Variety of mating systems Panmixia Assortative mating Disassortative

Variety of mating systems • Panmixia • Assortative mating • Disassortative mating • Outcrossing • Inbreeding • Mixed mating

Disassortative mating What is the equilibrium frequency of alleles S 1, S 2, S 3, and S 4? What is the fate of a new mutation that produces allele S 5?

Inbreeding What is inbreeding? Forms of inbreeding

Genetic consequences of inbreeding Loss of heterozygosity Do allele frequencies change (i. e. , does inbreeding result in evolution)?

Genetic consequences of inbreeding Heterozygosity: Homozygosity: Genetic variance within lines: Genetic variance among lines: Response to selection among lines: Fitness of inbred lines:

The inbreeding coefficient, Wright’s F Sewall Wright 1889 -1988

The inbreeding coefficient, Wright’s F F ranges from 0 -1 • 0 = completely outbred (H-W) • 1 = completely inbred (homozygous) F can be thought of as: • Proportional loss of heterozygosity • Probability of homozygosity • Half the coefficient of relatedness of the parents F is measured relative to some starting population, which is usually assumed to have F = 0

The inbreeding coefficient, Wright’s F From Hartl and Clark Fig. 4. 15

Genotype frequencies as a function of F AA: p 2 + pq. F Aa: 2 pq - 2 pq. F aa: q 2 + pq. F

How can F be estimated if we don’t have a pedigree for the whole population? HI = observed heterozygosity at neutral loci (usually with molecular markers) HT = heterozygosity expected under H-W (2 pq) Aa: 2 pq - 2 pq. F = HI 2 pq(1 -F) = HI HT(1 -F) = HI F = 1 -(HI/HT) F = (HT-HI)/HT

Inbreeding depression What is inbreeding depression? What causes inbreeding depression? How do organisms avoid inbreeding depression?

Inbreeding Scientists find no biological reason to stop first cousins from marrying • Risks to children born to cousins not as high as previously thought By Denise Grady The New York Times Thursday, April 4, 2002 Sarah Josiah Wedgwood I Wedgwood Robert Darwin Susannah Wedgwood Charles Darwin Josiah Wedgwood II Mrs. JWII Emma Wedgwood

Is inbreeding always bad? By exposing recessive alleles to selection in homozygotes: • Leads to rapid fixation of beneficial recessive alleles • Purges deleterious recessives; aka, lightens the genetic load