Instead of becoming extinct, why don't endangered species adapt to the changing environment and evolve? originally appeared on Quora - the knowledge sharing network where compelling questions are answered by people with unique insights.
There can be a couple of reasons. First, environmental change can happen too fast. Evolution requires generations to function, often many thousands of them, and environmental change can happen much faster than that. But sometimes even if the environmental change happens slowly enough a species still can't adapt to it. Imagine that the possible body shapes and the fitness of each are graphed like this:
The higher fitness shapes are in red while the lower ones are blue. As the environment changes, that peak is going to flatten and another is going to rise. In order to adapt, the species is going to have to traverse the fitness landscape to get to the new peak like this:
But remember, evolution is blind. It doesn't "know" that there's a peak in a particular direction. It only climbs hills that the population is already on. Often this will work fine, as the peaks are close enough or there's a series of peaks for the intermediate environments (effectively, the peak moves) and the population will traverse the landscape fine. But if the peaks are too far apart the population will never find the new one. Imagine that there's a lake (with very steep sides) that's drying up. As it shrinks, the aquatic fitness has decreased (there's less water) and the terrestrial fitness has increased (there's more land). But there's never a peak or ridge for the fish to traverse between the two, and so by the time the lake dries up, they're out of luck.