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Some misconceptions promulgated Discovery series: “Life” (3/21-10) • 1. female poison frog: “has to get tadpoles to safety.” • 2. female octopus: “blows water over her eggs to help them breath.” • 3. “amphibians have no scales, no claws, so they have had to evolve different (survival) strategies. • 4. female chameleon in Namib Desert: moving across the dunes “looking for love.” • 5. male African bullfrog: “tadpoles will begin to suffocate within one hour unless he can do something about it.” • 6. Sea krait: “needs to make sure eggs are safe.” “found a solution.” Life histories and reproductive mode • Two principles: • 1. Small genetic component in life history variation. – Flexibility is important • 2. Life history characteristics have not evolved in order to perpetuate a species. – Shaped by natural selection increasing fitness of individuals. • Life histories and reproductive maturity • E.g., insects Reproductive effort—Index of the proportion of biomass allocated to inflorescences—in annual (semelparous) and perennial (iteroparous) species of British grasses Semalparity in males of Antechinus sps. Trade-off between number of seeds and their size among Goldenrods (Solidago) Trade-off between age of reproductive maturity and adult mortality rate Guppies: selection for age at reproductive maturity ♂ ♀ Selection for size of female (first birth) and size of offspring ♀ Sex and Reproductive Success • Sex as an Adaptation • Since most species are bisexual (gonochoristic), there must be some advantage to sexual reproduction. • Resistance against parasites? • Sexual reproduction promotes genetic recombination • 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 (gAB*gab) – (gAb*gaB) = 0 Countering linkage equilibrium: Primulus vulgaris 1. Keep genes together that promote outcrossing Heterostylous G and A closely linked • • • • • • 2. Reproduce asexually e.g., unisexual vertebrates Shared features 1. Originate by hybridization 2. Exclusively female 3. There is an aberrant gametogenic mechanism – mitotic or meiotic • 4. Genetic recombination is usually absent so that inheritance is clonal. • 5. Polyploidy can occur if synapsis between nonhomologous chromosomes does not have to occur • or if homologous chromosomes can be produced. 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. • a proportion of parthenogens hatched in the lab have developmental deformities