1. BIOE 109
Summer 2009
Lecture 9- Part I
Sexual selection

2. Sexual dimorphism is very common in nature

3. What is sexual selection?

4. What is sexual selection?
Natural Selection:
Differential reproductive success due to variation among individuals in survival and reproduction.

5. What is sexual selection?
Darwin (1871) defined sexual selection as:
Differential reproductive success due to variation among individuals in success at getting mates.

6. What is sexual selection?
Darwin (1871) defined sexual selection as:
Differential reproductive success due to variation among individuals in success at getting mates.
• sexual selection refers to one component of fitness: mating success.
Fitness = Viability +Fecundity +Longevity +Mating success

7. What is sexual selection?
What is sexual selection?
 Fitness = Viability +Fecundity +Longevity +Mating success

8. What is sexual selection?
What is sexual selection?
 Fitness = Viability +Fecundity +Longevity +Mating success
• “trade-offs” between fitness components may occur!

9. What is sexual selection?
What is sexual selection?
 Fitness = Viability +Fecundity +Longevity +Mating success
• “trade-offs” between fitness components may occur!
• a trait improving mating success may increase total
fitness yet reduce viability!

10. If sexual selection is indeed the explanation for sexual dimorphism, then it will have to act on sexes differently…..
And it does!
Sexual reproduction creates different selection pressures for males and females.
How?   

11. If sexual selection is indeed the explanation for sexual dimorphism, then it will have to act on sexes differently…..
And it does!
Sexual reproduction creates different selection pressures for males and females.
• Through asymmetric parental investment! 

12. What is parental investment?
Energy and time expended in either or both:
-constructing an offspring
-caring for it.

13. What is parental investment?
Energy and time expended in either or both:
-constructing an offspring
-caring for it.
Producing eggs (or pregnancies) are more expensive than ejaculates.

14. What is parental investment?
Energy and time expended in either or both:
-constructing an offspring
-caring for it.
Producing eggs (or pregnancies) are more expensive than ejaculates.
In more than 90% of mammal species, females provide substantial parental care and males provide little to none.

15. What acts to limit the lifetime reproductive success of males and females?
females: limited by no. of eggs and/or pregnancies

16. What acts to limit the lifetime reproductive success of males and females?
females: limited by no. of eggs and/or pregnancies
males: limited by no. of females mated

17. What acts to limit the lifetime reproductive success of males and females?
females: limited by no. of eggs and/or pregnancies
males: limited by no. of females mated (access to mates)
 This sets up a conflict = sexual selection.
To understand sexual selection we must quantify the relationship between number of mates and reproductive success for both males and females

18. Sexual selection in rough-skinned newts

19. Sexual selection in rough-skinned newts

20. Sexual selection in rough-skinned newts
Access to females increases reproductive success in males

21. What does this mean?
Heritable traits associated with mating success will become common in males

22. What does this mean?
Heritable traits associated with mating success will become common in males
tall crests
that appear
during
breeding
season

23. What does this mean?
Heritable traits associated with mating success will become common in males
Heritable traits that are not associated with mating success will tend to disappear from the population

24. Sexual selection in pipefish

25. A baby pipefish emerging from Dad’s brood pouch

26. Sexual selection in pipefish

27. Based on Parental Investment:
Predictions:
Based on Parental Investment:
females: limited by no. of eggs and/or pregnancies
males: limited by no. of mates
Members of the sex subject to strong sexual selection will be competitive.
Members of the sex subject to weaker sexual selection will be choosy.  

28. Predictions:
1. Males should be competitive

29. Predictions: 1. Males should be competitive
1. Males should be competitive
• they should compete among themselves for access to females.

30. Predictions: 1. Males should be competitive
1. Males should be competitive
• they should compete among themselves for access to females.
• this is “male-male competition” or intrasexual selection.

31. Predictions: 1. Males should be competitive
1. Males should be competitive
• they should compete among themselves for access to females.
• this is “male-male competition” or intrasexual selection.
2. Females should be choosy

32. Predictions: 1. Males should be competitive
1. Males should be competitive
• they should compete among themselves for access to females.
• this is “male-male competition” or intrasexual selection.
2. Females should be choosy
• since her investment is larger, she has more to lose by making a bad decision.

33. Predictions: 1. Males should be competitive
1. Males should be competitive
• they should compete among themselves for access to females.
• this is “male-male competition” or intrasexual selection.
2. Females should be choosy
• since her investment is larger, she has more to lose by making a bad decision.
• this is “female choice” or intersexual selection.

34. Intrasexual selection
 (male-male competition)

35. Intrasexual selection
• occurs when individual males can monopolize access to females.
-Fight for control of mates and/or control of resources vital to mates

36. Intrasexual selection
• occurs when individual males can monopolize access to females.
-Fight for control of mates and/or control of resources vital to mates
Types of intra-sexual selection:
Combat
Sperm competition
Infanticide
Sneaky strategy

37. Intrasexual selection
• occurs when individual males can monopolize access to females.
1. Combat
• leads to sexual dimorphism in size

38. Intrasexual selection
• occurs when individual males can monopolize access to females.
1. Combat
• leads to sexual dimorphism in size
Example: northern elephant seal.
♂ ~ 4,000 lbs
♀ ~1500 lbs

39. Intrasexual selection
1. Combat
• leads to sexual dimorphism in size
Example: northern elephant seal.
• can also lead to the evolution of weaponry

40. Intrasexual selection
1. Combat
• leads to sexual dimorphism in size
Example: northern elephant seal.
• can also lead to the evolution of weaponry
Example: horns in ungulates and beetles.

41. Intrasexual selection
2. Sperm competition

42. Intrasexual selection
2. Sperm competition
• If a female mates with two or more males, the male whose sperm win the race to the eggs has higher reproductive success.

43. Intrasexual selection
2. Sperm competition
• If a female mates with two or more males, the male whose sperm win the race to the eggs has higher reproductive success.
Ex. of traits needed to be successful
Large ejaculates
More and longer mating occurrences

44. Intrasexual selection
2. Sperm competition
• If a female mates with two or more males, the male whose sperm win the race to the eggs has higher reproductive success.
Ex. of traits needed to be successful
Large ejaculates
More and longer mating occurrences
Other examples include Sperm plugs, scooping out sperm, prolonged copulation, guarding of mate, applying hormones that reduce female’s attractiveness to other males.

45. Sperm competition in damselflies
barbed horns on penis

46. Intrasexual selection
3. Infanticide
Example: the African lion

47. Intrasexual selection
4. Alternative male reproductive strategies.
Example: Sneaky strategy in Pacific salmon.
Female
Hooknoses (male- 18mon)
Jacks (male-6mon)

48. Intersexual selection
 (female choice)

49. Intersexual selection
• occurs when males “advertise” for mates and females choose among different males.
Elaborate courtship displays: singing, dancing, or showing off bright color
leads to sexual dimorphism in ornate features

50. Intersexual selection: dancing…...

51. Intersexual selection singing……

52. Intersexual selection
• occurs when males “advertise” for mates and females choose among different males.
Elaborate courtship displays: singing, dancing, or showing off bright color
leads to sexual dimorphism in ornate features
Types of intersexual selection:
1. Direct Benefits
-Acquisition of resources
-Good genes
2. Pre-sensory Bias
3. Runaway selection

53. Intersexual selection
1. Direct benefits- Acquisition of resources
• females directly benefit from choosing certain males.
Example: the common tern

54. Intersexual selection
1. Direct benefits- Acquisition of resources
• females directly benefit from choosing certain males.
Example: the common tern
• females show a strong preference for males that bring a lot of food.

55. Intersexual selection
1. Direct benefits- Acquisition of resources
• females directly benefit from choosing certain males.
Example: the common tern
• females show a strong preference for males that bring a lot of food.
• the amount of food a male brings during courtship is strongly correlated with the amount he brings to feed young.

56. 1. Direct Benefits- Good genes

57. 1. Direct Benefits- Good genes
Example: call length of male gray tree frog

58. 1. Direct Benefits- Good genes
Gerhardt et al. 1996

59. 1. Direct Benefits- Good genes
• females choose males with certain traits because they are honest “indicators” of overall genetic quality.

60. 1. Direct Benefits- Good genes
• females choose males with certain traits because they are honest “indicators” of overall genetic quality.
-calling in frogs
-calling in birds
-plumage color

61. 2. Sensory bias

62. 2. Sensory bias
• predicts that female preference for certain male traits evolves prior to the appearance of the male trait.

63. 2. Sensory bias
• predicts that female preference for certain male traits evolves prior to the appearance of the male trait.
• females have a pre-existing sensory bias for the trait and males exploit this preference.

64. 2. Sensory bias
• predicts that female preference for certain male traits evolves prior to the appearance of the male trait.
• females have a pre-existing sensory bias for the trait and males exploit this preference.
Example: Water mite (Neumania papillator)

65. Water mite (Neumania papillator)

66. 3. Runaway selection

67. 3. Runaway selection
• results from a genetic correlation between a male trait and female preference for that trait.

68. 3. Runaway selection
• results from a genetic correlation between a male trait and female preference for that trait.
Components needed:
-Assortative mating
-Heritable trait

69. 3. Runaway selection
• results from a genetic correlation between a male trait and female preference for that trait.
• the simplest model assumes two genetic loci in linkage disequilibrium:

70. 3. Runaway selection
• results from a genetic correlation between a male trait and female preference for that trait.
• the simplest model assumes two genetic loci in linkage disequilibrium:
a female preference gene (P locus) for the male trait.

71. 3. Runaway selection
• results from a genetic correlation between a male trait and female preference for that trait.
• the simplest model assumes two genetic loci in linkage disequilibrium:
1. a female preference gene (P locus) for the male trait.
2. a gene for the male trait (T locus).

72. 3. Runaway selection
• results from a genetic correlation between a male trait and female preference for that trait.
• the simplest model assumes two genetic loci in linkage disequilibrium:
1. a female preference gene (P locus) for the male trait.
2. a gene for the male trait (T locus).
P T

73. Example: stalk-eyed flies

74. Example: stalk-eyed flies
P T
Preference Eye-stalk
for long length
eye-stalks

75.  Example: stalk-eyed flies P T mutation increasing
Preference Eye-stalk
for long length
eye-stalks 
mutation increasing
♂ eye-stalk length…

76.   Example: stalk-eyed flies P T …will be quickly driven
Preference Eye-stalk
for long length
eye-stalks
 
…will be quickly driven
to fixation by ♀ choice
mutation increasing
♂ eye-stalk length…

77.   … resulting in continued exaggeration of ♂ trait
Example: stalk-eyed flies
P T
Preference Eye-stalk
for long length
eye-stalks
 
…will be quickly driven
to fixation by ♀ choice
mutation increasing
♂ eye-stalk length…
… resulting in continued exaggeration of ♂ trait

78. Runaway selection  ♀ preference for long eye-stalks Elongation of

79. LIMITS to Runaway selection:

80. Recap Sexual selection explains sexual dimorphism in nature
Asymmetry in parental investment leads to differential selection
pressures in males and females (sexual selection)
Sex that invests less is more “competitive”, sex that invests more
is “choosy”
Sexual selection is more potent force of evolution in males than
in females (with few exceptions like pipe-fishes)
Intersexual selection (male-male competition): leads to combat, sperm
Competition, infanticides, alternative male strategies
Intrasexual selection (female choice): due to direct benefits like
resources, good genes etc., female sensory bias, runaway selection

81. Interesting videos http://www.youtube.com/watch?v=GPbWJPsBPdA&NR=1
For nest decoration of a Australian Bowerbird