Transcript 4101SexualSelFemalex

SEXUAL CONFLICT
AND
SEXUAL SELECTION
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
(1728-1793)
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
(1731-1802)
Natural and Sexual Selection
Charles Darwin
1809 - 1882
Alfred Russel Wallace
1823 – 1913
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.
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. 1858. 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
Problems with Sexual Selection
- Traits involved are
- energetically wasteful
- overly elaborate
- contrary to ideas of natural selection
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
No sex
Sex but no males
(isogamy)
Sex with males
(anisogamy)
5 offspring each
100% relatedness
5 offspring each
50% relatedness
5 offspring
50% relatedness
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
Reproductive Behaviour
Benefits of sex
1. Produce new combinations
x
Unique offspring
can cope with
different niches
Reproductive Behaviour
Benefits of sex
2. Muller’s Ratchet
- in asexual species, deleterious mutations accumulate
A
A+B
Frequency of
mutant gene
Mutation A
Mutation B
A+B+C
B+C
Mutation C
C
Time
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
Darwin’s Dilemma
?
?
?
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, 1859. Origin of species
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, 1859. Origin of species
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.
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.
Reproductive Behaviour
Males and females have different reproductive interests
What does each sex have to provide to produce offspring?
Females
Males
1.
Manufacture eggs
(expensive)
1.
2.
Incubate eggs or fetus
(expensive)
3.
Post partum care
(expensive)
4.
Loss of mating opportunity
when pregnant
(expensive)
Manufacture sperm
(cheap)
Males invest less
in any offspring
Reproductive Behaviour
In more general terms
Females
Difference in reproductive investment
higher investment
Males
lower investment
lower reproductive rate
lower levels of mating activity
higher potential reproductive rate
higher levels of mating activity
Biased operational sex ratio
Selection among potential mates
Best mate = best fitness benefit
Competition for mates
Achieve greatest number of matings
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
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
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
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
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
OSR and strength of sexual selection
Increase
monopolization
OSR = 1.33
Mean = .75
Variance = 1.69
Is = 3.0
+1 Male
OSR = 1
Mean = 1
Variance = .67
Is = .67
Decrease
monopolization
OSR = 1.33
Mean = .75
Variance = .19
Is = 0.33
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
Intersexual Selection
Genetic Models for Mate Choice
1. Direct Benefits
-females choose males that give them a concrete
resource
e.g. Nuptial gifts
Intersexual Selection
Genetic Models for Mate Choice
1. Direct Benefits
Intersexual Selection
Genetic Models for Mate Choice
1. Direct Benefits
-females choose males that give them a concrete
resource
Crotolaria
Utethesia ornatrix
Intersexual Selection
Genetic Models for Mate Choice
1. Direct Benefits
-females choose males that give them a concrete
resource
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
Intersexual Selection
2. Good genes models (indirect
benefits)
Female should look for best genetic
complement in male
Traits should reflect
genetic quality
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
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
Red-collared widowbird
Collar functions
in territory defense
Tail is to
attract females
Intersexual Selection
2. Good genes models (indirect benefits)
Female should look for best genetic complement in male
Traits should reflect genetic quality
Intersexual Selection
Equilibrium
Female Preference
Female Preference
3. Runaway Selection
Male Character
Viability selection is strong
Male Character
Viability selection is weak
After Lande (1981)
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
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
Intersexual Selection
3. Runaway Selection
Example - stalk-eyed flies - Diopsidae
Variation in males
Female
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
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
Male
Strategies