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A Recap Regarding Senescence Selection is more powerful in the stages up to the age of peak reproductive value If a pleiotropic gene is introduced that gives an benefit early in life, at the expense of a cost much later on, than it will be selected Senescence is the outcome of such negative effects later in life The Evolution of Sex The Mathematics Urge Sublimated What Use is Sex? The Problem: Asexual: 100% copy in offspring Sexual: 50% (usually) copy in offspring So why do some species have sex? Some answers: Genetic “proofreading” Certainty of environment Parasite resistance The Genetic Manuscript “We are no4 ready for winter” What is the correct letter? A second copy from another organism of the same species may help After “proofreading,” some recombination takes place New genotypes are produced The Certainty of the Environment Offspring produced in environments similar to parents: Asexual reproduction Offspring reproduced in environments dissimilar to parents: Sexual reproduction Gambling theories are not well supported by the data Parasite Resistance Parasites are responsible for far more deaths than physical threats Correlation between high parasite infestation and amount of recombination Sex allows for new genetic “locks” to keep out old parasite “keys” Hermaphroditism Simultaneous hermaphroditism When sexual functions serve both sexes Sequential hermaphroditism Generally due to maturation and sizeadvantage Separate sexes When sexual functions do not serve both sexes Eggs & Sperm “Cheating” organisms may produce a greater number of smaller gametes Disruptive selection: organisms producing many small gametes or fewer large gametes vs. producing average gametes Minimum Investment Females in most species have a greater minimum investment in offspring than males Ova Gestation Lactation… Male & female “needs” differ due to minimum investment The Sex Ratio If males can inseminate several females, why do we have 1:1 ratio? Reproductive efficiency? Island with 90 men & 10 women vs. 10 men & 90 women Frequency dependent selection Sexual Dimorphism Intrasexual contests Lead to dimorphism of competing sex relative to other sex No intrasexual contests leads to females typically being larger Ova production The Wrap-Up Evolution of sex Proofreading Gambling Parasite resistance Sex ratio Minimum investment Sexual dimorphism Things to Come Long-term mating strategies Female Male Short-term mating strategies