Enhancement of prezygotic barriers to interspecific crosses.

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Presentation transcript:

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 A 1 is always associated with B 1 and A 2 with B 2, 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 A 1 B 1 / A 1 B 1 and A 2 B 2 / A 2 B 2 in the same population A 1 B 1 / A 2 B 2 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 (gA 1 B 1 *gA 2 B 2 ) – (gA 1 B 2 *gA 2 B 1 ) = 0

Countering linkage equilibrium: Primulus vulgaris Heterostylous G and A closely linked Keep genes together that promote outcrossing

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.