P10- Lecture 12- Sexual Selection- March 1 (1)

P10- Lecture 12- Sexual Selection- March 1 (1) - Sex and...

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Unformatted text preview: Sex and Sexual Selection cont. Do we need males? The value of males and thus, SEX s "Sex is the Queen of Problems in Evolutionary Biology" (Bell, 1982) s 1. The Tangled Bank Hypothesis x Sex provides genetic variability in offspring x Variability comes from new gene combinations (independent assortment and recombination) x Variability promotes chances of success in a complicated world niche differentiation Why is sex so common? s 2. The Red Queen Hypothesis x Sex provides varied offspring x Allows more rapid evolution x Can keep up in the evolutionary "race" 3 e.g. host/parasite evolution 3 Is sex a strategy to combat parasites? Advantage of Sex s Sex generates variability among offspring. s This diversity of offspring provides challenges to competitors, predators, parasites, etc. s Thus, survival chances are increased. Males are worth the trouble-- Setting the stage for sexual selection s Different levels of investment for males and females Generalizations: s Males x Sperm is cheap to produce x Male investment in each mating is small x Male reproductive success determined by # of matings (i.e. access to females) x There is great variability in male mating success Setting the stage for sexual selection s Females x Eggs relatively expensive to produce x Investment in each mating is large 3 Especially where female parental care is important and asymmetrical (e.g. most mammals) x Female reproductive success determined by # of eggs she can produce or nurture, not by access to males. x Relatively little variation in female mating success Consequences s Males will be competitive for opportunities to mate; Females will be choosy. s The sexes will differ in their characteristics due to different selection pressures. xSexual Dimorphism Sexual dimorphism Very common in animals s What is it? x Differences in characteristics between males and females of a species. Sexual dimorphism Very common in animals s What is it? x Differences in characteristics between males and females of a species Sexual dimorphism males have: x Brighter feathers or plumage x Larger weapons (e.g. antlers) x A greater tendency to vocalize x Larger muscles x Greater aggressive tendencies x A higher intensity in seeking mating opportunities s Darwin noted, when males and females differ, s How does this arise? Sexual Selection s Differential success among individuals in acquiring mates s Operates more strongly on males because relative reproductive success determined primarily by relative mating success s Can operate against survival interests! x I.e. There can be selection against survival if lower survival is compensated for by greater reproductive success. Intrasexual selection - Male control s Male-Male combat x Males monopolize access to females or staking out of territory (at least much of the time) Intrasexual selection - Male control s Male-Male combat x Access to mating opportunities determined by success in battle - very common 3 Often leads to elaborate weapons, large size Intrasexual selection - Male control s Male-Male combat Intrasexual selection - Male control s Sperm competition x Females mate repeatedly, but male success is determined by whose sperm gets to the eggs x 1. Produce large amounts of sperm 3 Also very common. 3 E.g. Chimps Intrasexual selection - Male control s Sperm competition x 2. Remove the competitor's sperm 3 Flush out the female's reproductive tract Intrasexual selection - Male control s Sperm competition x 3. Leave copulatory plug further sperm from entering oviduct of female 3 Prevents Intrasexual selection - Male control s Sperm competition x 4. Male guardingstay by mates and fight to prevent other males from mating. 3 Males Intrasexual selection - Male Control s Infanticide s Occurs when a single male or related males have access to one or more females, but only temporarily. Intrasexual selection - Male Control s Infanticide x When taking over access to females, new males may kill newborns or young fathered by other males x Reduces investment in another male's offspring x Brings female into estrous sooner x E.g. Gorillas, Lions Intrasexual selection - Male Control s Pretty good at explaining dimorphism in size and other factors directly related to dominance and/or sperm competition. s Costs of size, aggressiveness, sperm production, and weaponry are balanced by benefit to reproduction. s But what about conspicuous differences in color, ornamentation? x Return to our peacock discussion... Intersexual selection s Males often bear conspicuous colorations or ornaments: x May involve major costs to survival, but little or no benefit in male-male combat x E.g. Coloration & tails 3 Peacocks, Widowbirds Intersexual selection - Female Choice s Often occurs when males are unable to monopolize access to females s It pays to advertise! x Female choice exerts powerful control over male reproductive success x Females tend to be very discriminating 3 Reflects investment in eggs, offspring Intersexual selection - Female Choice s What, exactly, are males advertising, and why should females care? Intersexual selection - Female Choice s The Sexy Son Hypothesis x Males are advertising nothing more than "sexiness" itself x Females mating with sexy males will produce 3 Sexy sons 3 Daughters with a tendency to find these traits sexy x Runaway sexual selection 3 E.g. possibly in stalk-eyed flies Intersexual selection - Female Choice s Good Genes hypotheses x Ability to produce and maintain elaborate ornamentation indicates good genes 3 i.e. I can afford to take risks or waste energy x There are 3 variations to the hypotheses: 3 Handicap 3 Parasite 3 Developmental stability Intersexual selection - Female Choice s Good Genes hypotheses: x 1. Handicap hypothesis 3 Ability to survive despite costly advertisements indicates good genes 3 The bigger the handicap, the better the genes must be. 3 Females mating with males that are successful despite handicap are getting good genes in their offspring. Intersexual selection - Female Choice s Good Genes hypotheses x 2. Parasite hypothesis 3 Parasites reduce the ability to produce displays 3 Males that can produce/maintain elaborate displays must be resistant to parasites 3 Females mating with showy males will get parasiteresistant genes in their offspring Intersexual selection - Female Choice s Good Genes hypotheses x 3. Developmental stability 3 Developmental stress causes asymmetry 3 Advertisements that are large and symmetrical imply resistance to developmental stress 3 Females mating with symmetrical males get good genes for their offspring Summary s Sexual reproduction predominates, despite the costs increased variation in offspring vs. clones from asexual reproduction x Tangled Bank and Red Queen hypotheses s Differences in male vs. female investment in reproduction set the stage for sexual selection x Sperm is cheap - Mate often x Eggs are expensive - Be selective x There are often differences in parental care Summary s Sexual Dimorphism is common x Often arises via sexual selection x Sexual selection can act in opposition to natural selection s Intraspecific vs. Interspecific x Intraspecific - Combat, sperm competition, infanticide x Interspecific - Based on female choice 3 Sexy Sons 3 Good Genes Handicap Parasite Developmental stability ...
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