The neutral theory
Everywhere we look, we see evidence of natural selection: organisms are generally well-adapted to their environments. While this is certainly true, all populations have genetic variation that may or may not affect the fitness of the organism. 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. According to this theory, if a population carries several different alleles of a particular gene, odds are that each of those alleles is equally good at performing its job in other words, that variation is neutral: whether you carry allele A or allele B does not affect your fitness.
So according to the neutral theory, 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 theory applies is still the topic of much research.
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
Most importantly, the neutral theory is NOT an alternative to the theory of evolution; it is simply an elaboration of a small part of evolutionary theory. All the evidence discovered through investigations of the neutral theory also supports the theory of evolution.