Sexual selection, a brief review Some basic principles. Some examples. Difference between sexual and natural selection. Sex role reversal. Video -Why Sex (Evolution Series) Hey, look at me!!

Leads to: Mate Competition (male-male, for access to females) Mate Choice (female choice, for best quality males) In general, success at mating: For a male, is limited by the number of females he can convince to mate with him. For a female, is limited by the number of eggs/pregnancies she can produce.

Anisogamy = having gametes of very different size Small gamete is the male gamete (sperm). Large gamete is the female gamete (egg). Begins with difference in gamete size: Sperm are very small (“inexpensive”) Eggs are large and nutrient rich (“expensive”).

Eggs (or pregnancies) are more “expensive” than ejaculates. Sperm are very small (“inexpensive”) Eggs are large and nutrient rich (“expensive”). So, females INVEST more in gametes, and have more investment in each offspring compared to males; Asymmetry in sexual reproduction established… What effects does this have on mating behavior?

Males: fertilize as many eggs as possible (maximize number of matings). Females: make as many high-quality offspring as possible (maximize quality of each offspring through mate choice). Mate competition (usually male-male for access to females) Mate Choice (usually female choice among a selection of males)

How is sexual selection different from natural selection? For both sexes, reproductive success (number of offspring produced in a lifetime) is determined by process of natural selection (success at survival and reproduction).

Puzzle of sexual dimorphism (differences in males and females in appearance or behavior) Darwin “Sight of a peacock makes me sick.”

Sexual selection is a subset of natural selection that has to do with mating success. If there is variation in a trait that affects ability to mate successfully, and that trait is heritable, then individuals with that trait will become more common over time.

Sexual selection is a subset of natural selection that has to do with mating success. If there is variation in a trait that affects ability to mate successfully, and that trait is heritable, then individuals with that trait will become more common over time. What are some of these “sexually-selected traits?

Leads to: Mate Competition (male-male, for access to females) Mate Choice (female choice, for best quality males) In general, success at mating: For a male, is limited by the number of females he can convince to mate with him. For a female, is limited by the number of eggs/pregnancies she can produce.

Some species show sex-role reversal, then: Females compete for males, and Males are the “choosy” sex. When would you predict this would occur?

When would you predict sex-role reversal? When males invest more than females in offspring. (Examples: seahorses, giant water bugs, wattled jacanas) Happens under unusual ecological conditions.

Why Sex?Video Why do males and females look so different/ Why do male peacocks have that ridiculous tail? Why are so many birds monogamous (but also fooling around)? Why is there sex-role reversal in some species (jacanas)? Evolutionary psychology: how can we explain human mate choice (what is beauty, do we smell our mates)? Does sex ultimately explain art, music and humor?

What limited reproductive success for male peacocks? What limited reproductive success for female peacocks?

Why is monogamy common among many birds, such as the forest birds that Emlen studied in the video? Female promiscuity is also favored in these birds. What was the evidence for this, and how is it that a monogamous species is also promiscuous?

Why do males care for young in the jacana? Why do females have harems of males in the jacana?

What do you think of the studies done by the evolutionary psychologists on human mating preferences?