More Mendelian Violations of Hardy-Weinberg (Migration)

Hardy-Weinberg Conditions

No selection, no mutation, no migration (in or out), infinitely large population, completely random mating

Hardy-Weinberg Outcome

Allele frequencies do not change; genotype frequencies can be calculated from allele frequencies

Migration Definition

Movement of alleles between populations; dispersal of juveniles, transport of pollen/seeds, relocation of adults

Effect of Gene Flow on Allele Frequencies

Continental population large, island population small; creates essentially one-way gene flow

Gene Flow Example

One gene, two alleles A1 and A2; continental population fixed A1A1, island fixed A2A2

Continental Population Frequencies

A1=1.0, A2=0; A1A1=1.0, A1A2=0, A2A2=0

Island Population Frequencies

A1=0, A2=1.0; A1A1=0, A1A2=0, A2A2=1.0

Migration Event

200 individuals move from continent to island; migration is random

New Island Allele Frequencies

fA1=0.2, fA2=0.8

New Island Genotype Frequencies

gA1A1=0.8, gA1A2=0, gA2A2=0.2

Expected Hardy-Weinberg Frequencies After Migration

A1A1=(0.8)^2=0.64, A1A2=2(0.8)(0.2)=0.32, A2A2=(0.2)^2=0.04

Migration Effect

Migration is a strong mechanism of evolution; causes deviations from expected genotype frequencies

Single Round of Random Mating

Restores Hardy-Weinberg equilibrium after isolated migration

Recurrent Migration

Continuous unidirectional migration or nonrandom mating keeps changing allele frequencies

Unidirectional vs Bidirectional Migration

Unidirectional: frequencies become more similar; Bidirectional: frequencies may eventually be identical

Algebraic Models of Migration

Predict migration between populations with different allele frequencies will eventually equalize frequencies, assuming no other evolutionary forces

Giles and Goudet (1977) Study

Perennial wildflower S. dioica in Skeppsvik Archipelago, Sweden; islands rising ~1 cm/year; new islands constantly emerge; existing islands different ages

S. dioica Gene Flow

Genes flow via wind- and water-carried seeds; founding populations grow quickly; initial unidirectional migration

Bidirectional Migration in S. dioica

Initially high gene flow between islands due to seed dispersal and insect pollination

Long-Term Limits in S. dioica

Habitat invasion by more successful species; pollinator-borne disease; after hundreds of years recruitment declines, gene flow reduces, population size dwindles

Succession in Skeppsvik Archipelago

Allelic diversity heterogeneous in young populations; more homogeneous in medium-aged populations; heterogeneous in older populations due to low gene flow and stabilizing selection

Allelic Diversity Components

Number of different alleles within a population; similarity of allele distributions between populations

Allelic Diversity Example

Locus with six alleles A-F; Population 1: 0.5 A, 0.2 C, 0.3 D; Population 2: 0.3 A, 0.1 B, 0.1 D, 0.5 F; diversity considers differences in alleles present and differences in frequencies of shared alleles

S. dioica Results

Analyzed different proteins to determine genotypes and allele frequencies; results confirmed hypothesis that migration homogenizes allele frequencies, but selection can offset

Lake Erie Water Snakes Example

Two populations: mainland and island; two alleles: banded and unbanded; three phenotypes: banded (BB), unbanded (bb), intermediate (Bb); migration bidirectional

Water Snake Allele Frequencies

Mainland mostly banded, island mixed; migration not fully homogenized due to habitat differences and selection

Selection vs Migration in Snakes

Mainland shoreline highly vegetated, island mostly exposed rocks; juvenile camouflage favors unbanded on island, banded on mainland; equilibrium exists between migration homogenization and selection