When Eyes Become Expensive
The Mexican blind cavefish, Astyanax mexicanus, is the same species as its sighted river-dwelling relatives. Put them side by side and you see almost identical bodies, except the cave population has no eyes at all, just smooth skin where the sockets would be. This happened across roughly 20,000 generations. Eyes are metabolically costly organs. In permanent darkness, maintaining them offers no return, so evolution stopped paying the bill. The genes responsible for eye development didn't vanish, they were redirected. Research published in the journal Current Biology found that the same genetic pathways that would have built the cavefish's eyes were repurposed to expand its lateral line system, a network of pressure-sensitive cells along the body that detects water movement with extraordinary precision. The animal didn't simply lose something. It reinvested the biological budget.
