Transcript Sexual Selection and Reproductive Behaviour

Sexual Selection and
Reproductive Behaviour
Sex is the queen of problems in evolutionary biology. Perhaps no other
natural phenomenon has aroused so much interest; certainly none has
sowed as much confusion. The insights of Darwin and Mendel, which
have illuminated so many mysteries, have so far failed to shed more
than a dim and wavering light on the central mystery of sexuality,
emphasizing its obscurity by its very isolation.
Graham Bell
The Masterpiece of Nature: The Evolution of Genetics and Sexuality
Sex vs. Reproduction
Sex - process by which a genome is changed by
rearrangement and combining of genes
Reproduction - process by which individuals are added to a
population
Sex -changes in kinds of individuals in populations
Reproduction -changes in the number of individuals in populations
Reproductive Behaviour
1. What is ‘sex’?
2. What are some of the costs and benefits of sex?
3. Why are the sexes generally in a 1:1 ratio?
Reproductive Behaviour
What is the one feature that serves to separate male from female in all animals?
Anisogamy
Anisogamy (heterogamy) - a form of sexual reproduction
involving the union or fusion of two dissimilar gametes
Reproductive Behaviour
Anisogamy – how did it come about?
Reproductive Behaviour
Anisogamy
Frequency of
producers
Gamete size
Reproductive Behaviour
Anisogamy
- lots are produced by not enough mass for cell division
- fewer produced but are competent
- very few produced but are very competent
Reproductive Behaviour
Anisogamy
+
+
+
+
- lots of contact but won’t divide properly
- will divide but fewer contacts
- will divide but very few contacts
-greatest success
-best combination of numbers and competence
Reproductive Behaviour
Costs of sex
“Sex is not only unnecessary, but it ought to be a recipe for
evolutionary disaster. For one thing, it is an inefficient way to
reproduce…And sex carries other costs as well…By all rights, any
group of animals that evolves sexual reproduction should be
promptly outcompeted by nonsexual ones. And yet sex reigns…
Why is sex a success, despite all its disadvantages?”
Carl Zimmer
Evolution: The Triumph of an Idea
Reproductive Behaviour
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
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
Frequency
of mutant
gene
A+B
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
Reproductive Behaviour
Sex ratio -why 1:1?
Excess males
-some males won’t mate
-females that produce
more females are favoured
1:1 sex ratio
-some females won’t mate
-females that produce
more males are favoured
Excess females
Time
Contributing Elements to Darwin’s theory
1. Charles Lyell (1797 - 1875)
Gradualism (Uniformitarianism)
All change through time can be
explained by processes at work
today
No need to invoke catastrophic
events
Contributing Elements to Darwin’s theory
1.
2.
Charles Lyell (1797 - 1875)
Thomas Malthus (1766 - 1834)
Populations of organism will grow
faster than their food supply
Population
Number
Food supply
Time
Contributing Elements to Darwin’s theory
1.
2.
3.
Charles Lyell (1797 - 1875)
Thomas Malthus (1766 - 1834)
Plant and Animal Breeders
-showed that the form of a species
could be changed over time
Darwin - Theory of sexual selection
Natural selection - Observations and deductions
Observation
- all species can increase exponentially but don’t
- more are produced than can survive
Deduction
- there is a struggle for existence
Observation
- there is variation in all organisms that is heritable
Deduction
- some are better equipped to survive than others
- advantage is passed to offspring
Differential survival = Natural selection
Problem for Darwin
?
?
?
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
Reproductive Behaviour
Operational Sex Ratio
Males
Remove males for:
Females
Remove females for:
1. Pre-puberty
2. Sterile
( Postmenopause)
3. Age
4. Pregnant
5. Lactating
Operational sex ratio
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
Evidence?? - Lions
Females
% of
breeders
10
0
# of surviving offspring
Reproductive Behaviour
Evidence?? - Lions
Females
Males
% of
breeders
10
0
# of surviving offspring
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
Size of prey is critical
20 min
Duration of
copulation
# sperm
transferred
16 mm
Size of prey
8
25
Time in copulation
Intersexual Selection
2. Good genes models
Female should look for best genetic complement in male
Traits should reflect genetic quality
Intersexual Selection
2. Good genes models
Fluctuating Asymmetry
Horns in oribi - Ourebia ourebia
Arese. 1994. Anim.Beh. 48:1485
Intersexual Selection
2. Good genes models
Fluctuating Asymmetry
3
2
Harem size
1
0
Symmetric
Asymmetric
Damaged
Intersexual Selection
2. Good genes models
Fluctuating Asymmetry
Symmetry
Ability to fight parasites
(nematodes)
Lagesen & Folstad, 1998. Beh.Ecol.Sociobiol.
Intersexual Selection
2. Good genes models
Fluctuating Asymmetry
Intersexual Selection
2. Good genes models
Good genes and parasites
Number
of
matings
Clean
Infected
No choice
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
Intersexual Selection
Other factors affecting mate choice Learning
1.
Sexual imprinting -already discussed this
2.
Mate choice copying
- chances of a male being preferred as a mate at time 2
increases as a result of being preferred at time 1.
Male 1
Model
female
Test
female
Male 2
Intersexual Selection
Other factors affecting mate choice Learning
1.
Sexual imprinting -already discussed this
2.
Mate choice copying
Frequency
Near previously
‘chosen’male
Near previously
‘non-chosen’male
Time spent by test fish
Intrasexual Selection - Male/Male Competition
Competition before mating
Fiddler Crab (Uca)
1. Direct Fighting
Female
Female digs burrow
Male enters burrow and mates
Male
Male defends burrow and female
Male leaves when female ovulates
No evidence of female choice
Intrasexual Selection - Male/Male Competition
Competition before mating
2. Interference
-male tries to interfere with copulating pair
Intrasexual Selection - Male/Male Competition
Competition before mating
2. Interference
Elephant seals (Mirounga)
-females incite males to fight via interference
Males fight for dominance
-8.3% of males mate
Intrasexual Selection - Male/Male Competition
Competition before mating
2. Interference
Logic - if subordinate male tries to mate
- female protests
- dominant interferes
-female gets ‘best’ mate
% of matings
protested by
female
Alpha
Adult male
Rank of male
Subadult male
Intrasexual Selection - Male/Male Competition
Competition before mating
3. Cuckoldry
Bluegill sunfish
Lepomis macrochirus
Males set up territories
where females lay eggs
Sunfish Territory
Intrasexual Selection - Male/Male Competition
Competition before mating
3. Cuckoldry
Three kinds of males
1. Parental - larger, aggressive territory holders
2. Sneaker - smaller
3. Satellite - look like females
spawning
Male digs nest
Female lays eggs
Male fertilizes eggs
Intrasexual Selection - Male/Male Competition
Male digs nest
Female lays eggs
Parental - larger, aggressive territory holders
Male fertilizes eggs
Parental male
Female
Sneaker - smaller - rush in before parental male
and fertilize eggs
Satellite - look like females - spawn with pair
Satellite
Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition
Females of many species can
1) Store sperm
2) Mate with several males before fertilization
Raises probability that sperm will compete for fertilizations
Males can a) reduce the chances that a second male’s sperm is used
(first male adaptations)
b) reduce the chances that a first male’s sperm is used
(second male adaptations)
Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition - ‘First male adaptations’
a. Postcopulatory mate guarding
Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition - ‘First male adaptations’
a. Postcopulatory mate guarding
Stick insects
Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition - ‘First male adaptations’
a.
b.
c.
Postcopulatory mate guarding
Antiaphrodisiacs
Mating plugs
Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition - ‘Second male adaptations’
a. Sperm removal
Dunnock
Damselfly (Argia) Penes
Intrasexual Selection - Male/Male Competition
Competition after mating
2. Bruce effect
- Mice - strange male (or male odour)
-females abort fetuses and become receptive
Intrasexual Selection - Male/Male Competition
Competition after mating
3. Infanticide
Summary - Sexual Selection
Intersexual Selection
1. Direct Benefits
2. Good Genes
3. Runaway Selection
Intrasexual Selection
1.
Premating
a. Male fighting
b. Cuckoldry
c. Interference
2. Postmating
a. Sperm Competition
b. Bruce Effect
c. Infanticide