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 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?


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?


Anisogamy – how did it come about?


Gamete size

Frequency of producers

Anisogamy


Anisogamy

- lots are produced by not enough mass for cell division

- very few produced but are very competent

- fewer produced but are competent


Anisogamy

- lots of contact but won’t divide properly

- will divide but very few contacts

- will divide but fewer contacts

+

+

+

+

-greatest success-best combination of numbers and competence


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


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


Costs of sex

2. Cost of recombination

-take a functioning genome, split it and combine it with another one

-chance of deleterious combinations


Benefits of sex

1. Produce new combinations

xUnique offspring can cope with different niches


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


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


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


1. Charles Lyell (1797 - 1875)2. Thomas Malthus (1766 - 1834)

Populations of organism will growfaster than their food supply

Population

Food supply

Number

Time


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

Problem for Darwin

?

?

?

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.


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.


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.


Two parts

2. Intersexual selection (Female choice)

All those structures and behaviour patterns employed by males to attract females

- plumage, song.


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)


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


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



Numerical Sex Ratio (close to 1:1)

Males Females


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



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


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


Evidence?? - Lions

# of surviving offspring

% of breeders

Females

0 10


Evidence?? - Lions

# of surviving offspring

% of breeders

Females

0 10

Males

Intersexual Selection

Genetic Models for Mate Choice

1. Direct Benefits

-females choose males that give them a concreteresource

e.g. Nuptial gifts


Size of prey is critical

Size of prey

Duration ofcopulation

20 min

16 mm

Time in copulation

# sperm transferred

258


2. Good genes models

Female should look for best genetic complement in male

Traits should reflect genetic quality


2. Good genes models

Fluctuating Asymmetry

Horns in oribi - Ourebia ourebia

Arese. 1994. Anim.Beh. 48:1485


2. Good genes modelsFluctuating Asymmetry

Symmetric Asymmetric Damaged

Harem size

3

2

1

0



Lagesen & Folstad, 1998. Beh.Ecol.Sociobiol.

Symmetry

Ability to fight parasites(nematodes)


2. Good genes modelsGood genes and parasites

Numberof matings

Clean Infected No choice


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



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


3. Runaway SelectionExample - stalk-eyed flies - Diopsidae

Variation in males

Female



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



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


Competition before mating

2. Interference

-male tries to interfere with copulating pair



2. Interference Elephant seals (Mirounga)-females incite males to fight via interference

Males fight for dominance-8.3% of males mate



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



3. Cuckoldry

Sunfish Territory

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



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



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)



1. Sperm Competition - ‘First male adaptations’

a. Postcopulatory mate guarding




a. Postcopulatory mate guarding

Stick insects




a. Postcopulatory mate guardingb. Antiaphrodisiacsc. Mating plugs



1. Sperm Competition - ‘Second male adaptations’

a. Sperm removal

Dunnock

Damselfly (Argia) Penes



2. Bruce effect

- Mice - strange male (or male odour)

-females abort fetuses and become receptive



3. Infanticide

Summary - Sexual Selection

Intersexual Selection1. Direct Benefits

2. Good Genes


Intrasexual Selection

1. Premating

a. Male fighting

2. Postmating

b. Bruce Effect

a. Sperm Competition

b. Cuckoldry

c. Interference

c. Infanticide

Sexual Selection and Reproductive Behaviour

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