Anatomical features that took millennia to evolve can revert in a single generation. Specifically, sex between blind cavefish, if done right, can lead to progeny that can see.
The blind, albino, cave-dwelling form of the Mexican tetra (Astyanax mexicanus) evolved from ancestors living near the water’s surface whose eyesight withered after they descended into complete darkness roughly one million years ago. These cavefish, which thrive in the freshwater caves of northeastern Mexico, can reach about 12 centimeters in length, and skin grows over their useless eyes.

Studies revealed that eye loss evolved independently at least three times among the cavefish; in each case, blindness resulted from mutations at several gene sites. At least some of the mutations responsible for this loss of vision differed between the 29 populations of cavefish known to exist. This mutational variety suggested that breeding different cavefish lineages together might result in offspring that could see, because the genetic deficiencies of one group might be compensated for by working genes in another.

In the January 8 Current Biology, evolutionary biologist Richard Borowsky of New York University found that the hybrid progeny of different cave populations all indeed had some fish with smaller than normal yet functional eyes capable of following a series of moving stripes. The farther apart the home caves of the blind parents were, the more likely their progeny had sight, Borowsky adds.

This pattern supports the notion that lineages separated by greater distances are probably more distantly related and so have less overlap in terms of the genes behind their blindness. Identifying the specific mutations underlying vision loss in the fish could help illuminate human eye development and blindness.