Assortative mating in the Gouldian Finch (Erythrura gouldiae)

    Gouldian Finches occur in two color morphs, black- or red-headed, in both sexes. Recall that in avian sex determination, females are heterogametic (ZW) and males homogametic (ZZ). Head color is determined by a single locus on the Z chromosome, with Z^R dominant to Z^b. Both sexes prefer mates with the same head color. As shown here, black-headed females ( Z^bW ) [center] are preferentially attracted to black-headed males (Z^bZ^b ) [left], rather than red-headed males ( Z^RZ^R or Z^RZ^b ) [right]. All offspring (male and female) of such crosses are necessarily black-headed ( Z^bZ^b males, or Z^bW females). Similarly, all offspring of red-headed females ( Z^RW ) who choose homozygous red-headed males ( Z^RZ^R ) will be red-headed. However, it they choose heterozygous red-headed males ( Z^RZ^b ), all of the male offspring will be red-headed (  Z^RZ^R or Z^RZ^b  ), whereas half of the female offspring will be black-headed (  Z^bW  vs Z^RW ).

    Such assortative mating schemes may lead to runaway sexual selection: female choice and male phenotype drive each other in the same direction, and the divergence of the phenotypes evolves rapidly.