1. “The sight of a feather in a peacock’s tail,
whenever I gaze at it, makes me sick.”
--Charles Darwin

3. Sexual Selection
Natural selection ---> viability or fecundity
Sexual selection ---> mating success, fertilization success
Only traits which affect courtship and mating
Occurs when all individuals do not have equal probability of mating
Usually stronger on males because:
1) greater female investment in young
2) single mating sufficient to fertilize
female’s clutch

4. Taricha salamanders

5. Sexual Selection
Natural selection ---> viability or fecundity
Sexual selection ---> mating success, fertilization success
only traits which affect courtship and mating
Occurs when all individuals do not have equal probability of mating
Usually stronger on males because:
1) greater female investment in young
2) single mating sufficient to fertilize
female’s clutch
Usually greater variance in male reproductive success, often related
to age, size

6. Red Deer, Cervus elaphus
Reproductive Success Hinds Stags
Reproductive lifespan 27% 7%
Fecundity/Mating success
Offspring survival

7. Northern elephant seal
#pups sired
and weaned
#pups
weaned

8. Selection asymmetries lead to
Competitive males
Choosy females
[Assumes: sperm are cheap; Females can determine the ‘best’ male]
Two mechanisms:
Male-male competition
Female choice
Sexual selection shapes variation in traits much like natural selection, although it may frequently conflict with natural selection
Different types of selection may act at different stages in the
life cycle

9. Sexual selection can force a trait away from the natural selection optimum

11. Male-male competition
Winners: bigger, more heavily armed, more vigorous
Horns, antlers -- ungulates
“Horns” -- beetles
Song/display, territory defense -- birds
Body size (males >>> females) -- mammals
Read the example from your text: marine iguanas pp

12. Males Females

13. Male-male competition
winners: bigger, more heavily armed, more vigorous
horns, antlers -- ungulates
“horns” -- beetles
song/display, territory defense -- birds
body size (males >>> females) -- mammals
Read the example from your text: marine iguanas pp ,
including box on alternate male strategies

15. Male-male competition
Sperm competition
Rapid divergent evolution of male genitalia in animals with
internal fertilization
Arnqvist Nature 393:784
“Lock-and-key” hypothesis -- selection against inferior hybrids
Sexual selection hypothesis -- post-insemination % competition
Compare related clades of insects that differ in the
mating frequency of females (Single vs. Multiple)
Morphometric distance ratio: (interspecific divergence in
morphology within a polyandrous clade) / (interspecific
divergence in morphology within a related single
mating clade) H0: MR=1

16. (Arnqvist 1998)

17. Genitalia are significantly more divergent (MR = 2.19, p < 0.001)
than other traits (MR = 0.72, p > 0.38)

18. Male-male competition
Sperm competition
longer copulation duration
transfer more sperm
displace previous males’ sperm

19. Fertilization success increase with copulation duration in Scatophaga stercoria
Simmons 2001

20. Male-male competition
Sperm competition
longer copulation duration
transfer more sperm
displace previous males’ sperm
sperm morphology
anucleate (apyrene) sperm in Lepidoptera

21. Drosophila bifurca
58mm

22. Sperm competition in Pieris napi Cook and Wedell 1999 Nature 397:486
mate virgin females to either 1) virgin males or 2) recently mated males
allow females the opportunity to remate for 10 consecutive days
pr(remating) affected by amount of apyrene sperm
eupyrene
apyrene

23. Accessory gland proteins reduce female survival in Drosophila

24. Female Choice
Females prefer males with elaborate ornaments
Moller 1994 female choice in barn swallows
-- correlation between male phenotype and male
mating success
-- manipulation of male trait ---> effect on male
reproductive success

26. Female choice
Direct benefits
Increased viability +/or fecundity
Territory quality -- Rana catesbiana

28. Female choice
Direct benefits
Increased viability +/or fecundity
territory quality -- Rana catesbiana
“nuptial gift” -- insects

30. Female choice
Direct benefits
Increased viability +/or fecundity
territory quality -- Rana catesbiana
“nuptial gift” -- insects
parental care -- birds, insects
Decreased probability of disease transmission

32. Female choice
Indirect benefits -- ‘good genes’
Higher quality offspring
Male offspring with higher mating success

34. Tail length is a
costly trait

35. Parus major
Females prefer males
with a large breaststripe
Breaststripe size has a
heritability of ~0.7

36. breaststripe size is
strongly positively
correlated with
offspring viability

37. Female choice
Sensory bias -- females have an existing preference due to
a pleiotropic sensory system
Females prefer males that elicit the greatest amount of stimulation
from the female’s sensory system
example: Anolis lizards
frequently, ‘sit-and-wait’ predators
respond to small movements by prey
male courtship displays– “pushups”
inflate colored throat sac

38. grackle
zebra finch

47. Sexual selection acts on traits which affect mating and
fertilization success
Shapes phenotypic variation in traits much like natural selection
but may frequently conflict with natural selection
Operates via: male-male competition, including sperm competition
mate choice by females
Females may be choosy because of:
- direct benefits
- indirect benefits
- sensory bias
Correlation between female preference and male phenotype may
cause the male trait to evolve to phenotypic extremes