To adapt to their eternally dark surroundings, Mexican cavefish have gone blind, lost their pigment, and changed their sleep cycle. You can see one in the photo up top. Here's the weird thing though...all those fish are the same species.
The cavefish, known as Astyanax mexicanus, is really the same species as a surface-dwelling fish known as the tetra. While they don't look much alike anymore, the two types of fish can still interbreed, meaning that they're just isolated populations. Researchers had long thought that these were two different species. Long ago, the ancestors of the cavefish went underground, beginning the evolutionary processes that robbed their descendants of their sight and color. A second, completely distinct fish species then colonized the now empty surface waters.
The actual story is far messier. Different populations of tetra went underground at different points, and all of these distinct cave-dwelling populations came up with the same solutions, all going blind and becoming albino. This is an example of a phenomenon known as convergent evolution, and yet in all cases the changes weren't drastic enough to cause the fish to evolve into a distinct species.
Indeed, not only do the different cave and surface populations - there's about a dozen of each throughout Mexico - remain connected, the different types of tetra still interbreed. Some caves that are relatively close to the surface see a huge genetic influx from surface-dwelling fish. However, despite the constant mixing of the surface and cave phenotypes, you're unlikely to find many sighted fish deep inside the caves. Research leader Dr. Martina Bradic of New York University explains:
"Despite interbreeding and gene flow from the surface populations the eyeless 'cave phenotype' has been maintained in the caves. This indicates that there must be strong selection pressure against eyes in the cave environment. Whatever the advantage of the eyeless condition, it may explain why different populations of A. mexicanus cave fish have independently evolved the same eyeless condition, a striking example of convergent evolution."
Via BioMed Evolutionary Biology. Image by Professor Richard Borowsky.