FÖ8 Reproductive Behaviour & Evolution of Mating Systems

Intra-sexual

Sexual selection within a sex

• Ex. males fighting over access to females

Inter-sexual

Sexual selection between sexes

• Ex. females being choosy, female birds liking the prettiest feathers

Anisogamy

The difference in size between the gametes of males and females

• Makes it so that a female’s fitness increases if she has a high quality mate while a male’s fitness increases if he has many mates

• The smaller sperm will always outnumber the larger eggs in a population → competition between males to fertilize

Fisher’s principle on sex ratio

Explains why population sex ratio is 1:1

• Negative frequency-dependent selection due to individuals of the rare sex realize greater reproductive opportunity than those of the more common sex.

Mating system

Males and females associating in some systematic way during breeding

Monogamy

One male and one female

• Ex. some birds

• Mainly happens if the male has low chances of finding another female, benefits from defending the female or the offspring require the care of both parents.

Polyandry

One female and multiple males

• Ex. honeybees

• Benefits include: fertility insurance, good genes, genetic compatibility, more resources, protection and care as well as infanticide reduction.

Polygyny

One male and multiple females

• Ex. elephant seals, gorillas

• Maximizes the male’s fitness

• Female defense, resource defense, lek or scramble competition

Polygynandry (promiscuity)

Both sexes have multiple partners

• Ex. chimps, pigs

Operational sex ratio

the ratio of sexually active males to sexually receptive females