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slide2

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

slide3

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?

Reproductive Behaviour

3. Why are the sexes generally in a 1:1 ratio?

reproductive behaviour1

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

reproductive behaviour3

Anisogamy

Reproductive Behaviour

Frequency of producers

Gamete size

reproductive behaviour4

Anisogamy

- lots are produced by not enough mass for cell division

Reproductive Behaviour

- fewer produced but are competent

- very few produced but are very competent

reproductive behaviour5

Anisogamy

+

- lots of contact but won’t divide properly

Reproductive Behaviour

+

- will divide but fewer contacts

+

- will divide but very few contacts

+

  • greatest success
    • best combination of numbers and competence
reproductive behaviour6

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?”

Reproductive Behaviour

Carl Zimmer

Evolution: The Triumph of an Idea

reproductive behaviour7

Costs of sex

1. Cost of meiosis

Reproductive Behaviour

  • 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 behaviour8

Costs of sex

2. Cost of recombination

Reproductive Behaviour

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

-chance of deleterious combinations

reproductive behaviour9

Benefits of sex

1. Produce new combinations

Reproductive Behaviour

Unique offspring can cope with different niches

x

reproductive behaviour10

Benefits of sex

2. Muller’s Ratchet

Reproductive Behaviour

- in asexual species, deleterious mutations accumulate

A

A+B

Frequency of mutant gene

Mutation A

A+B+C

Mutation B

B+C

Mutation C

C

Time

reproductive behaviour11

Benefits of sex

2. Muller’s ratchet

Reproductive Behaviour

- in sexual species, such mutations tend to be eliminated

Frequency of mutant gene

Mutation A

Mutation B

Mutation C

Time

reproductive behaviour12

Sex ratio -why 1:1?

Excess males

Reproductive Behaviour

-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

slide17

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

slide18

Contributing Elements to Darwin’s theory

Charles Lyell (1797 - 1875)

Thomas Malthus (1766 - 1834)

Populations of organism will grow

faster than their food supply

Population

Number

Food supply

Time

slide19

Contributing Elements to Darwin’s theory

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

Darwin - Theory of sexual selection

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

slide22

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 selection1

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.

Darwin - Theory of sexual selection

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 selection2

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

Darwin - Theory of sexual selection

- claws, antlers etc.

darwin theory of sexual selection3

Two parts

2. Intersexual selection (Female choice)

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

Darwin - Theory of sexual selection

- plumage, song.

reproductive behaviour13

Males and females have different reproductive interests

What does each sex have to provide to produce offspring?

Reproductive Behaviour

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

reproductive behaviour14

In more general terms

Females

Males

Difference in reproductive investment

Reproductive Behaviour

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

reproductive behaviour15

Operational vs Numerical Sex ratio

Numerical Sex Ratio

The ratio of the number of males to the number of females in the population

Reproductive Behaviour

Operational Sex Ratio

The ratio of the number of males to the number of females in the population who are available for reproduction

reproductive behaviour16

Operational vs Numerical Sex ratio

Numerical Sex Ratio (close to 1:1)

Males

Females

Reproductive Behaviour

reproductive behaviour17

Operational Sex Ratio

Males

Females

Reproductive Behaviour

Remove females for:

Remove males for:

1. Pre-puberty

2. Sterile

( Post-menopause)

3. Age

4. Pregnant

5. Lactating

Operational sex ratio

reproductive behaviour18

Operational vs Numerical Sex ratio

Numerical Sex Ratio (close to 1:1)

Males

Females

Reproductive Behaviour

Operational Sex Ratio (very different from 1:1)

Competition in more abundant sex

Selectivity in less abundant sex

reproductive behaviour19

Evidence??

Bateman (1948)

-Drosophila melanogaster - nearly all females mated

but not all males

Reproductive Behaviour

-variance in male reproductive success - higher

Males

Bateman gradient

# of offspring

Females

# of mates

reproductive behaviour20

Evidence?? - Lions

Females

Reproductive Behaviour

% of breeders

10

0

# of surviving offspring

reproductive behaviour21

Evidence?? - Lions

Females

Males

Reproductive Behaviour

% of breeders

10

0

# of surviving offspring

slide35

Intersexual Selection

Genetic Models for Mate Choice

1. Direct Benefits

-females choose males that give them a concrete

resource

e.g. Nuptial gifts

slide36

Intersexual Selection

Size of prey is critical

20 min

Duration of

copulation

# sperm transferred

8

25

16 mm

Size of prey

Time in copulation

intersexual selection
Intersexual Selection

2. Good genes models

Female should look for best genetic complement in male

Traits should reflect genetic quality

intersexual selection1
Intersexual Selection

Fluctuating Asymmetry

2. Good genes models

Horns in oribi - Ourebia ourebia

Arese. 1994. Anim.Beh. 48:1485

intersexual selection2
Intersexual Selection

3

2

1

0

Fluctuating Asymmetry

2. Good genes models

Harem size

Symmetric

Asymmetric

Damaged

intersexual selection3
Intersexual Selection

Fluctuating Asymmetry

2. Good genes models

Symmetry

Ability to fight parasites

(nematodes)

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

intersexual selection4
Intersexual Selection

Fluctuating Asymmetry

2. Good genes models

intersexual selection5
Intersexual Selection

Good genes and parasites

2. Good genes models

Number

of matings

Clean

Infected

No choice

intersexual selection6
Intersexual Selection

Assume 2 genes

Gene A - Trait ‘X’ in males - more colourful feathers

3. Runaway Selection

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 selection7
Intersexual Selection

Assume 2 genes

Gene A - Trait ‘X’ in males - more colourful feathers

3. Runaway Selection

Gene B - In females - preference for trait ‘X’

Frequency

in males

Frequency in females

intersexual selection8
Intersexual Selection

Example - stalk-eyed flies - Diopsidae

3. Runaway Selection

Variation in males

Female

intersexual selection9
Intersexual Selection

Example - stalk-eyed flies - Diopsidae

Two experiments

3. Runaway Selection

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 selection10
Intersexual Selection

Example - stalk-eyed flies - Diopsidae

Two experiments

3. Runaway Selection

2. Female choice

Female offspring prefer

short stalked males

Female offspring prefer

long stalked males

Stalk length

intersexual selection11
Intersexual Selection

Sexual imprinting -already discussed this

Other factors affecting mate choice - Learning

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 2

Model female

Test female

intersexual selection12
Intersexual Selection

Sexual imprinting -already discussed this

Other factors affecting mate choice - Learning

2. Mate choice copying

Frequency

Near previously

‘chosen’male

Near previously

‘non-chosen’male

Time spent by test fish

intrasexual selection male male competition
Intrasexual Selection - Male/Male Competition

Competition before mating

Fiddler Crab (Uca)

1. Direct Fighting

Female

Female digs burrow

Male

Male enters burrow and mates

Male defends burrow and female

No evidence of female choice

Male leaves when female ovulates

intrasexual selection male male competition1
Intrasexual Selection - Male/Male Competition

Competition before mating

2. Interference

-male tries to interfere with copulating pair

intrasexual selection male male competition2
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 competition3
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

Subadult male

Rank of male

slide54

Intrasexual Selection - Male/Male Competition

Competition before mating

3. Cuckoldry

Bluegill sunfish

Lepomis macrochirus

Males set up territories

where females lay eggs

slide56

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

slide57

Intrasexual Selection - Male/Male Competition

Male digs nest

Female lays eggs

Male fertilizes eggs

Parental male

Parental - larger, aggressive territory holders

Female

Sneaker - smaller - rush in before parental male

and fertilize eggs

Satellite

Satellite - look like females - spawn with pair

slide58

Intrasexual Selection - Male/Male Competition

Competition after mating

1. Sperm Competition

Females of many species can

Store sperm

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)

slide59

Intrasexual Selection - Male/Male Competition

Competition after mating

1. Sperm Competition - ‘First male adaptations’

a. Postcopulatory mate guarding

slide60

Intrasexual Selection - Male/Male Competition

Competition after mating

1. Sperm Competition - ‘First male adaptations’

a. Postcopulatory mate guarding

Stick insects

slide61

Intrasexual Selection - Male/Male Competition

Competition after mating

1. Sperm Competition - ‘First male adaptations’

Postcopulatory mate guarding

Antiaphrodisiacs

Mating plugs

slide62

Intrasexual Selection - Male/Male Competition

Competition after mating

1. Sperm Competition - ‘Second male adaptations’

a. Sperm removal

Dunnock

Damselfly (Argia) Penes

slide63

Intrasexual Selection - Male/Male Competition

Competition after mating

2. Bruce effect

- Mice - strange male (or male odour)

-females abort fetuses and become receptive

slide64

Intrasexual Selection - Male/Male Competition

Competition after mating

3. Infanticide

summary sexual selection
Summary - Sexual Selection

Intersexual Selection

1. Direct Benefits

Intrasexual Selection

Premating

2. Good Genes

a. Male fighting

3. Runaway Selection

b. Cuckoldry

c. Interference

2. Postmating

a. Sperm Competition

b. Bruce Effect

c. Infanticide