SEXUAL SELECTIONPowerPoint PresentationSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Sperm CompetitionSlide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28SEXUAL SELECTIONI. Sexual Selection as an evolutionary forceA. What types of traits are the targets of sexual selection?1. Exaggerated nature of many secondary sexual traits1. Energetically expernsive2. Increase risk of predation3. Payoff- attract mates2. Sexual dimorphism exists for many of these traits-males are more ornate than femalesB. What factors drive sexual selection?1. Cost is not always equal between the sexes-cost of gametes: eggs are usually more expensive to produce (fewer, larger) -parental investment: cost of caring for young differs bt the two sexesfemales often provide more parental carein polygynous (male gives no parental care) systems, males often have very elaborate secondary sexual traits2. Differences in costs lead to different ways to maximize fitness between the sexes-males maximize number of matings because often their investment is lessmales increase their fitness by fathering as many offspring as possible (sexual selection should favor traits tht enable them to get more matings) -females minimize number of mates making them the “choosier” sexfemales increase their fitness by choosing fathers who will sire the most fit offspringC. How do we define sexual selection1. Variation in mating success of one gender that is produced by an association of a trait value with its ability to obtain matescurrency of sexual selection: mating successsexual selection favors traits tht increase an individuals ability to acquire mates2. Can operate in two ways-INTRASEXUAL selection: traits are favored that increase the ability of one sex to compete directly with one another for matingsuaually male-male competition Males that copulated were larger in body size than those that did not mate (Wikelski and Trillmich 1997).-INTERSEXUAL selection: traits are favored in one sex that makes them more attractive to the opposite sex, thereby increasing their mating successusually female choice Males with shortened tails had fewer nests (Andersson 1989, 1991)D. Male-male competition: What do males compete for?1. Resources via territories-nesting sites-food for young or female(Howard 1978)2. Access to females-prairie chickens have leks tht they use to attract females -mate guarding in elk-harems: animals tht make guard3. Sperm Competition-assurance of paternity(1) Sperm displacement: some insects develop ways to pull out another male's sperm and deposit theirsSperm Competition(2) Alternative male mating strategies: small males fertilize nest while larger fish fight HOOKNOSEJACKJACK051015202520-2425-2930-3435-3940-4445-4950-5455-5960-6465-6970-7475-7980-8485-8990-94Standard length (mm)frequencyP. veliferaAlternative male mating types in sailfin molliesSperm Competition(3) Infanticide: new male will kill all young tht arent his in his new prideE. Female choice: When does it pay to shop around?1. Immediate benefit: females receive direct benefit from mating with particular males -certain males are better fathers-males provide nutritional benefit (hanging fly brings his female a bug to eat during sex)Female choice in Hanging Flies:Bigger gift (food), longer copulation, more sperm transferred (bigger size is favored in males as result of this)Thornhill 1997Immediate Benefit:*female preference is under direct sexual selection -female gains an immediate benefit in terms of survival or increased fecundity from choosing certain males2. Male traits are indicators of good genetic quality (Indicator or Good Genes Models)-exaggerated traits are energetically costly for males to make and maintain-males who carry these traits must be more “fit” than males who do not-these males have superior genes that confer resistance to disease or parasites-the male traits serve as “indicators” to females of superior male genetic quality-female choice in under INDIRECT selection (all offspring directly benefit)Long calls are energetically expensive to produceFemales prefer long calls to short callsDo males that produce long calls have “Good Genes?”Female choice in Hyla versicolorGerhardt et al. 1996Experimental design of Welch et al. 1998YES--Offspring of long-calling males had 1) faster growth rates, 2) shorter larval periods, 3) higher larval survival and postmetamorphic growth3. Fisher’s Runaway Process--the Sexy Sons Hypothesis-females choose arbitrary traits (may initially have indicated better genetic quality)-genetic correlation between the male trait genes and the female preference genes (linkage disequilibrium) -positive association between male and female genes drives the male trait to extreme values (nat sel will form road block) -females that mate with these males insure that their sons inherit the trait (INDIRECT SELECTION on female preference) (only sons benefit)-natural selection usually stops the “runaway” process- bird cant have too long of tail bc he will get eaten by predatorFemale choice in Stalk-eyed Flies (Cyrtodiopsis whitei)(Wilkinson and Reillo 1994)Females prefer males with longer eye stalks1. Is eye-stalk length heritable in males? sexual selection cant cause evolution unless traits are heritable 2. Is female preference for long eye-stalks heritable?3. Is there genetic correlation between the male and female traits?Yes--genetic variation for male trait and female preferenceYes--genetic correlation between son’s eyestalk length and daughter’s preference level for long eyestalks (positive slope)Yes--selection on one trait (male) causes a correlated response in the other trait (female)F. Interaction between Sexual Selection and Natural Selection1. Does it always pay to advertise?Example: Túngara frog Physalaemus pustulosus (has complex call tht invloves species recognition and chucks)Males mating success increases with more complex callsMales make more ‘chucks’ at higher chorus densitiesMales risk of predation decreases with increased chorus size but detection probability is higher, forced to use more ‘chucks’Leads to overall stabilizing selection on number of ‘chucks’Calls with more ‘chucks’Calls with fewer ‘chucks’Ryan et al. 1981Calls without