How Supergenes Beat the Odds—and Fuel Evolution

Supergenes can also complicate the process of mating. In some species, supergenes create a breeding system that in effect has four sexes. Because of a supergene in the North American birds called white-throated sparrows, for example, there are two “morphs” with dissimilar coloration and behaviors. Not only do males have to find females, but they must find a partner from the opposing morph. Otherwise, offspring will die either from inheriting supergenes from both parents or from inheriting none. Only chicks that receive a “balanced lethal” inheritance of one supergene and one ordinary segment of chromosome survive.

With such a steep price, it’s a wonder that supergenes evolved at all, Berdan says. “Any set of variants is going to be really hard to maintain, especially over millions of generations,” she said. “That’s one of the big mysteries of supergenes.” She suggested that multiple types of selection might be working together to preserve supergenes, and that certain environments might be most conducive to their persistence in the population.

Ironically, one of…