Announcements EXAM II next Wednesday (March 14th) Review Monday (March 12th) No Class Friday (March 16th)

Why did ansiogamy evolve? Can lead to conflict among sexes (which will invest more). Bigger gametes (more parental investment - be choosy when selecting mates) More gametes (less parental investment - mate as many times as possible)

Sexual Selection occurs in two forms Intrasexual: Male-male competition Intersexual: Female choice Sexual selection: “the result of a non-random association between a (secondary sexual) character and a component of mating success." Møller (1994)

secondary sexual character: physical or behavioral characteristics that appear in sexually mature animals differentiate between the sexes without having a direct reproductive function (e.g, horns, color patterns, facial hair, breasts)

Some exaggerated ornaments or displays do not give a competitive advantage (in competition for mates) but are directly selected for by females. Many may even incur a cost to the male with the display!

Long tailed widowbirds

Pied flycatchers

Sexual selection through female choice… … what benefits do females get?

Evolution of female preferences for male traits: The Fisher Process - “runaway sexual selection” Indicator Mechanisms - of heath, good genes Sensory Bias - pre-existing perceptual mechanism Species Recognition

Evolution of female preferences for male traits: The Fisher Process - two step process 1.Trait starts by conferring an advantage (slightly longer tail = better flight) 2.Preference for trait that confers advantage evolves Females that mate with males with long tails produce offspring with advantage (longer tail) and with preference for trait! This leads to a “runaway process” in which the preference continues even though the exaggerated trait no longer confers a fitness advantage.

Evolution of female preferences for male traits: Indicator Mechanisms - of heath, good genes Handicap Principle (Zahavi) signals are expensive to produce, can be costly, are therefore accurately convey information about quality

Geoffrey Hill and colleagues showed that house finch color determined by carotenoids gotten from diet. Better foragers = brighter red.

evidence for healthy mate hypothesis

Marion Petrie 1994 evidence for good genes

The “Sexy-Son” Hypothesis - Weatherhead and Robertson (1979) Females choose males with particular traits because their male offspring will later be preferred (either because they out compete other males in a dominance hierarchy or are more attractive to females).

Current signal (context A) exploits a sensory system that evolved in a different context (for example foraging). Evidence - preference may precede evolution of a signal! Evolution of female preferences for male traits: Sensory Bias - pre-existing perceptual mechanism

“chase-away sexual selection”

What is preference surpasses the trait’s range? Use the Environment. Bower birds

What is preference surpasses the trait’s range? Bring in a “wing man”. In some animal societies, cooperation occurs in close-knit family groups and kin selection explains apparently selfless behavior. Not so for the lance-tailed manakin. Males of this little tropical bird cooperate in spectacular courtship displays with unrelated partners, and the benefits of lending a helping wing may only come years down the line. March 7, 2007

Additional reading: Sexual Selection Malte Andersson The Red Queen: Sex and the Evolution of Human Nature Matt Ridley Why is sex fun? The evolution of human sexuality Jared Diamnod