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Announcements Why Sex? Two features that distinguish sexual from asexual reproduction: meiosis and syngamy Meiosis: process by which diploid organisms produce haploid gametes Syngamy is the fusion of two haploid gametes to produce a diploid zygote. Sex typically associated with anisogamy Ansiogamy = differences in size between two gametes Sex = Recombination Recombination Species with separate sexes in separate individuals are dioecious. (includes most animals) Animals in which the same individual can produce both eggs and sperm are either simultaneous hermaphrodites or sequential hermaphrodites. Hermaphrodites Simultaneous: many snails, annelid worms Sequential: echinoderms, some fish (e.g., gobies, wrasses, parrot fish) Asexual Reproduction Common in plants, less common in animals. Found in all groups of vertebrates, except mammals Many organisms capable of both sexual and asexual reproduction hydra, Aphids, Daphnia, nematodes Sexual reproduction is costly Gonads are expensive organs to produce and maintain. Mating is risky and costly, often involving elaborate structures and behaviors, risk of injury and disease. Recombination breaks up beneficial combinations of alleles at different loci. So why does sexual reproduction exist at all? Asexual reproduction is evolutionarily advantageous Assume: Reproductive mode does not affect how many offspring a female can produce. Reproductive mode does not affect the survival or reproductive success of the offspring. Is asexual reproduction is evolutionarily advantageous? Does reproductive mode affect how many offspring a female can produce? Does reproductive mode affect the survival or reproductive success of the offspring? (Dunbrack et al. 1995) Evolutionary advantage to sex: Deleterious mutation and drift --> “Muller’s ratchet” Faster adaptive evolution Muller’s Ratchet Model In asexual populations, mutations will accumulate in lineages because there is no recombination. Faster adaptive evolution If a different beneficial mutation occurs in two different families in a population. If the population is sexual, the two different mutations can end up in the same organism through meiosis and syngamy. Why did ansiogamy evolve? Why did ansiogamy evolve? Once we have sexual reproduction, two selection pressures on gametes: Bigger gametes (increase zygote size and survival) More gametes (increase potential number of offspring) Both strategies increase reproductive potential, just differently Why did ansiogamy evolve? Can lead to conflict among sexes (which will invest more). Bigger gametes (more parental investment - be choosy when selecting mates) More gametes (less parental investment - mate as many times as possible) Darwin (1871), The Descent of Man, and Selection in Relation to Sex Sexual Selection: “the advantage which certain individuals have over others of the same sex and species, in exclusive relation to reproduction.” Sexual Selection occurs in two forms Intrasexual: Male-male competition Intersexual: Female choice