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Sex and Sex Ratio • • • • What is sex? Why sex? Mechanisms of sex determination Sex ratio allocation What is sex? Genetic recombination What determines gender? Why Sex? The Costs. • Cost of males - could produce 2 times as many females • Cost of meiosis - favorable gene complexes recombine • Energetic and time costs of finding a mate Why Sex? The Benefits. • Muller’s Ratchet – deleterious mutations accumulate in asexual lineages • Genetic Recombination – novel gene combinations are created – Red Queen Hypothesis: need to create new gene arrangements to combat pathogen evolution Evolutionary Arms Race • Some spp of snails are both sexual and asexual • Sexual proportion (freq. of males) increases with parasite infection Sex determination • Influences the degree to which a female can alter the sex ratio of her offspring • Types – Chromosomal – Environmental – Social Chromosomal Sex Determination • Heterogamety – Male: (XY) mammals, flies – Female: (ZW) Birds, Butterflies Chromosomal Sex Determination • Haplodiploidy: all hymenoptera, thrips, scale insects, some beetles – Haploid males develop from unfertilized eggs – Diploid females develop from fertilized eggs Creates asymmetries in relatedness Environmental Sex Determination • Incubation temperature – Terrapins – Turtles – Alligators Social Sex Determination • Some fish undergo sex reversal • Depends on mating system • Ex: dominant female in sea bass harem changes into male when territorial male disappears Sex Ratio Allocation: Null Model • R. A. Fisher: Parental strategies should evolve towards equal investment in offspring of the two sexes • If sex ratio falls below 50%, increased production of rare sex is favored • Assuming random mating, rare sex will experience greater reproductive success • Frequency-dependent selection leads to an ESS of 1:1 sex ratio Adaptive Sex Ratio Bias • Maternal condition influences offspring investment (Trivers-Willard Effect) 2. Local mate competition 3. Local resource competition 4. Local resource enhancement Trivers-Willard Effect • Population sex ratio is 1:1, but individual sex allocation depends on condition • If moms in good condition transfer competitive ability to sons more than daughters • Then, they should produce more sons than daughters assuming that • Dominant individuals sire more offspring Condition-dependent sex allocation in red deer Dominance dependent sex allocation in yellow baboons • Dominant females have more daughters than sons (pass social rank to daughters) Local Mate Competition • Mating b/w siblings takes place near hatching site • Males compete with each other for matings. • Solution: Produce few sons. Expect most offspring of first-laying female to be daughters • What if a second female arrives to lay eggs? • Ex: Nasonia wasps; haplodiploid ‘superparasites’ on fly pupae Depends on proportion of eggs that belong to the second female Local Resource Competition • Offspring that stay near their birth site may compete with their parents for resources • In many species, one sex disperses farther or at a greater rate than the other sex • Solution: Produce more of the dispersing sex (Ex: galagos produce more males) Local Resource Enhancement • In some spp., offspring of one sex delay dispersal and remain at the natal site to help parents raise their siblings • Benefits of helpers must be greater than cost of increased competition • Ex: Red-cockaded woodpecker male-biased • Prediction: Sex-ratio biased towards helping sex on high-quality territories? Seychelles Warblers As territories fill up, produce fewer males (which disperse) but more females (which help)