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 traits-Energetically expensive to maintain-Increase risk of predication-Payoff- attracting 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-parental investment: cost of caring for young differs between the 2 sexesfemales often provide more parental carein polygynous systems, males often have very elaborate secondary sexual traits2. Differences in costs lead to different ways to maximize fitness between the sexes-males maximize because often their investment is lessmales increase their fitness by fathering as many offspring as possible-females minimize 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 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 matingsMales 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 success 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-leks-mate guarding-harems3. Sperm Competition-assurance of paternity(1) Sperm displacementSperm Competition(2) Alternative male mating strategiesHOOKNOSEJACKJACK051015202520-2425-2930-3435-3940-4445-4950-5455-5960-6465-6970-7475-7980-8485-8990-94Standard length (mm)frequencyP. veliferaAlternative male mating types in sailfin molliesSperm Competition(3) InfanticideE. Female choice: When does it pay to shop around?1. Immediate benefit certain males are better fathersmales provide nutritional benefitFemale choice in Hanging Flies:Bigger gift, longer copulation, more sperm transferredThornhill 1997Immediate Benefitfemale 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 qualityLong 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-positive association between male and female genes drives the male trait to extreme values-females that mate with these males insure that their sons inherit the trait (INDIRECT SELECTION on female preference)-natural selection usually stops the “runaway” processFemale 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?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 eyestalksYes--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 pustulosusMales 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