1. 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.

2. 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?

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

5. Natural Selection
Ecological selection
Sexual selection

6. 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?

7. Butterflies

10. Pipe fish
Giant Water Bug
Midwife Toad

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

12. Intra-sexual selection

13. 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.)

14. Intersexual Selection

15. Sexually selected behavior

16. Artificial head feather in zebra finches

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

18. Direct benefits: hanging flies and scorpion flies

19. (Thornhill 1976)

20. 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.

21. 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).

22. A test for
“Good Genes”
Fitness differences?

23. 1995 1996 Fitness Component High Food Low Food Larval growth LC
Fitness Component
High Food
Low Food
Larval growth LC
Development time
Mass at metamorphosis
Larval survival
Postmetamorphic growth

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

25. 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.

27. 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

28. Genus Xiphophorus
Male swordtail Male platy

29. Natural Selection
Ecological selection
Sexual selection

30. Tradeoffs between natural and sexual selection