What is sexual selection? Natural Selection: Differential reproductive success due to variation among individuals in survival and reproduction.
What is sexual selection? Darwin (1871) defined sexual selection as: Differential reproductive success due to variation among individuals in success at getting mates.
Sexual selection is non-random variance in reproductive success. • Two forms of sexual selection: • Intrasexual selection: direct competition for mates between members of the same sex, usually male-male competition. • Intersexual selection: differences in attractiveness to the opposite sex, usually non-random mate choice by females.
Mating systems Monogamy One male mates exclusively with one female Polygamy Individuals mate with more than one partner PolygynySome males mate with more than one female Polyandry Some females mate with more than one male Promiscuity Males mate with more than 1 female and vice versa Promiscuity Monogamy Polygyny
Mating systems Mating systems influence sexual selection Strength of sexual selection Variance in mating success Some males = 0 mates Some males = 1 mate Some males = 2 mates All males have same mating success = 1 mate Most males = 0 mates One male = 8 mates Strong Polygyny Monogamy Moderate Polygyny
Harem polygyny Elephant seals (Mirounga)- breeding females cluster together on beaches - allows males to defend a harem of many females at once Bull male elephant seals engage in violent, bloody fights over females – large size confers an advantage in male combat Male elephant seals weigh up to 3x more than females! Male reproductive success is highly variable: 8 individual males inseminated 348 females in one study!
Polyandry When is polyandry favored? When males become limiting resource for reproduction This often occurs when low offspring survival requires male parental care, so that males have the greater Reproductive effort and lower reproductive rate What is the consequence? Sexual selection is stronger on females than males Saddleback Tamarins, Spotted Sandpipers, and Red-necked Phalaropes are examples of polyandrous species
The form of Sexual Selection is directly related to the relative investment in offspring production. • Bateman’s principle: The sex that invests more in offspring production has fewer reproductive opportunities. • Thus, the sex that has a higher investment • Should be more discriminating (choosier). • Become a limiting resource for the opposite sex. • This is usually the female sex
ANISOGAMY FEMALES: Sex that produces relatively few, large well-provisioned gametes (eggs) MALES: Sex that produces many, “cheap” gametes (sperm)
Females are more invested • Females also often invest more than males in other forms of care • Females bare the burden of pregnancy • Care for offspring • Only in fish is male care more common than female care • Males guard the eggs in their territory so they can also secure multiple mates
Why don’t males care for offspring more often? • Females can sometimes mate multiply (polyandry) • Therefore, not all offspring may be sired by the male
Why don’t males care for offspring more often? • Females can sometimes mate multiply (polyandry) • Therefore, not all offspring may be sired by the male (paternity sharing) • Opportunity costs • A male’s mating success is more driven by how many pairings he can achieve • Especially because females are choosy
Why do females invest more in offspring care? • If females invest more prior to mating, there is competition amongst males for female investment • This causes variability amongst male success • As long as females continue to provide care, it is less likely males will contribute (a positive feedback loop)
The form of Sexual Selection is directly related to the relative investment in offspring production. • Bateman’s principle: The sex that invests more in offspring production has fewer reproductive opportunities. • Thus, the sex that has a higher investment • Should be more discriminating (choosier). • Become a limiting resource for the opposite sex. • Evolution of gender and sex resources • This is usually the female sex
The asymmetric nature of sexual selection often leads to dramatic sexual dimorphism in characters directly related to male-male competition and/or female choice. Peacock Peahen
Selection for combat in males • Natural selection selects for male morphologies that make them more competitive • Larger body size • Greater strength • Weaponry
Selection in Males: Body Size • Larger male copepods can carry larger spermatophores • Mating with large males allows females to fertilize more eggs and reduces the need for frequent matings
Weaponry • Narwhals!
And in terrestrial birds • Some really awesome courtship behaviors • Marine birds
Sexual selection can be very strong and often opposes natural selection. • This can lead to exaggerated and sometimes maladaptivedevelopment of male traits. Irish Elk (Megalocerosgiganteus)
Natural selection (predation) favors spot patterns that match background Sexual selection (mate attraction) favors male patterns that contrast background Sexual vs natural selection Guppies (Poecilia)- sexual selection can favor traits that reduce survival - laboratory selection studies by John Endler
Intersexual selection Intersexual selection has always been controversial We know mate choice occurs because we can observe it directly However, it is debated if and how some sexually dimorphic traits evolve in response to mate choice Intra- and inter-sexual selection Intrasexual selection All of the examples thus far have involved interactions within a sex - Darwin’s theory of intra-sexual selection was readily accepted, even in his time
Are females really choosy? • Females can prefer elaborate ornaments • Long tailed widowbirds
Why are females choosy? • Two broad benefits: • Good resources • Competitive males often secure the best territories • Good genes • Body size and other competitive characteristics will be inherited by the females offspring • Humback whale size and mating calls
But what are the genetic benefits? • 1) Fisher’s hypothesis: gaining attractive sons • Females want sexy sons to attract more mates in the future • Often produces runaway sexual selection • Usually begins with a trait that confers some adaptive benefit and then is further exaggerated because it is selected for by females
Survival Selection Sexual Selection Female choice adaptive for survival Total male fitness (survival + mating) Fitness Fitness due to survival Tail length Fisher’s runaway model
But what are the genetic benefits? • 1) Fisher’s hypothesis: gaining attractive sons • Females want sexy sons to attract more mates in the future • Often produces runaway sexual selection • Usually begins with a trait that confers some adaptive benefit and then is further exaggerated because it is selected for by females • 2) Handicap hypothesis
GOOD GENES MODEL ELABORATED MALE TRAITS MAY BE INDICATORS OF HERITABLE GENETIC QUALITY (I.E. FITNESS). The Handicap Principle (Zahavi 1975) • Some males may have a heritable trait that reduces viability. • Only males with “Good Genes” can survive despite the handicap. • Females that mate with these males will have offspring with higher fitness.
GOOD GENES MODEL ELABORATED MALE TRAITS MAY BE INDICATORS OF HERITABLE GENETIC QUALITY (I.E. FITNESS). Hamilton and Zuk added to this hypothesis: • Sexual displays are reliable indicators for genetic resistance to disease or parasites • Advertisements of healthy animals
Evidence of females choosing for sexy sons • Females prefer brighter red colors • And the intensity of red in sons is correlated with the red in his father • But is the only benefit of this trait increased matings? Males with bright red color should not show increased viability, just increased mating success
Evidence of females choosing for good genes • Reddish color is derived from carotenoids in a fish diet • Better color, better foraging, • Even subordinate males express this color early in the mating season, but only more fit males are able to maintain it though the breeding season
Healthier male fish are more red • Red coloration declines in males infected with parasites • Healthy red males were also likely to have a strong MHC profile
Genetic Benefits • 1) Sexy sons and daughters • Only the sexy are selected • 2) Good genes as fitness indicators • Only good quality males that can afford elaborate ornaments can survive • But then how is variation maintained in a population? • Mutations • Traits are quantitative (many genes are responsible) • Variation in choice over time
Female behavior can also help to maintain genetic variation • Female copying- females imitate the choices of other females • 3 morphed isopod • Female isopods prefer to mate with other females and can copy matings of other females • This may account for the maintenance of the 3 male morphotypes
Sex Role Reversal • When males make large contributions to parental investment, males may become the choosy ones • Seahorse mating
Sex Role Reversal • When males make large contributions to parental investment, males may become the choosy ones
Sexual Conflict Over Mating • If males need to maximize matings and females need to choose the best partner • Then females need to resist mating and males need to be aggressive • Results in a sexual selection arms race • Males competing for a female sea turtle
Sexual Conflict Over Mating • The male guppy grows claws on its gonopodium (anal fin) to counteract sexually-selective females.
Chase Away Sexual Selection • This sexual conflict can drive antagonistic co-evolution between the sexes • Females will evolve resistance to male ploys because acquiescence is costly
Sexual Conflict • Selection has favored Littorinasaxatilisfemale snails that do not signal their gender in their slime trails