MODULE 7: Migration, Drift & Non-Random Mating

Migration (gene flow)

The movement of alleles between populations through dispersal or gamete movement.

When migration affects evolution

When migrants differ genetically from residents and successfully reproduce.

Effect of migration on populations

Migration homogenizes allele frequencies and reduces genetic differentiation.

One-island model outcome

Gene flow makes island and mainland populations increasingly similar over time.

Genetic drift

Random changes in allele frequencies caused by sampling error between generations.

Key feature of drift

Does not lead to adaptation; operates by chance.

When drift is strongest

Drift has the greatest effect in small populations.

Effect of drift on heterozygosity

Drift reduces heterozygosity over time and eventually causes fixation or loss of alleles.

Founder effect

A small group starts a new population with allele frequencies different from the original population due to chance.

Heterozygosity

The frequency of heterozygotes in a population.

Fixation index (FST)

A measure of genetic differentiation among populations.

Interpretation of FST

FST ≈ 0: populations similar; FST ≈ 1: populations highly differentiated.

Effective population size (Ne)

The genetically relevant population size; typically smaller than the census size.

Formula for Ne

Ne = 4NmNf / (Nm + Nf).

Effect of small Ne

Strengthens drift and increases loss of genetic variation.

Interaction of drift and selection

Drift can overpower weak selection, especially in small populations.

Neutral theory

Most fixed mutations are effectively neutral, and drift dominates molecular evolution.

Silent vs replacement sites

Silent sites often neutral; replacement sites often deleterious or sometimes neutral.

dN/dS ratio

Test for selection:

Non-random mating

Mating that does not occur randomly; changes genotype frequencies but not allele frequencies.

Inbreeding

Mating between relatives that increases homozygosity and reduces heterozygosity.

Inbreeding coefficient (F)

Probability two alleles are identical by descent.

Genotype frequencies with inbreeding

AA = p²(1-F) + pF; Aa = 2pq(1-F); aa = q²(1-F) + qF.

Heterozygosity under inbreeding

HF = H0(1 - F).

Consequences of inbreeding

Increases expression of deleterious recessive alleles; can cause inbreeding depression.

Examples of inbreeding depression

Florida panthers, small isolated populations, selfing plants.

Inbreeding avoidance

Mate choice, dispersal, and self-incompatibility mechanisms.