1. SEXUAL CONFLICT AND
SEXUAL SELECTION

2. Pre-Darwinian writings on Sex.
John Hunter
-distinguished primary sexual characteristics (those present at birth)
from
secondary sexual characteristics (those that appear with sexual maturity)
John Hunter
( )

3. Erasmus Darwin
Would it be too bold to imagine, that in the great length of time, since the earth began to exist, perhaps millions of ages before the commencement of the history of mankind, would it be too bold to imagine, that all warm-blooded animals have arisen from one living filament, which THE GREAT FIRST CAUSE endued with animality, with the power of acquiring new parts, attended with new propensities, directed by irritations, sensations, volitions, and associations; and thus possessing the faculty of continuing to improve by its own inherent activity, and of delivering down those improvements by generation to its posterity, world without end!
Erasmus Darwin
( )

4. Natural and Sexual Selection
Charles Darwin
Alfred Russel Wallace
1823 – 1913

5. The Ternate Letter
Ternate Island
   We believe we have now shown that there is a tendency in nature to the continued progression of certain classes of varieties further and further from the original type--a progression to which there appears no reason to assign any definite limits--and that the same principle which produces this result in a state of nature will also explain why domestic varieties have a tendency to revert to the original type. This progression, by minute steps, in various directions, but always checked and balanced by the necessary conditions, subject to which alone existence can be preserved, may, it is believed, be followed out so as to agree with all the phenomena presented by organized beings, their extinction and succession in past ages, and all the extraordinary modifications of form, instinct, and habits which they exhibit.

6. Besides this natural means of selection, by which those individuals are preserved, whether in their egg, or larval, or mature state, which are best adapted to the place they fill in nature, there is a second agency at work in most unisexual animals, tending to produce the same effect, namely, the struggle of the males for the females. These struggles are generally decided by the law of battle, but in the case of birds, apparently, by the charms of their song, by their beauty or their power of courtship, as in the dancing rock-thrush of Guiana. The most vigorous and healthy males, implying perfect adaptation, must generally gain the victory in their contests. This kind of selection, however, is less rigorous than the other; it does not require the death of the less successful, but gives to them fewer descendants. The struggle falls, moreover, at a time of year when food is generally abundant, and perhaps the effect chiefly produced would be the modification of the secondary sexual characters, which are not related to the power of obtaining food, or to defence from enemies, but to fighting with or rivalling other males. The result of this struggle amongst the males may be compared in some respects to that produced by those agriculturists who pay less attention to the careful selection of all their young animals, and more to the occasional use of a choice mate.
Darwin, C. & A. Wallace On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection. J. Proc. Linn. Soc. Lond. 3:45-50

7. Problems with Sexual Selection
- Traits involved are
- energetically wasteful
- overly elaborate
- contrary to ideas of natural selection

9. Costs of sex 1. Cost of meiosis
must combine genes with another organism
lose 50% every generation relative to asexuals
-often referred to as cost of males
- females can always do better asexually
Sex but no males (isogamy)
Sex with males (anisogamy)
No sex
5 offspring each
100% relatedness
5 offspring each
50% relatedness
5 offspring
50% relatedness

10. Reproductive Behaviour
Costs of sex
2. Cost of recombination
take a functioning genome, split it and combine it with another one
-chance of deleterious combinations

11. Reproductive Behaviour
Benefits of sex
1. Produce new combinations
Unique offspring can cope with different niches x

12. Reproductive Behaviour
Benefits of sex
2. Muller’s Ratchet
- in asexual species, deleterious mutations accumulate A
Frequency of mutant gene
A+B
Mutation A
A+B+C
Mutation B
B+C
Mutation C C
Time

13. Reproductive Behaviour
Benefits of sex
2. Muller’s ratchet
- in sexual species, such mutations tend to be eliminated
Frequency of mutant gene
Mutation A
Mutation B
Mutation C
Time

14. Darwin’s Dilemma ? ? ?

15. And this leads me to say a few words on what I call Sexual Selection
And this leads me to say a few words on what I call Sexual Selection. This depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring. Sexual selection is, therefore, less rigorous than natural selection. Generally, the most vigorous males, those which are best fitted for their places in nature, will leave most progeny.
Charles Darwin, Origin of species

16. Darwin - Theory of sexual selection
Two parts
1. Intrasexual selection (Male-male competition)
But in many cases, victory will depend not on general vigour, but on having special weapons, confined to the male sex.
The war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special weapons.
2. Intersexual selection (Female choice)
…but if man can in a short time give elegant carriage and beauty to his bantams, according to his standard of beauty, I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect.
Charles Darwin, Origin of species

17. Darwin - Theory of sexual selection
Two parts
1. Intrasexual selection (Male-male competition)
All those structures and behaviour patterns employed by males to fight other males for the chance to mate with females
- claws, antlers etc.

18. Darwin - Theory of sexual selection
Two parts
2. Intersexual selection (Female choice)
All those structures and behaviour patterns employed by males to attract females
- plumage, song.

19. Reproductive Behaviour
Males and females have different reproductive interests
What does each sex have to provide to produce offspring?
Females
Males
Manufacture eggs
(expensive)
Manufacture sperm
(cheap)
2. Incubate eggs or fetus
(expensive)
3. Post partum care
(expensive)
4. Loss of mating opportunity when pregnant
(expensive)
Males invest less in any offspring

20. Reproductive Behaviour
In more general terms
Females
Difference in reproductive investment
Males
higher investment
lower investment
lower reproductive rate
higher potential reproductive rate
lower levels of mating activity
higher levels of mating activity
Biased operational sex ratio
Selection among potential mates
Competition for mates
Best mate = best fitness benefit
Achieve greatest number of matings

21. Reproductive Behaviour
Operational vs Numerical Sex ratio
Numerical Sex Ratio
The ratio of the number of males to the number of females in the population
Operational Sex Ratio
The ratio of the number of males to the number of females in the population who are available for reproduction

22. Reproductive Behaviour
Operational vs Numerical Sex ratio
Numerical Sex Ratio (close to 1:1)
Males
Females
Operational Sex Ratio (very different from 1:1)
Competition in more abundant sex
Selectivity in less abundant sex

23. Reproductive Behaviour
Evidence??
Bateman (1948)
-Drosophila melanogaster - nearly all females mated
but not all males
-variance in male reproductive success - higher
Males
Bateman gradient
# of offspring
Females
# of mates

24. Reproductive Behaviour
Maximum # offspring produced (lifetime)
Species
Male
Female
Ratio
Elephant seal
100 8
12.5
Red deer 24 14
1.7
Human
888 69
12.9
Kittiwake gull 26 28
0.96

25. (usually male-biased)
OSR and strength of sexual selection
Is = measure of strength of directional sexual selection
= (coefficient of variation in mating success)2
Assumption: Is determined by number of individuals available for mating
(usually male-biased)
Also need to know the ability of males to monopolize mates

26. OSR and strength of sexual selection
Mean = .75
Variance = 1.69
Is = 3.0
Increase monopolization
+1 Male
OSR = 1.33
Mean = .75
Variance = .19
Is = 0.33
OSR = 1
Mean = 1
Variance = .67
Is = .67
Decrease monopolization

27. Intersexual Selection
Why are females choosy? ✓
How is this choosiness exhibited?
Genetic models for mate choice
1. Direct Benefits
2. Good Genes
3. Runaway Selection

28. Intersexual Selection
Genetic Models for Mate Choice
1. Direct Benefits
-females choose males that give them a concrete
resource
e.g. Nuptial gifts

29. Intersexual Selection
Genetic Models for Mate Choice
1. Direct Benefits

30. Intersexual Selection
Genetic Models for Mate Choice
1. Direct Benefits
-females choose males that give them a concrete
resource
Crotolaria
Utethesia ornatrix

31. Intersexual Selection
Genetic Models for Mate Choice
1. Direct Benefits
-females choose males that give them a concrete
resource

32. Intersexual Selection
Alkaloid retained on molt to adult male
Larva ingests alkaloid
Some alkaloid converted to male pheromone
Attracts females
Mating
Females mate with males will more pheromone
Greater amount of alkaloid transferred
Greater protection for eggs

33. Intersexual Selection
2. Good genes models (indirect benefits)
Female should look for best genetic complement in male
Traits should reflect genetic quality

34. Intersexual Selection
2. Good genes models (indirect benefits)
Female should look for best genetic complement in male
Traits should reflect genetic quality
Cost to males
Year 1
-mated faster
-preferred by females seeking EPC’s
-poorer foragers
-caught small prey
Year 2
-shorter tails
-poor quality plumage
-lower mating success

35. Intersexual Selection
2. Good genes models (indirect benefits)
Female should look for best genetic complement in male
Red bishop
Traits should reflect genetic quality
Cost to males
Collar functions
in territory defense
Red-collared widowbird
Tail is to
attract females

36. Intersexual Selection
2. Good genes models (indirect benefits)
Female should look for best genetic complement in male
Traits should reflect genetic quality

37. Intersexual Selection
Equilibrium
3. Runaway Selection
Female Preference
Female Preference
Male Character
Male Character
Viability selection is strong
Viability selection is weak
After Lande (1981)

38. Intersexual Selection
3. Runaway Selection
Assume 2 genes
Gene A - Trait ‘X’ in males - more colourful feathers
Gene B - In females - preference for trait ‘X’
A+ B
Expressed in males as the trait
A+ B
Expressed in females as a preference for the trait

39. Intersexual Selection
3. Runaway Selection
Assume 2 genes
Gene A - Trait ‘X’ in males - more colourful feathers
Gene B - In females - preference for trait ‘X’
Frequency
in males
Frequency in females

40. Intersexual Selection
3. Runaway Selection
Example - stalk-eyed flies - Diopsidae
Variation in males
Female

41. Intersexual Selection
3. Runaway Selection
Example - stalk-eyed flies - Diopsidae
Two experiments
1. Standard disruptive selection
every generation - allow shortest and longest stalked
males to mate with randomly selected females
Short stalked
Long stalked
13 generations
Stalk length
Stalk length

42. Intersexual Selection
3. Runaway Selection
Example - stalk-eyed flies - Diopsidae
Two experiments
2. Female choice
Female offspring prefer
short stalked males
Female offspring prefer
long stalked males
Stalk length

43. Male Strategies