It might seem like everywhere we look, we see evidence of natural selection: organisms seem to be pretty well adapted to their environments. But the neutral theory of molecular evolution suggests that most of the genetic variation in populations is the result of mutation and genetic drift and not selection.

Basically, the theory suggests that if a population carries several different versions of a gene, odds are that each of those versions is equally good at performing its job — in other words, that variation is neutral: whether you carry gene version A or gene version B does not affect your fitness.

The neutral theory is easily misinterpreted. It does NOT suggest:

• That organisms are not adapted to their environments
• That all morphological variation is neutral
• That ALL genetic variation is neutral
• That natural selection is unimportant in shaping genomes

The main point of the neutral theory is simply that when we see several versions of a gene in a population, it is likely that their frequencies are simply drifting around. The data supporting and refuting the neutral theory are complicated. Figuring out how widely the neutral theory applies is still the topic of much research.