Aatish Bhatia in his excellent new blog, Empirical Zeal:
A world without light is quite an alien place. There are many examples of fish that live in completely dark caves. Remarkably, if you compare these fish to their relatives that live in rivers or in the ocean, you find that the cavefish often undergo a similar set of changes. Their eyes do not fully develop, rendering them essentially blind. They lose pigmentation in their skin, and their jaws and teeth tend to develop in particular ways. This is an example of what is known as convergent evolution, where different organisms faced with similar ecological challenges also stumble upon similar evolutionary solutions.
The changes mentioned above are all about appearance, but what about changes in behavior? In particular, when animals sleep, they generally line up with the day and night cycle. In the absence of any daylight, how do their sleep patterns evolve?
A recent paper by Erik Duboué and colleagues addressed this question by comparing 4 groups of fish of the same species Astyanax mexicanus. Three of the populations (the Pachón, Tinaja, and Molino) were blind cavefish that inhabited different dark caves, whereas the fourth was a surface-dwelling fish.