What causes geographic populations to become differentiated? Natural Selection? Genetic Drift? (limited gene flow)

Genetic drift Changes in allele (and genotype) frequencies due to random sampling of breeding individuals in small (finite) populations Leads to: Loss of genetic variation (heterozygosity) increase in homozygosity (inbreeding) Fixation (or loss) of alleles Key Concept:Effective population size (N e ) Number of breeding individuals contributing genes to next generation

Genetic drift is STRONG in SMALL populations Genetic drift is WEAK in LARGE populations

Experimental demonstration of drift among sub population (demes)

Buri experiment: experimental demonstration of genetic drift Average allele frequency does not change Variance among sub-populations increases

Loss of heterozygosity (H) H= heterozygosity = 2pq (from p 2, 2pq, q 2 ) = proportion of heterozygotes in population = (1 - sum of all homozygotes) = 1 -  x i 2, where x i = frequency of ith allele if there were 1, 2, 3, …i alleles in popln. H decays due to drift (and inbreeding) H t+1 = H t [1 - 1/(2N e )] Inbreeding coefficient = F = H expected - H observed H expected

Drift causes: Change in the genetic make up of the population (evolution) Loss of heterozygosity; increase in homozygosity Differentiation among populations Fixation of alleles

Probability transition matrix for 2N = 4 Probability of i A 2 alleles in generation t+1 given j A 2 alleles in generation t

Large population sizes of marine invertebrates (shrimp)

Small(er) population sizes of some mammal species

Estimating Ne Inbreeding effective size -increase in inbreeding over time Variance effective size -increase in the variance among demes Eigenvalue effective size -loss of heterozygosity over time

Fur seal lion and harem: Unequal sex ratio of breeding individuals Effective population size is lower than census size

Sex ratio effects on effective population size Skewed sex ratio reduces N e below census size N f = number of breeding females N m = number of breeding males N e = 4N f N m / (N f + N m ) Example: sea lions, 1  -male and a harem of females N f = 100, N m = 1 N e = (4 x 100 x 1) / ( ) = 4 sea lions!

Examples of population size fluctuation. The low points have a disproportionately large effect on reducing the effective population size below the average census size over the time interval. Population size in Egypt

Effect of population fluctuation on effective population size Bottlenecks have a disproportionate effect on reducing N e Generationtt+1t+2t+3 N e Average N e = 320 / 4 = 80 (not correct) Real N e = harmonic mean 1/N e = 1/(# of generations) x ∑ 1/N e = 1/4 x [1/ / /20 + 1/100] 1/N e = 1/4 x [0.08] = 0.02 Ne= 50 LESS THAN THE SIMPLE AVERAGE (arithmetic mean)

Variance in reproductive success Poisson distribution

Ne/N ratio Effective / census population size