12: Population Structure (Gene Flow)

Natural populations

Due to spatial variation; spread over large areas, with barriers to movement, varying density, and environmental conditions.

Not single, well-mixed gene pools.

Not panmictic.

Panmictic

Each individual is equally likely to mate or interact with every other; all individuals experience the same conditions.

Clines

Patterns where allele frequencies or quantitative traits vary graudally (not erratically).

There are intermediate phenotypes between two different-looking populations.

Gene flow

A gene can change its location (to another population) from one generation to the next.

This can be a result of animals searching for food / mates; or pollen / seeds being moved by wind or animal hosts.

Has a homogenizing effect.

Also called migration or introgression.

Homogenizing effect

The movement of genes from place to place (gene flow) makes different parts of a population more similar (homogeneous) to each other.

Island model

A single local population (deme) receives migrants from some big, outside “mainland” population; this model represents gene flow (migration) and genetic drift.

In each generation, a fraction of genes are derived from the mainland.

This mixing has the effect of reducing differences in allele frequency between the mainland and island.

Island model parameters

m = migration rate (fraction of individuals on island that migrated from mainland).

t = time; time that has passed since before the migration is t — 1.

Deme

A single, local population.

Genetic drift

Always connected to effective population size (N_e).

Generates geographic variation; causes divergence.

Genetic drift vs gene flow

Genetic drift — causes divergence / variation; diversifies demes.

Gene flow — leads to homogeneity; makes demes more similar.

Both processes balance each other to generate variance in allele frequency; this variance can be used to estimate the relative rates of these two processes.

Metapopulation

A group of spatially separate populations (same species) that interact; not a mainland population, but a pool of migrants contributed equally by all demes.

The population as a whole will settle to an equilibrium distribution (normal) of allele frequencies as gene flow balances random drift.

Genetic variation (F_ST)

The F_ST coefficient measures the genetic divergence between populations (or demes).

F_ST  =  Var_p / [ Mean_p × (1 — Mean_p) ]

p = allele frequency in all populations.

When F_ST = 1, it is very divergent.

Drift in species vs populations

Species — populations as a whole drift very slowly.

Individual populations — drift can be relatively fast, especially if the population is low.

Rates of gene flow

Can be estimated using the measure of genetic variation, the coefficient F_ST.

F_ST  =  1 / (1 + 4N_em)