sexual selection. a difference between the sexes is called a sexual dimorphism

Sexual selection

A difference between the sexes is called a sexual dimorphism

Natural selection cannot always explain sexual dimorphism

Asexual reproduction Sexual reproduction

live long enough live long enough

reproduce reproduce

persuade partner to mate

Asymmetries in sexual reproduction

Sexual selection - differential reproductive success due to variation among individuals in success at getting mates

Sexual reproduction imposes different selection pressures on females versus males


Females

Pregnant for 8 months

Nurses for 3 years

Protects until 7 or 8 yrs old

Orangutan mating

Males

A few mL of semen

Sperm is cheap!

reproductive success

potential realized

females small egg (preg) limited

males large mate limited


male reproductive success limited by access to mates

female reproductive success limited by capacity to produce offspring


Sexual selection theory predicts that males will compete with each other over access to mates and that females will be choosy


Intra-sexual selection - outcome determined within a sex

male-male competition

Inter-sexual selection - outcome determined by opposite sex

female choice

Male-male competition

combat over access to females

sperm competition

infanticide

Intra-sexual selection

combat - elephant seals


sperm competition - damselflies


sperm competition - other examples


large ejaculates

mate guarding

prolonged copulation

copulatory plug

repellant pheromones applied to female

sperm competition - other examples


large ejaculates

infanticide - lions


infanticide - lions

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Female choice

better genes

resource acquisition

preexisting sensory bias

‘sexy sons’

Inter-sexual selection

Female choice

barn swallows

better genes - gray tree frogs

Female choice

long-call males had higher fitness offspring

resource acquisition - hanging flies

Female choice

nuptial gifts

pre-existing sensory biases - tungara frogs

Female choice

females prefer low frequency calls

pre-existing sensory biases - swordtails & grassfinches

Female choice

females prefer males with artificial enhancements

‘sexy sons’

Female choice

stalk-eyed flies

peacocks

Runaway selection

-females prefer some trait

-males with that trait leave more offspring

-male offspring will possess that trait and female offspring will prefer males that possess that trait

-leads to a positive feedback loop, get exaggeration of the trait

-if trait becomes a detriment to survival, sexual selection balanced by natural selection

Species in which males invest more in each offspring, and are thus a limiting resource for females, are the exceptions that can prove the rules of sexual selection

Role reversal

male pipefish brood their young, and access to males is a limiting resource for females.

females fight among themselves and males are choosy

Role reversal

Role reversal

sexual selection. a difference between the sexes is called a sexual dimorphism

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