6a. Sexual Selection: survival of the sexiest? Flashcards
Name the processes by which sexually selected traits evolve
Runaway selection
Good genes
Examples of harmful traits that hinder survival
Peacock’s tail
Irish Elk’s antlers
Why is reproduction important for evolution?
Pass on genes to offspring
Traits that aid X can be adaptive even if they harm survival.
What is X?
reproduction
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
True
When does sexual selection occur?
When a selection pressure acts to increase success in mating scenarios
TRUE OR FALSE: Sexual selection pressures act in the same way on males and females.
False
How do sexual selection pressures act differently on males and females?
Can affect QUANTITY of matings (males), QUALITY in matings (females) and EFFICIENCY in matings
What do sexual selection pressures act on?
Physical structures, behaviours and ‘extended phenotypes’
What are extended phenotypes?
Not a physical trait or behaviour but something an organism produces externally
What is a female peacock called?
A peahen
How do peahens judge the quality of a peacock?
Its tail
The peacock’s tail
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!
The Irish Elk’s antlers
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
Bowerbirds
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)
Two forms of sexual selection
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)
Runaway selection
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)
‘Sexy Sons’ and runaway selection
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
The problem of female choice
Fisher’s account can explain the existence of ‘runaway’ male traits
But does not explain why FEMALES are the choosy ones
According to Trivers (1972), why are females the choosy ones?
Sex differences in parental investment
Parental investment theory
In most species, females invest more in offspring due to anisogamy (results in a sex difference in fitness variance)
Females have more to lose
What do females invest in their offspring in most species?
They have a limited number of large, immobile gametes (eggs)
They host the embryo for 9 months
Breast feeding
What is the exception for ‘males court and females choose’?
Seahorses:
Males invest more in offspring - incubate eggs
Female seahorses behave aggressively and competitively
This exception proves the rule
Male choice
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
The handicap principle
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
Runaway selection vs. good genes
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.
Sexually selected behavioural traits
Aggression Risk-taking Humour use Altruism Creativity
Humour doesn’t aid survival - must be sexually selected for
Differences in motivation/use, not in abilities
Intelligence as a sexually selected behavioural trait
‘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’
Is sexual selection a form of natural selection?
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)
