Chapter 17 Population Genetics and Evolution, part 3 Jones and Bartlett Publishers © 2005

Bottlenecks and Founder Effects Temporary periods of small population size = bottleneck. This results in a loss of alleles. Elephant seals – dropped to 20 animals, now up to 30,000. Now there is no variation at 24 enzyme loci. Founder effect – new area of colonization.

Consequences of inbreeding for genotype & allele frequencies at F = 1 and F = 0

Genotype frequencies under H-W and with complete inbreeding (F = 1) p = 0.4, q = 0.6

Decrease in heterozygosity with successive generations of inbreeding

Effect of inbreeding on the genotype frequencies F = Inbreeding Coefficient. Reduction in heterozygosity due to inbreeding (HI) = 2pq (1-F)

The frequency of heterozygotes is reduced as inbreeding increases

A pedigree showing inbreeding Calculating inbreeding coefficient using allelic identity by descent in an inbred pedigree A pedigree showing inbreeding A closed rectangle in a pedigree indicates inbreeding

Calculation of the probability that the alleles indicated by the double-headed arrows are identical by descent

The logic behind calculation of allelic identity by descent in a pedigree For example, the probability of producing 2 blue gametes for individual A is 1/2 x1/2 = 1/4. Similarly, the probability of producing 2 red gametes is also 1/4, but the probability of producing a red and a blue gamete is 1/2 (1/4 + 1/4). FA is the inbreeding coefficient of the individual producing the gametes.

A complex pedigree in which the individual I received genes from different ancestors through multiple paths Calculation of inbreeding coefficient in a complex pedigree is more involved because each path contributes to the final inbreeding coefficent.

Inbreeding increases the chance of having progeny that are homozygous for a rare recessive trait

Effect of autozygosity on viability Drosophila 2nd chromosome

Inbreeding depression in rats

inbreeding depression in the titmouse

Gene flow in corn F = proportion of offspring of recessive plants, grown at different distances from a dominant strain, that were fathered by the dominant strain

Three generalized forms of selection Directional Stabilizing Disruptive

Natural selection increases adaptation Differential reproductive ability of alternative genotypes is natural selection. Fitness describes the reproductive efficiency of a genotype in relationship to others. (W=fitness) Selection coefficient – S=1-W. Peppered moth example.

Positive frequency-dependent selection on Heliconius color patterns

Frequency of melanic moths of B. Gonodontis bidentata A. Biston betularia B. Gonodontis bidentata

Mutations The source of all genetic variation is mutations. Random mutations occur at a background level (replication error, radiation, etc.) Some mutations in response to stress may be caused by transposons.

Effect of mutation (irreversible or reversible) on allele frequency Allele frequency is changed very slowly by mutation. In the case of reversible mutation, an equilibrium state is reached where the allele frequency becomes constant.

Effect of selection for a favored allele (A) in a haploid (Escherichia coli)

Results of selection for a favored allele in a diploid depends upon whether the allele is dominant or recessive

Effect of the degree of dominance in a diploid on the equilibrium frequency of a recessive lethal allele h = degree of dominance. If the deleterious allele is completely recessive, then h= 0

The heterozygote is favored over the homozygous Geographic distribution of the diseases sickle cell anemia and falciparum malaria The heterozygote is favored over the homozygous dominant genotype (overdominance) in areas where malaria is prevalent. The homozygous recessive is usually lethal.

Speciation has a genetic basis Speciation may occur suddenly. Polyploidy is a good example of a sudden reproductive barrier. Translocations also isolate populations. Neutralist vs. selectionist debate.