1. BIOE 109
Summer 2009
Lecture 10- part I
Mating systems

2. Types of Mating Systems
Mode
Parental care
Monogamy
One male and one female form bond
both
Polygyny
Male mates with multiple females
female
Polyandry
Female mates with multiple males
male
Promiscuity
Both sexes mate with multiple partners
Both, either, or neither

3. Mating Systems In Nature

4. Mating Systems In Nature
Monogamy
Monogamy
Monogamy
Polygyny
Polyandry
Polygyny

5. Mating Systems In Nature?

6. Mating Systems In Nature
Promiscuous

7. Mating Systems In Nature
Promiscuous

8. Hypothesis for the evolution of mating systems
Based on parental care and ecological constraints

9. Hypothesis for the evolution of mating systems
Based on parental care and ecological constraints
Who can ditch first?

10. Hypothesis for the evolution of mating systems
Based on parental care and ecological constraints
Who can ditch first?
Is ditching worth it?

11. Sex allocation

12. Sex allocation
the allocation of resources to male versus female production in sexual species (Charnov 1982).
Sex Ratio?

13. What is sex ratio?
• sex ratio is defined as the proportion of males to females.

14. What is sex ratio?
• sex ratio is defined as the proportion of males to females.
• two distinct sex ratios exist:

15. What is sex ratio?
• sex ratio is defined as the proportion of males to females.
• two distinct sex ratios exist:
1. the population sex ratio
i.e., the proportion of males to females in the population

16. What is sex ratio?
• sex ratio is defined as the proportion of males to females.
• two distinct sex ratios exist:
1. the population sex ratio
i.e., the proportion of males to females in the population
2. the individual sex ratio

17. What is sex ratio?
• sex ratio is defined as the proportion of males to females.
• two distinct sex ratios exist:
1. the population sex ratio
i.e., the proportion of males to females in the population
2. the individual sex ratio
i.e., the sex ratio of progeny from a female

18. The evolution of sex ratio
-In many species sex chromosomes cause 1:1 sex ratio

19. The evolution of sex ratio
Mammals: females are homogametic (XX)
males are heterogametic (XY)
 Birds: males are homogametic (ZZ)
females are heterogametic (WZ)

20. Mammals: females are homogametic (XX)
males are heterogametic (XY)
 Birds: males are homogametic (ZZ)
females are heterogametic (WZ)
 Sex chromosomes do not guarantee a 1:1
sex ratio!

21. Why equal numbers of males and females?
• R.A. Fisher (1930) provided a genetic explanation for the evolution of a stable sex ratio of 1:1.

22. Why equal numbers of males and females?
• R.A. Fisher (1930) provided a genetic explanation for the evolution of a stable sex ratio of 1:1.
• since every individual has one mother and one father, each sex contributes equally, on average, to subsequent generations.

23. Why equal numbers of males and females?
• R.A. Fisher (1930) provided a genetic explanation for the evolution of a stable sex ratio of 1:1.
• since every individual has one mother and one father, each sex contributes equally, on average, to subsequent generations.
• therefore, males and females must have the same average fitness.

24. Suppose:
25% male males will have high fitness
75% female because they mate with multiple females
75% male females will have high fitness
25% female because they mate with multiple males
Members of the rarer sex will experience increased reproductive success relative to common sex
frequency-dependent selection results in stable equilibrium sex ratio of 1:1.

25. Exceptions to Fisher’s theory
NOT ALWAYS 1:1

26. Exceptions to Fisher’s theory
Local mate competition (Hamilton 1967)
2. Condition-dependent sex allocation (Trivers and Willard 1973)

27. Exceptions to Fisher’s theory
1. Local mate competition (Hamilton 1967)
• proposed to account for female-biased sex ratios (e.g., parasitoid wasps).

28. Exceptions to Fisher’s theory
1. Local mate competition (Hamilton 1967)
• proposed to account for female-biased sex ratios (e.g. parasitoid wasps).
• here, a single foundress produces a small group of closely related individuals that mate among themselves. 

29. Exceptions to Fisher’s theory
1. Local mate competition (Hamilton 1967)
• proposed to account for female-biased sex ratios (e.g. parasitoid wasps).
• here, a single foundress produces a small group of closely related individuals that mate among themselves.  
• females invest heavily in daughters and don’t “waste” effort in producing sons.

30. Exceptions to Fisher’s theory
1. Local mate competition (Hamilton 1967)
Mother
Male
1 son to 20 daughters
Females
Dust mites (Acarophenox)

31. Exceptions to Fisher’s theory
2. Condition-dependent sex allocation (Trivers and Willard 1973)
Red deer, Cervus elaphus

32. Exceptions to Fisher’s theory
2. Condition-dependent sex allocation (Trivers and Willard 1973)
• occurs in polygynous species when females invest heavily in producing and caring for their young.

33. Exceptions to Fisher’s theory
2. Condition-dependent sex allocation (Trivers and Willard 1973)
• occurs in polygynous species when females invest heavily in producing and caring for their young.
 • a good mother can produce larger, or healthier, individuals when they mature.

34. Exceptions to Fisher’s theory
2. Condition-dependent sex allocation (Trivers and Willard 1973)
• occurs in polygynous species when females invest heavily in producing and caring for their young.
 • a good mother can produce larger, or healthier, individuals when they mature.
• theory predicts that females in extremely good condition should produce males.

35. Exceptions to Fisher’s theory
2. Condition-dependent sex allocation (Trivers and Willard 1973)
• occurs in polygynous species when females invest heavily in producing and caring for their young.
 • a good mother can produce larger, or healthier, individuals when they mature.
• theory predicts that females in extremely good condition should produce males.
• Why?

36. Exceptions to Fisher’s theory
2. Condition-dependent sex allocation (Trivers and Willard 1973)
• occurs in polygynous species when females invest heavily in producing and caring for their young.
 • a good mother can produce larger, or healthier, individuals when they mature.
• theory predicts that females in extremely good condition should produce males.
• Why? Because sexual selection (usually) occurs more strongly in males and condition matters!

37. How is sex ratio adjusted by mother?
Not known

38. Sex Allocation Recap Sex ratio Why we see an unbiased sex ratio
Sex chromosomes
Frequency dependent selection
Exceptions to sex ratio:
Local mate competition
Condition-dependent sex allocation

39. Sex in Plants

40. Sex in Plants ♂ ♀

41. Sex in Plants
Why and how do they outbreed?
Why do they inbreed?

42. The evolution of inbreeding and outbreeding
• many plant species have evolved traits to avoid inbreeding.

43. The evolution of inbreeding and outbreeding
• many plant species have evolved traits to avoid inbreeding.
1. Asynchronous male and female functions
• pollen shed after or before plant’s stigmas are receptive.

44. The evolution of inbreeding and outbreeding
• many plant species have evolved traits to avoid inbreeding.
1. Asynchronous male and female functions
• pollen shed after or before plant’s stigmas are receptive.

45. The evolution of inbreeding and outbreeding
• many plant species have evolved traits to avoid inbreeding.
1. Asynchronous male and female functions
• pollen shed after or before plant’s stigmas are receptive.
2. Monoecy
• male and female flowers separated on same plant.

46. The evolution of inbreeding and outbreeding
 2. Monoecy
• male and female flowers separated on same plant.

47. The evolution of inbreeding and outbreeding
• many plant species have evolved traits to avoid inbreeding.
1. Asynchronous male and female functions
• pollen shed after or before plant’s stigmas are receptive.
2. Monoecious
• male and female flowers separated on same plant.
3. Dieocy
• sexes are separated in different individuals.

48. The evolution of inbreeding and outbreeding
3. Dieocy
• sexes are separated in different individuals.

49. The evolution of inbreeding and outbreeding
4. Self-incompatibility loci
• prevent selfing or breeding with close relatives.

50. The evolution of inbreeding and outbreeding
4. Self-incompatibility loci
• prevent selfing or breeding with close relatives.
5. Heterostyly
• two (distyly) or three (tristyly) forms of flowers exist in a species (on different plants).

51. The evolution of inbreeding and outbreeding
4. Self-incompatibility loci
• prevent selfing or breeding with close relatives.
5. Heterostyly
• two (distyly) or three (tristyly) forms of flowers exist in a species (on different plants).
• pollen is more effectively transferred between, rather than within, morphs.

52. The evolution of inbreeding and outbreeding
4. Self-incompatibility loci
• prevent selfing or breeding with close relatives.
5. Heterostyly
• two (distyly) or three (tristyly) forms of flowers exist in a species (on different plants).
• pollen is more effectively transferred between, rather than within, morphs.
• acts to maximize outcrossing.

53. Heterostyly in Primula polynera
Thrum flowered Primula polyneura
Pin flowered Primula polyneura

54. Why inbreed?

55. Why inbreed?
• partial selfers have a fitness advantage over exclusive outcrossers because genes can be transmitted:

56. Why inbreed?
• partial selfers have a fitness advantage over exclusive outcrossers because genes can be transmitted:
1. through its ovules

57. Why inbreed?
• partial selfers have a fitness advantage over exclusive outcrossers because genes can be transmitted:
1. through its ovules
2. through its pollen by selfing

58. Why inbreed?
• partial selfers have a fitness advantage over exclusive outcrossers because genes can be transmitted:
1. through its ovules
2. through its pollen by selfing
3. through its pollen by outcrossing.

59. Why inbreed?
• partial selfers have a fitness advantage over exclusive outcrossers because genes can be transmitted:
1. through its ovules
2. through its pollen by selfing
3. through its pollen by outcrossing.
• another advantage of selfing is reproductive assurance

60. Why inbreed?
• partial selfers have a fitness advantage over exclusive outcrossers because genes can be transmitted:
1. through its ovules
2. through its pollen by selfing
3. through its pollen by outcrossing.
• another advantage of selfing is reproductive assurance
• if pollinators are scarce, then a plant can produce at least some seeds by selfing.

61. OVERALL Mating Systems Sex allocation Sex ratios
Exceptions to 1:1 sex ratio
Mating in plants