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Sexual selection

Evolutionary Psychology

Adam Putz

on 5 March 2015

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Transcript of Sexual selection

Sexual selection
Evolutionary Psychology
Charles Darwin
Natural selection
"the survival of the
Males and females have faced the same ecological pressures throughout the evolution...
Why don't they look the same?!
Males are usually...
attracting predators
hinder movement and/or flight
attracting females
Darwin proposed a new selective force...
Sexual selection
It applies to those characteristics that provide individuals with advantages in gaining access to mates (sexual partners).
"the survival of the sexiest"
Sexual selection
Intrasexual selection
Intersexual selection
= consists of individuals competing with members of the
same sex
for access to the opposite sex.

usually males
= consists of members of one sex attempting to impress members of the other.
female choice
Sexual ornaments
Courtship behavior
males court
less choosy
they fight for females' favor


Why is that so?
Trivers ('72) -
parental investment
Even at the level of gamete formation...
... females invest much more effort into producing offspring than do males.
Trivers suggests that...
... sexual selection is directly related to asymmetries between the sexes in the amount of effort that each parent puts into raising the offspring.
Females should be very choosy
about which males they allow to fertilise their eggs.
Males have little to loose
less discrimination and choosiness
Theories of sexual selection
'Runaway selection'
Parasite theory
The handicap principle
Fisher (1930)
Females choose a certain feature because it signals survival advantage
e.g. large tail feathers -> better flight

with each generation
It's called
'runaway selection'
since the feature runs away from its original function and becomes selected purely on the basis of its attractive qualities.
But how does it work?
For how long can a feature be exaggerated?
Until the counteracting costs of natural selection.
Trade off mechanism
Attracting females
Attracting predators
Zahavi (1975)
Males develop ornaments in order to demonstrate their abilities to survive despite having such a handicap (-> signaling good genes)
Do you see how fit I am?
Honest signaling
Can I trust this male?
Male adornments correlate with good health indeed
Costly adornments are signs of genetic quality
Yes, you can.
Hamilton & Zuk (1982)
Male adornments evolved to demonstrate to females that they are free from parasites
good genetic quality
They should
pass on disease resistance
to their offspring
Different levels of mite infection of young swallows
Honest signals...
Why bother with sex?!
What else can sexual selection influence?
Courtship and mating
Fighting for females
The cost of producing males
e.g. elephant seals
In contrast to females, most males do not reproduce at all
The cost of meiosis
= you throw half of your genes away
Why don't we all reproduce asexually?
Evolution is faster with recombination than with mutation only
Host-parasite co-evolution
The Red Queen model
Leigh Van Valen (1973)
The host produces variable offspring in retaliation to parasites so that, by chance, some of them will have
Parasites and hosts are in a continual evolutionary battle or
arms race
Sexual selection and behavior
Looking sexy is not enough (morphology)
Behavioral patterns
have both survival and reproductive value

female choice
as the prime mover of sexual selection
How does it work?
1. There is a variation in traits.
e.g. some beetles are green, some are brown
2. Differential reproductive success.
3. There is heredity.
i.e. brown beetles produce brown baby beetles
i.e. predators prefer green beetles
4. Few generations later...
better adaptation to environmental pressures -> more offspring
shorter life expectancies
Age related gender differences in different causes of death
The distribution of injuries and deaths in adult male rhesus monkeys throughout the calendar year
Dominant males fertilize the majority of females
How to increase one's reproductive success?
copulate with as many females as possible
males fight with each other (
intrasexual rivalry

females choose the winner
Sexual dimorphism
Alternative strategies
= the degree to which the sexes differ in physical characteristics
(usually males are bigger than females)
the greater the difference
between males and females,

more intense the fight
becomes among males for females
who are on the top of the male dominance hierarchy
Sneaking behavior
bluegill sunfish
3 male morphs:
Parental male
Sneaker male
Satellite male
- maintain territory, mate with female
- no territory, sneak to fertilize eggs when parental mates with female
- looks like a female; fools male (who tries to mate with satellite) and fertilizes eggs
Increase reproductive fitness AFTER copulation
Sperm competition
Mate guarding
Good genes
Alpha male
= offspring will be resistant against parasites and diseases
High male parental investment
Males present with gifts
look for...
Why sexy is sexy?
Ritualized aggression
(e.g. courtship behavior)
Wilson & Daly
Young Male Syndrome
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