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Mating Systems Recombination Common, Not Universal Sexual Reproduction: Recombinant Genotypes Basic Questions: Sex
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Basic Questions: Mating Systems 1. Sex Versus Non-Sex Mitotic Replicates, No Recombination Clonal Growth Isogamy, Recombination Equal Gametes Anisogamy, Sexual Reproduction Unequal Gametes Parthenogenesis
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Anisogamy
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Sex versus Asexuality
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Asexual Lineages Disappear; Sexual Lineages Persist “Bet Hedging” Against Environmental Unpredictability Thwart Specialized Pathogens Group Selection, Possible
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Basic Questions: Mating Systems 2.Given Anisogamy, Sex Ratio: Males/Female Zygotic, Birth, Sexual Maturity Operational Sex Ratio: During Mating
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Basic Questions: Mating Systems 3. Given Sex Ratio, Mating System Polygyny, Monogamy, Polyandry “Parental Investment,” Correlative/Comparative Approach Sexual Selection, Predictive Approach
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Sex Ratio Given Evolved Anisogamy, Female & Male “Strategies” Ratio Males/Female Human Zygotic 1.15 Birth 1.05 15 Yrs 1.0
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Sex Ratio Sex Ratio Evolution, R.A. Fisher Economically Rational Grandparent (Male and Female: Autosomal) Assign Sex to Offspring Maximize No. Grandoffspring, Given Competition with Others ESS: Sex Ratio
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Sex Ratio Sex Ratio Evolution: Rarer Sex Has Fitness Advantage Assume Population: F females, M males Each Female: 2 Offspring Diploid Genetics
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Sex Ratio Individual Fitness: Gene Copies Female Fitness 2 F (1/2)/F = 1 Independent of Sex Ratio M/F 2 Offspring, (1/2) Genome
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Sex Ratio Individual Fitness: Gene Copies Male Fitness 2 F (1/2)/M= F/M Inverse of Sex Ratio
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Sex Ratio IF F < M (Females Rare) Males Fitness = F/M < 1 = Female Fitness Natural Selection Favor Gene: More Daughters Since Females More Fit than Males
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Sex Ratio IF F > M (Males Rare) Males Fitness = F/M > 1 = Female Fitness Natural Selection Favor Gene: More Sons Since Males More Fit than Females
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Sex Ratio IF F = M (Sexes Equally Frequent) Males Fitness = F/M = 1 = Female Fitness Sex Ratio Selection at Equilibrium Each Sex Has Same Fitness Rational Grandparents Produce Females & Males Equally
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Sex Ratio General Sex Ratio Theory Stable Equilibrium: Invest Parental Resources Equally in Daughters & Sons Resources Equally in Daughters & Sons IF Sexes Have Different Cost: Produce Fewer of Costly Sex
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Sex Ratio Sex Ratio Theory Gene with Sex-Linked Transmission XX, XY: Heterogametic Sex Y Chromosome: Appears in Sons Only Bias Sex Ratio Extinction, “Marriage Squeeze”
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Parental Investment “PI” and Mating Systems PI: Investment in Current Offspring Reduces Capacity to Produce Future Offspring Investment Costly: Time, Energy, Limited Nutrient(s), Exposure to Predation
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Parental Investment “PI” and Mating Systems Given Anisogamy, Two Sexes Often Differ in Pattern of Parental Investment Define Pattern in Mating Systems, Sexual Selection
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Parental Investment Many Animals: Males Lower PI Males Compete for Chance to Mate with Females, Greater PI Females Exercise Mate Choice, Generate (Incite) Competition Among Males for Access to Females
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Parental Investment Many Animals: Males Lower PI Variance in Reproductive Success Greater Among Males Males Larger, More Aggressive Polygynous Mating Systems
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Polygyny
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Polygyny
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Parental Investment Some Animals: Sexes (app.) Equal PI Each Sex, Low Variance in Reproductive Success Body-Size Differences Reduced Monogamy
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Parental Investment Few Animals: Females Lower PI Females Compete for Chance to Mate with Males, Greater PI Variance in Reproductive Success Greater Among Females Females More Aggressive Polyandry
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Polygyny Common Mating System Access to Females (female gametes) Limits Male Reproductive Success Rep. Success: More Variable in Males Male PI < Female PI
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Polygyny Three Polygyny Models Resource Defense Female Defense Male Dominance (lek)
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Polygyny Resource Defense Females Unpredictable in Both Space and Time Resource Required by (Mating) Females Highly Predictable [Food, Water,…] Males Compete to Defend Resource & So Acquire Chance to Mate
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Polygyny Female Defense Females Predictable in Both Space and Time Males Compete to Defend Harems, Gain Mating Opportunities
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Polygyny Male Dominance Neither Females nor Required Resources Sufficiently Predicable Males Form “Lek,” Compete for Locations Where Females Arrive to Mate
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