Biology 280/545: Evolution Lecture: Population Genetics

Background: Freeman and Herron: Chapter 6

Outline:

I. Migration

II. Nonrandom Mating

III. Genetic Drift

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I. Migration:

A. Migration vs. gene flow

B. Migration and allele frequencies (one island model)

Look at the “island” population after migration:

- proportion of migrants into the island population = m

- genes descended from the local population = 1-m

- genes descended from the migrants = m

Then the frequency of A₁ in the island population after migration will be:

When does it stop?

II. Nonrandom Mating

A. Definitions:

Assortative Mating:

Disassortative Mating:

Inbreeding:

B. Example: Inbreeding

1. Self-fertilization

2. Effects of Inbreeding

3. Coefficient of Inbreeding = F

a) F is the proportionate reduction in heterozygosity , (H=proportion of heterozygotes), compared to random mating

b) F is also the probability that 2 alleles in an individual are identical by descent)

c) Some inbreeding coefficients:

Selfing………1/2

Full-sib matings……1/4

First cousins………1/16

Second cousins…….1/64

d) In a population of inbreeders, the genotypic frequencies will change accordingly:

f(AA):

f(Aa):

f(aa):

e) Example: Albinism in humans

1/20,000 of American caucasian births is an albino (aa)

q² = 1/20,000 = 0.00005 à q = 0.007

For first cousin marriages, the frequency of albino children will be:

4. Inbreeding Depression

III. Genetic Drift:

1. Effects of Genetic Drift

2. Effective Population Size

3. Special cases of drift

1. Founder Effect:

2. Bottleneck:

4. Neutral Theory

1. “Neutral Mutations”

2. Evolution by drift

3. Predictions of Neutral Theory