Gary Marcus explains how the blind watchmaker designs:
Evolution is often likened to a process of climbing a mountain. If one follows the strategy of taking small steps, only going up (never down) and is willing to keep at it long enough, one will inevitably make it to the top of the mountain.
Alas, the strategy of small upward steps can only take a mountain climber so far. If the mountain is a single smooth surface with just one peak (like Mount Fuji), all is well. But if the mountain range looks more like the Himalayas, rough, uneven and full of peaks and valleys, there’s a distinct chance of getting stuck on a low peak that is by no means the highest peak in the range.
Neither evolution nor Darwin ever promised anything like perfection. Evolution is not about creating perfect or optimal creatures, which would require forethought, but only about the fact that the genes of creatures with modest advantages (“fittest” among those that happen currently to be alive) tend to spread throughout the population.
Darwin’s theory of natural selection tells us that a one-eyed creature may outcompete a blind creature, but that doesn’t mean that a creature with two eyes couldn’t come along later.
This seemingly subtle difference — between “fittest among the choices that happen to be lying around” and “fittest imaginable” — makes all the difference in the world.
Biologist PZ Myers agrees: “evolution does not produce rational, perfect, finely-tuned beings. It makes organisms that are good enough.”