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Enhancement of prezygotic barriers to interspecific crosses Peripatric speciation: founder effect? Paradise kingfishers Parapatric speciation in progress across c. 300 feet of ground Grass: Anthoxanthum odoratum Self-pollination vs. cross pollination • Sex as an Adaptation • Most species are (gonochoristic) bisexual. • Therefore, there must be some advantage to sexual reproduction. • Previous lecture: gonochoristic lizard species have fewer parasites than syntopic parthenogenetic congeners. • Sexual reproduction involves genetic recombination • The trade-off: separating favorable gene combinations. • If A1 is always associated with B1 and A2 with B2, the genes are in linkage disequilibrium. • Linkage equilibrium is achieved by crossing over. • Eventual result = association of 2 or more alleles at the frequency predicted by their individual frequencies. Bringing together A1 B1/ A1 B1 and A2 B2 / A2 B2 in the same population A1 B1/ A2 B2 Decay of linkage disequilibrium Continues until alleles at The 2 loci are randomly associated with each other A and B are in linkage equilibrium if (gA1B1*gA2B2) – (gA1B2*gA2B1) = 0 Countering linkage equilibrium: Primulus vulgaris Keep genes together that promote outcrossing Heterostylous G and A closely linked Four Reproductive Modes • An hypothesis (Maynard Smith): Asexual reproduction (all females) should be twice as advantageous as sexual reproduction--where half of the population are males. • Theoretical basis – Female fitness is maximized if her genes are not “wasted” in the production of males. • However, when actual asexual populations are studied e.g., (parthenogenetic vertebrates) there are some obvious disadvantages. • 1. An asexual mechanism in vertebrates is difficult to produce. • 2. The two sexual species that hybridize to produce successful asexual entities: – A. must be sufficiently divergent genetically to modify normal meiosis but – B. can’t be too divergent genetically or development will be abnormal. – Even if successful, a individuals can be produced with developmental deformities.