sexual selection and reproductive behaviour. sex is the queen of problems in evolutionary biology....
TRANSCRIPT
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 BellThe Masterpiece of Nature: The Evolution of Genetics and Sexuality
Sex vs. Reproduction
Reproduction - process by which individuals are added to a population
Sex - process by which a genome is changed by rearrangement and combining of genes
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
Anisogamy
Anisogamy (heterogamy) - a form of sexual reproduction involving the union or fusion of two dissimilar gametes
What is the one feature that serves to separate male from female in all animals?
Reproductive Behaviour
Anisogamy
- lots are produced by not enough mass for cell division
- very few produced but are very competent
- fewer produced but are competent
Reproductive Behaviour
Anisogamy
- lots of contact but won’t divide properly
- will divide but very few contacts
- will divide but fewer contacts
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-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 ZimmerEvolution: 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
xUnique offspring can cope with different niches
Reproductive Behaviour
Benefits of sex2. Muller’s Ratchet
- in asexual species, deleterious mutations accumulate
Frequency of mutant gene
Time
Mutation A
Mutation B
Mutation C
A
A+B
A+B+C
B+C
C
Reproductive Behaviour
Benefits of sex2. Muller’s ratchet
- in sexual species, such mutations tend to be eliminated
Frequency of mutant gene
Time
Mutation A
Mutation B
Mutation C
Reproductive Behaviour
Sex ratio -why 1:1?
Time
1:1 sex ratio
Excess females
Excess males-some males won’t mate
-females that produce more females are favoured
-some females won’t mate-females that produce more males are favoured
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. Charles Lyell (1797 - 1875)2. Thomas Malthus (1766 - 1834)
Populations of organism will growfaster than their food supply
Population
Food supply
Number
Time
Contributing Elements to Darwin’s theory
1. Charles Lyell (1797 - 1875)2. Thomas Malthus (1766 - 1834)3. 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
Charles Darwin, 1859. Origin of species
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.
Darwin - Theory of sexual selection
Two parts
1. Intrasexual selection (Male-male competition)
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.
But in many cases, victory will depend not on general vigour, but on having special weapons, confined to the male sex.
Charles Darwin, 1859. Origin of species
The war is, perhaps, severest between the males of polygamous animals, and these seem oftenest provided with special weapons.
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
Females
1. Manufacture eggs (expensive)
2. Incubate eggs or fetus (expensive)
3. Post partum care (expensive)
Males
1. Manufacture sperm (cheap)
What does each sex have to provide to produce offspring?
Males invest less in any offspring
4. Loss of mating opportunity when pregnant(expensive)
Reproductive Behaviour
In more general terms
Difference in reproductive investmentFemales Males
higher investment lower investment
higher potential reproductive ratelower 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
Reproductive Behaviour
Operational vs Numerical Sex ratio
Numerical Sex RatioThe ratio of the number of males to the
number of females in the population
Operational Sex RatioThe 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
Males Females
Operational Sex Ratio
Remove males for:
1. Pre-puberty
2. Sterile
3. Age
Remove females for:
4. Pregnant
( Post-menopause)
5. LactatingOperational 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 matedbut not all males
-variance in male reproductive success - higher
# of mates
# of offspringFemales
Males
Bateman gradient
Intersexual Selection
Genetic Models for Mate Choice
1. Direct Benefits
-females choose males that give them a concreteresource
e.g. Nuptial gifts
Intersexual Selection
Size of prey is critical
Size of prey
Duration ofcopulation
20 min
16 mm
Time in copulation
# sperm transferred
258
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 modelsFluctuating Asymmetry
Symmetric Asymmetric Damaged
Harem size
3
2
1
0
Intersexual Selection
2. Good genes modelsFluctuating Asymmetry
Lagesen & Folstad, 1998. Beh.Ecol.Sociobiol.
Symmetry
Ability to fight parasites(nematodes)
Intersexual Selection
2. Good genes modelsGood genes and parasites
Numberof 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
A+ B
Expressed in males as the trait
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 females
Frequencyin males
Intersexual Selection
3. Runaway SelectionExample - stalk-eyed flies - Diopsidae
Variation in males
Female
Intersexual Selection
3. Runaway SelectionExample - stalk-eyed flies - Diopsidae
Two experiments1. 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 SelectionExample - stalk-eyed flies - Diopsidae
Two experiments2. Female choice
Stalk length
Female offspring prefershort stalked males
Female offspring preferlong stalked males
Intersexual SelectionOther 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 Male 2Model female
Test female
Intersexual SelectionOther 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
1. Direct Fighting
Competition before matingFiddler Crab (Uca)
Male
Female
Female digs burrow
Male enters burrow and mates
Male defends burrow and female
Male leaves when female ovulatesNo 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
Rank of male
Alpha Adult male Subadult male
Bluegill sunfishLepomis macrochirus
Males set up territorieswhere females lay eggs
Intrasexual Selection - Male/Male Competition
Competition before mating
3. Cuckoldry
Three kinds of males1. Parental - larger, aggressive territory holders2. Sneaker - smaller3. Satellite - look like females
spawning
Male digs nest Female lays eggs Male fertilizes eggs
Intrasexual Selection - Male/Male Competition
Competition before mating
3. Cuckoldry
Parental - larger, aggressive territory holders
Male digs nest Female lays eggs Male fertilizes eggs
Sneaker - smaller - rush in before parental male and fertilize eggs
Satellite - look like females - spawn with pair
Parental male
Female
Satellite
Intrasexual Selection - Male/Male Competition
Intrasexual Selection - Male/Male Competition
Competition after mating
1. Sperm Competition
Females of many species can
1) Store sperm2) 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. Postcopulatory mate guardingb. Antiaphrodisiacsc. 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