6a. Sexual Selection: survival of the sexiest?

Question 1

Name the processes by which sexually selected traits evolve

Answer 1

Runaway selection

Good genes

Question 2

Examples of harmful traits that hinder survival

Answer 2

Peacock’s tail

Irish Elk’s antlers

Question 3

Why is reproduction important for evolution?

Answer 3

Pass on genes to offspring

Question 4

Traits that aid X can be adaptive even if they harm survival.
What is X?

Answer 4

reproduction

Question 5

TRUE OR FALSE: Costs of producing large physical traits or complex behaviours are less than the benefit of increased numbers of offspring i.e. higher inclusive fitness

Answer 5

True

Question 6

When does sexual selection occur?

Answer 6

When a selection pressure acts to increase success in mating scenarios

Question 7

TRUE OR FALSE: Sexual selection pressures act in the same way on males and females.

Answer 7

False

Question 8

How do sexual selection pressures act differently on males and females?

Answer 8

Can affect QUANTITY of matings (males), QUALITY in matings (females) and EFFICIENCY in matings

Question 9

What do sexual selection pressures act on?

Answer 9

Physical structures, behaviours and ‘extended phenotypes’

Question 10

What are extended phenotypes?

Answer 10

Not a physical trait or behaviour but something an organism produces externally

Question 11

What is a female peacock called?

Answer 11

A peahen

Question 12

How do peahens judge the quality of a peacock?

Answer 12

Its tail

Question 13

The peacock’s tail

Answer 13

Sexual ornament - only function
Petrie (1994): peacocks with most elaborate tail most successful in reproduction
A male with a small tail may easily escape predators but will not reproduce!

Question 14

The Irish Elk’s antlers

Answer 14

No functional reason for the size
Detrimental effects (e.g. heavy to carry around, must have strong neck muscles, get caught on things)
But females select males with the largest antlers

Question 15

Bowerbirds

Answer 15

Papa new Guinea
Create large structures called ‘bowers’
Up to 10 matings a day if impressive bower
Bower is their extended phenotype - displays creativity and intelligence (traits that females want to pass onto offspring)

Question 16

Two forms of sexual selection

Answer 16

INTRASEXUAL: competition between members of same sex for mate (e.g. male physiological weaponry)

INTERSEXUAL: attempts to impress/attract members of the opposite sex (e.g. male ornamentation and display)

Question 17

Runaway selection

Answer 17

Fisher (1930): the feature chosen by females originally had some survival advantage
Therefore desirable to females
Exaggerated forms become more desirable
Feature of trait that is desirable (e.g. size) leads to selection BEYOND function (runaway)

Question 18

‘Sexy Sons’ and runaway selection

Answer 18

Females mating with males with desirable features have ‘sexy sons’ –> more genes left behind
This is RS - all females want to mate with males with the trait
NS limits how far this can go

Question 19

The problem of female choice

Answer 19

Fisher’s account can explain the existence of ‘runaway’ male traits
But does not explain why FEMALES are the choosy ones

Question 20

According to Trivers (1972), why are females the choosy ones?

Answer 20

Sex differences in parental investment

Question 21

Parental investment theory

Answer 21

In most species, females invest more in offspring due to anisogamy (results in a sex difference in fitness variance)
Females have more to lose

Question 22

What do females invest in their offspring in most species?

Answer 22

They have a limited number of large, immobile gametes (eggs)
They host the embryo for 9 months
Breast feeding

Question 23

What is the exception for ‘males court and females choose’?

Answer 23

Seahorses:
Males invest more in offspring - incubate eggs
Female seahorses behave aggressively and competitively
This exception proves the rule

Question 24

Male choice

Answer 24

Emphasis since Darwin has been on female choice
Males are portrayed as not choosy, but that only applies in few cases (some species, types of mating)
Not so in species with less asymmetry in parental investment
High QUALITY males can be choosy

Question 25

The handicap principle

Answer 25

Females choose because they invest more
If females choose, need to assess a male’s quality
Runaway selection cannot do this (it is arbitrary)
Zahavi (1975): not all male ornaments develop just to look desirable
Also develop impediments to prove they can survive with such handicaps
E.g. peacock’s tail evolved BECAUSE it was detrimental to survival
Display genetic fitness (quality) - i.e. good genes

Question 26

Runaway selection vs. good genes

Answer 26

Debate about extent to which female choice is explained
‘Fisherians’ must show that females choose males for ‘sexiness’ to pass onto sons
‘Good geners’ must show that ornaments are real signs on health etc.

Question 27

Sexually selected behavioural traits

Answer 27

Aggression
Risk-taking
Humour use
Altruism
Creativity

Humour doesn’t aid survival - must be sexually selected for

Differences in motivation/use, not in abilities

Question 28

Intelligence as a sexually selected behavioural trait

Answer 28

‘The Mating Mind’ - Geoffrey Miller
Human intelligence is sexually selected
It may be essentially a male ornament - female intelligence may have evolved as an ‘assessment tool’

Question 29

Is sexual selection a form of natural selection?

Answer 29

Much debated
Both ultimately increase inclusive fitness
Some features used for survival AND reproductive benefits (e.g. intelligence?)
Perhaps easier to consider them separately as they often drive in different directions (e.g. handicap principle)