Extra credit question Sexual selection theory predicts that ___________ pipefish should compete with each other for access to mates, and that ___________ will be the choosy sex.

Darwin’s Problem If selection favors alleles that increase survival and reproduction, how does one explain conspicuous traits such as bright colors, horns, and displays of many species that seemingly lower survival of the bearers?

Darwin’s Solution: Sexual Selection Selection that arises from differences in mating success (# of mates that bear progeny over a defined time interval).

Natural Selection Ecological selection Sexual selection

Arises because one sex is a limiting resource for the other sex Sexual Selection Arises because one sex is a limiting resource for the other sex Q: How can one sex be a limiting resource for the other? Q: Which sex will be a limiting resource? Q: Can both sexes be a limiting resource?

Butterflies

Pipe fish Giant Water Bug Midwife Toad

Types of Sexual Selection Intrasexual Contests between members of one sex for access to members of the other sex

Intra-sexual selection

Types of Sexual Selection Intersexual Selection via mate choice: Individuals of one sex preferentially mate with individuals of the other sex based on particular phenotypes (size, color pattern, behavior, etc.)

Intersexual Selection

Sexually selected behavior http://tolweb.org/tree/eukaryotes/animals/arthropoda/arachnida/araneae/salticidae/++salticidae/movies/spiderdances.html

Artificial head feather in zebra finches

Why do these preferences exist? Direct Benefits Indirect Benefits Sensory Bias

Direct benefits: hanging flies and scorpion flies

(Thornhill 1976)

Direct benefits cannot explain mate choice when one sex provides nothing except gametes Indirect Benefits Models Good genes: females prefer traits that indicate genetic quality; high-quality mates pass on ‘good genes’ to their offspring. Runaway: females prefer arbitrary traits; preference for arbitrary traits leads to ‘runaway’ process.

Indirect Benefits: Good Genes Females prefer traits that indicate genetic quality; high-quality mates pass on ‘good genes’ to their offspring. Females choose based on some trait that indicates high fitness. Assumption: both trait and fitness must be heritable, (h2 > 0).

A test for “Good Genes” Fitness differences?

1995 1996 Fitness Component High Food Low Food Larval growth LC 1995 1996 Fitness Component High Food Low Food Larval growth LC Development time Mass at metamorphosis Larval survival Postmetamorphic growth

Fitness Component Larval growth LC Development time Mass at metamorphosis Larval survival

Indirect Benefits: Runaway Genotypes Females P1 (no preference) P2 (preference) Males T2 (no trait) T2 (trait) P2 females tend to mate with T2 males. P1 females mate randomly. Offspring of P2 females inherit both T2 & P2, ==> “genetic correlation” between preference & trait. T2 males have higher mating success than T1, so T2 increases in frequency. P2 increases in freq b/c it is co-inherited with T2. Elaboration (runaway) of trait & preference until mating advantage of trait outweighed by survival cost.

Sensory bias Preference for a trait may be a by-product of the sensory system that has adapted to be sensitive to certain kinds of signals for reasons that are unrelated to mating. Swordtails Guppies Zebra finch

Genus Xiphophorus Male swordtail Male platy

Natural Selection Ecological selection Sexual selection

Tradeoffs between natural and sexual selection