Lecture 7. Sexual Selection

Costs of sex

• Cost of meiosis• Cost of producing

males• Cost of courtship and

mating

Benefits of sex• Protection against

mutations (Muller’s ratchet)

• Protection against environmental changes (Raffle hypothesis)

• Protection against biotic fluctuations (Red Queen hypothesis)

”Now here, you see, it takes all the running you can do to keep in the same place.”

The Red Queen hypothesis

Why do male and female gametes differ in size?

Geoffrey Parker et al.:divergent evolutionary selectionfavoured two types of gametes:

small and mobile (sperm)sedentary and packed with nutrients (eggs)

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Darwin (1871, p256):

“We are, however, here concerned only with that kind of selection, which I have called sexual selection. This depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction.”

Sexual selection

1. More individuals are produced than manage to reproduce

2. Individuals differ in their ability to compete with others

for mates or to attract members of the opposite sex

Result: the evolution of traits that enhance reproductive success while decreasing survivorship

What about these

Inrasexual Selection

Members of one sex compete among themselves for access to members of the other sex

Competition before mating

Competition after mating

Competition following conception

Bruce effect in rodents

The pregnancy of a recently inseminated female is terminatedupon exposure to an unfamiliar male

Male infanticide in langur monkeys.

Male infanticide in lions

Explain This ?

Intersexual Selection

Members of one sex (mostly males) ‘advertise’ that they are worthy of an investment.

Members of the other sex (usually females) choose among them.

Sexually selected “ornaments” of males

Darwin argued that sexual selection via female choice was responsiblefor the evolution of male displays and plumage

The widowbird (Euplectes progne),a species the size of a blackbirdthat is endowed with a half-meter long tail

Sexual selection for tail length in long-tailed widowbirds

(Andersson 1982)

Mate-choice copying in the guppy (Poecilia reticulata)

One female’s choice of a mate affects another female’s choice

Male zebra finch

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Fluctuating asymmetry: The extent of asymmetry in bilaterally symmetrical traits

Sailfin Molly (Poecilia velifera)

Normal High Perfect