A scientific team consisting of husband and wife ornithologists has gathered years of research about the white-throated sparrow, Zonotrichia albicollis. The bird has a tan morph and a white morph, where there are males and females with white stripes on their head and males and females with tan stripes on their head. This difference in coloration is caused by a genetic mutation in Chromosome 2. Somehow, the chromosome was inverted so a large section of genetic information could not link up with a partner chromosome in reproduction. This led to the formation of a "supergene", allowing the chromosome and its information to evolve like how a sex chromosome would.
Over time, it was discovered that the sparrow acted as if it had four sexes instead of two. The difference in chromosomes allowed the birds to evolve into the different morphs with their own characteristic physical attributes and behaviors. The tan-striped birds are monogamous and overprotective of their offspring, while the white-striped birds are promiscuous and poor parents. The sparrow morphs do not mate with each other; tan-striped birds do not mate with tan-striped birds. Instead, in what is called disassortive mating, tan-striped birds almost always mate with white-striped birds, creating a further divide and establishment in the evolution of the four sexes.
This development in research allows so many different questions to be pursued. The role of genes in behavior can be investigated; the ability of an autosome to mutate and evolve like it is a separate sex chromosome can be analyzed; and finally, we can see if the formation of "supergenes" could possibly be used to benefit us or other organisms somehow.