96-102: More on eyes
From Carl Zimmer, Eyes, Part Two: Fleas, Fish, and the Careful Art of Deconstruction. Different populations of cave fish in Mexico lose their eyes the same ways. The different populations each evolved from the same eyed morph, and had the same evolutionary pathway to eyelessness. Why? Because the same gene that causes eyes to develop correctly slows other growth. This would be a poor design for a system, but as an evolutionary hypothesis it gets us testable predictions and a whole host of fascinating new questions about development, ecology and evolution.
Also, fleas have simple eyes, but are descended from an ancestor with sophisticated eyes. Why is there still evidence of stabilizing selection on the function of photosensitive chemicals? Because photosensitivity is useful outside the eyes. In addition to producing evolutionary questions, and using evolutionary hypotheses, it shows how eye functions can be reduced to simpler parts.
Updated 2/17/05 8:42am: Zimmer points out that it's different populations, not different species.