Biology Lecture No. 15: Sexual Selection
(Wednesday, November 2, 2011)
Ecological Explanations For Sex:
-Parasites place negative frequency-dependent selection on host genotypes.
-Advantageous for host to be rare genotype, parasites prefer common host genotypes simply due to the
large number of individuals.
-Sex (which produces a diversity of offspring genotypes) thus becomes favoured in individuals living in
parasite-rich environments (Red Queen Hypothesis).
Sex Is Costly And Inefficient, But Incredibly Successful:
-Long-term advantage (capacity to purge harmful mutations in a species)
-Short-term advantage (hedge bets against a quickly-changing environment, in other words the
possibility of developing new alleles better adapted to any inevitable shifts in environmental patterns).
Sex Has Important Consequences:
-It explains sexually dimorphic traits, and traits seemingly incompatible with natural selection.
Intrasexual V.s. Intersexual Selection:
-Intrasexual selection: Sexual selection that occurs between competing mates of the same sex for a
common mate (trying to discourage competitors), e.g. weapons like tusks and horns, dominance
-Intersexual selection: Sexual selection that occurs between possible mates (trying to impress or win
opposite sex’s approval), e.g. feather colour, mating songs, etc.
-Distinction between intrasexual and intersexual selection can overlap and become blurred; such is the
case of the mating behaviours of the Australian bowerbird, which exhibits both intersexual selection
(with males creating elaborate bowers specifically for female’s criticism) and intrasexual selection (with
other competing males sabotaging each other’s bowers in the hopes of minimizing competition).
Females Are Often Pickier Than Males, Why?: