SEXUAL SELECTION Combining behavior and evolution

Follow up to H-W Why do I look like this? Drift?Gene Flow?Selection?

Why Sex? Sex takes partners Sex take time and energy Sex is risky Sex is not even a requirement for reproduction!

Why face the costs of sex? Having two copies of each gene can “mask” bad recessive genes Recombination/crossing over makes new combinations of old genes (linkage) Multiple forms (alleles) of genes are more likely to “stick around” in a population until they are useful, when the environmental conditions change Two word answer: Genetic Diversity

Mating systems 1 FemaleMultiple Female 1 MaleMonogamy (rare- birds, few mammals) Polygyny (somewhat common) Multiple MalesPolyandry (very rare) Promiscuity (really common) Note- many living things don’t have much of a system- lots of these are external fertilizers.

Sex can be… complicated Animals show a variety of mating systems These can be surprising, bizarre, and, frankly, entertaining. How do we “make sense” of these? By studying them in evolutionary context. - What are the benefits (and costs)?

Natural selection “Selection” of features that contribute to survival of some individuals and their contribution of genes to the next generation. Features that don’t promote survival should not last.

So how do you get this guy?

Definition of sexual selection Selection that is driven by the competition for mates and that is considered an adjunct to natural selection. Selection for features that promote reproduction, but not necessarily “survival”

Intrasexual selection Competition between members of the same sex, for access to mates. Usually male-male and often associated with polygyny (single male mates with multiple females) Often a direct contest or fight over territory, dominance, or resources. Can result in large size, horns, antlers, & other exaggerated traits.

Classic male-male competition Elephant seals- winners get harems of females, losers wait ‘till next mating season. Everybody gets nasty scars on their necks.

What are costs to a male? Time and energy spent competing with other males Resources needed for growth of “fighting features” Fighting can be dangerous or even deadly

Benefits to male? Reproductive Success Sometimes, competition can result in better survival (secondary benefit)

“Sperm competition”- an extreme example of intrasexual competition In some organisms, male-male competition continues after mating is done! Some strategies: 1.Make more sperm, or bigger, or faster sperm than your male competitors 2.Make sperm that don’t even fertilize! -Some sperm may actively “fight” against other males’ sperm -May ‘fill space’ so other males’ sperm is blocked out

Sperm competition in primates

Sperm Swimming Speed- Why are humans “intermediate” between monogamous gorillas, and promiscuous monkeys and chimp females? Quote from the author: “Maybe humans haven’t always been as monogamous as we had thought.” Human ‘mating systems’: 83% of societies are polygynous, 16% monogamous, only 1% polyandrous (multiple males per female)

Extra-pair copulations (EPCs) Occur in monogamous species, polygynous species, & polyandrous species In many monogamous songbirds, the percentage of extra-pair young has been found to be about %. (as high as 80%)

Int ra sexual competition summary A direct contest between members of same sex (usually males) Can happen at large scale (such as fighting) or small (sperm competition)

Intersexual selection Competition for mates decided by the preference of the potential mate. Generally, by female choice. Often results in very showy features: large tails, bright colors, impressive courtship dances and rituals! Competition is more “indirect.” Focus is on “ornaments”

Costs to a male Time and energy spent on a display Resources must be devoted to growing & maintaining fancy features Features may even be dangerous to the male: - attract predators -slow down escape responses Benefits to male Reproduction Avoid costly or dangerous direct fighting/confrontation

Intersexual selection: an example Male swordtail ‘Sword’ Hypothesis: Sword is a sexual signal- sword size equals male attractiveness.

Evolutionary benefits to female “Honest Advertisement” hypothesis: Large or attractive ornaments may be a sign of health (e.g. low parasite load) Highly fit individuals may compensate for large ornaments’ “bad” effects Female’s offspring should carry both the relative vigor, and mating advantages, passed on from father – “Sexy Son Hypothesis” Daughters should share Mom’s “preference” for fancy males as well.

“Runaway” sexual selection? When a system goes just a bit farther than you might expect… these are lance-tailed manakins

Lance-tailed manakin mating- what’s going on? Are they brothers or even, relatives? - No more than any randomly selected population members. Do betas get to “sneak” matings? - Not really- only 2% have Beta-dads Do betas become alphas later? - Yes but it’s not automatic – only 46% take over, the rest go do the “beta” dance for another alpha!

Apprenticeship hypothesis Betas may eventually learn needed skills to become alphas. This may benefit females a couple of ways – Betas that can learn from alphas may just be good learners = smart – Cooperation between males may reveal cooperative tendencies with females, possibly good parenting skills

Alternate Strategies for Males… Side-blotched lizards: males fight for territory and display for females

Alternate Strategies for Males… Mating is not always “winner take all” with one best strategy Yellow: female-like, “sneaky” Orange: Ultra-dominant, “aggressive” Blue: Mate-guarders “defensive” Sinervo & Lively 1996

Could this be beneficial? Sinervo & Lively 1996 The best strategy is always the rarest strategy!

In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population Selection can favor whichever phenotype is less common in a population The side-blotched lizards are an example of Frequency-Dependent Selection

Fig a “Right-mouthed” “Left-mouthed”

Fig “Right-mouthed” 1981 “Left-mouthed” Frequency of “left-mouthed” individuals Sample year ’82 ’83 ’84 ’85 ’86 ’87 ’88 ’89’90

Fig b 1981 Frequency of “left-mouthed” individuals Sample year ’82 ’83 ’84’85 ’86’87’88’89’90

Moral of the stories Mating systems, even weird ones, usually make sense in evolutionary context, when we ask the right questions about: - costs & benefits - resource acquisition & protection -genetics: parentage & diversity – Density-dependent effects