. However, the effects of immigration
are opposing natural selection.
c. The result of constant introductions of alleles from the mainland is that this tends to
homogenize the allele frequencies on the island.
d. If natural selection did not oppose the effects of immigration, then the allele
frequency on the island would come to resemble that on the mainland.
III. Genetic Drift – Chance in a small population can lead to evolution.
1. Evolution is not a chance event.
2. Drift on the other hand, is a random process. Whether an individual has an appropriate
phenotype or not, it may be lost from the population.
a. Hence, genetic drift is a chance event that leads to changes in the allele frequency of a
population without leading to adaptation of the population.
A. A classroom example of genetic drift –red and black beans in a bag. Did your sampling
violate one of these conclusions of H-W?
1. The allele frequencies in a population will not change, generation after generation.
2. If the allele frequencies in a population are given by p and q, the genotype frequencies
will be given by (p2 + 2pq + q2).
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