Assortative mating in the Gouldian Finch (Erythrura gouldiae)

The finches occur in two color morphs, black- & red-headed, in both sexes. Both sexes prefer mates of the same head color. Here, the black-headed female [center] is attracted to the black-headed male [left] rather than the red-headed male [right]. Avian sex determination is the opposite of mammals: females are heterogametic (ZW) and males homogametic (ZZ). Female offspring always have the head color of the male parent, consistent with a single-locus trait located on the Z chromosome with two alleles for red and black, Z^R dominant to Z^B.  Z^RZ^R or Z^RZ^B red-headed males mates with red-headed Z^RW females, and Z^BZ^B black-headed males mates with black-headed Z^BW females. Note than even if heterozygous Z^RZ^B red-headed males mate only with Z^RW females, all male offspring will be red-headed but half the female offspring will be black-headed.

Such assortative mating schemes are an example of runaway sexual selection: the alternative female and male preferences drive each other, and the divergence of the phenotypic marker evolves rapidly.

HOMEWORK: Show that the expected frequency of male heterozygotes will be lower than expected, as driven by assortative mating.