Argument that most of the mutations that become within populations are neither beneficial nor detrimental.
Basically, beneficial mutations are considered to be rare, relatively to neutral ones, while detrimental mutations, though common, are less likely to become fixed. Thus, neutral mutations will tend to predominate in terms of accumulation within populations, and this tendency is a consequence of natural selection (which is responsible for both limiting the likelihood of fixation of detrimental alleles as well as, arguably, the potential for an allele to be beneficial).
Though not always the case, neutral theory stands as a reasonable default assumption as well as in contrast to , including in terms of the accumulation of neutral variation within populations.
Neutral theory underlies the concept of molecular clocks.