Population Genetics - Migration

Effect of migration on allele frequencies

• The main effect of gene flow between populations is to homogenize allele frequencies until they are equal
• If we have two populations (i and j), with migration rate $m$ the change in allele frequency per generation is

• $\mathrm{\Delta }p$ is larger (faster) with higher $m_{ij}$ and a bigger difference in allele frequency between i and j.

Migration and Selection

• Both migration and selection change allele frequencies in a population
• Migration can favor spreading a beneficial mutation in space, but can slow the rate of a beneficial mutation within a population
• Migration and selection can have opposite effects when selection varies withing a population

Example: Water Snakes in Lake Erie

• Banded and unbanded morphs - the mainland population is usually banded and larger
• Island population unbanded
• Bands provide camouflage on mainland, but strongly counter selected on some islands because of rocks
• There is migration between populations
• Islands further from the mainland are less banded

Balance between migration and selection

• important to understand maintenance of local adaptaitons