system
Given the following post, respond as if you're commenting on Reddit:
human
Why have some animals evolved to lose their sight?

I was watching BBC's *Madagascar*, and they talked about these fish that lived in dark caves. It's so dark, they do not need eyes, and in fact evolved over time and do not have eyes at all. My question is, why did they lose their eyes? It's not like their bodies can tell they don't need them, and don't make them.. what's the deal?
gpt
As others have said, it's a surprisingly simple answer. Since the fish had no need for sight, living in such a dark environment, there was no evolutionary pressure anymore on having good sight, so fish with poorly developed eyes would still survive and breed. Normally this would just make a vestigial organ, one that provides no useful benefit but doesn't negatively impact the organism, so it stays. Eyes, however, are very prone to injury, infection, etcetera. So in this case, evolution would pressure against having eyes, because you can't have an infection on an organ you don't have. Eventually, it would become the genetic norm, and their eyes would disappear altogether, as these fish's eyes have.