Topic 9 part 3 (Migration and gene flow)

movement of individuals from one population to another, and subsequent mating -general impact: reduces the level of population differentiation (as measured by Fst, etc)

• assumes ONE way migration from a large continental pop’n to a small island pop’n • it can also be adapted to other situations where gene flow is unidirectional

describes the case where a large number of islands randomly exchange individuals • islands can represent: actual islands, lakes, fragments of habitats, mountains separated by valleys, etc. • assume no genetic drift (by assuming pop’s on islands are infinitely large) • good model for organisms that can move very easily from place to place • ->equal prob. that any pop’n exchanges migrants w/ any other pop’n

-accounts for the fact that adjacent pop’ns are much more likely to to exchange migrants than more distant pop’ns

in this pattern, geographic distance is correlated to genetic distance • the greater the geog. distance, the greater the genetic diff b/w islands • the smaller the geog. distance, the smaller the genetic diff b/w islands -continuously distributed organisms: can also exhibit isolation by distance pattern

• movement of migrants into a pop’n can change the alleles frequencies • initially cause HW disequ’m • one round of random mating restores HW eq’m (based on new allele freq’s)

-Diseq’m may never break down if you continuously introduce migrants • consider a pop’n with only AAbb, and you introduce aaBB • results in Dmin (non-random genotypic associations among the loci) • AAbb and aaBB are only able to produce repulsion phase gametes

• drift and migration oppose each other - drift causes pop’s to diverge and gene flow acts to homogenize pop’s

• finite population sizes, but the number of islands is infinitely large • assumes the same effective population size on each island

• if gene flow is low, drift will be the dominant force on each islands and they will diverge • If gene flow is high, migration will be the dominant force