intersexual and intrasexual selection

intersexual selection

the selection between the sexes i.e. the strategies that males use to select females and vice versa.

strategies of mate selection are different between the sexes

female mate selection

prefer intersexual selection - quality over quantity

because the woman produces far fewer ova in her lifetime than a man produces sperm

women also use more energy producing ova

this is why females need to mate with the best males

Trivers 1972

emphasises that females need to invest more time and resources before, during and after the birth of offspring

So females try and choose a genetically fit partner who is able and willing to provide resources.

This leaves males to compete for the opportunity to mate with fertile females.

Both sexes are highly selective in their approach because they are both disadvantaged if they invest in substandard partners.

But the consequences of making a bad choice are more serious for females.

inherited characteristics

the preference for a fit male determines the features that are passed onto the offspring

e.g. if height is considered attractive, then over generations, it will increase in the male population

because females will mate with taller males, producing greater preference for taller partners

Fisher 1930

proposed sexy sons hypothesis

females mate with males who have desirable characteristics

their sons inherit the sexy train which increases the likelihood that the successive generation will mate with the females’ offspring

this is the ‘runaway’ process

intrasexual selection

competition between males to mate with females

male mate selection

intrasexual selection is the most common strategy of males and reflects male preference for quantity over quality

the winner of the competition gets to pass the characteristics that contributed to his victory onto his offspring

dimorphism

the obvious differences between males and females

e.g. because larger males are more likely to win physical competitions with other males, being a larger male is an advantage

but because females do not compete for reproductive rights, there is no evolutionary drive favouring larger females

consequences of intrasexual selection

when males get fertile females, they protect them from competing males. this means they can benefit from behaving aggressively

anisogamy

suggests that the best male reproductive strategy is to mate with as many fertile females as possible

this is because they need relatively low levels of energy to produce sperm and they have less responsibility in caring for any offspring