8.Population Genetics - Neutral Theory
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Mutations introduce new alleles in populations and the evolutionary forces, mainly selection and genetic drift determine the fate of these mutations. Kimura’s neutral theory of evolution describes that effectively neutral mutations that become fixed in populations by genetic drift overwhelmingly outnumber beneficial mutations that become fixed by natural selection. In other words, genetic drift, not natural selection, is the mechanism responsible for most molecular evolution.
With respect to effect on fitness, there are three kinds of mutations. Some mutations are harmful, some are beneficial, and some are neutral. Mutations that do not affect fitness - do not affect individual survival and reproduction are neutral mutations.
Harmful mutations tend to be eliminated by natural selection and thus contribute little to molecular evolution. Beneficial mutations are often lost to drift while still at low frequency or tend to rise to fixation as a result of a very large selective advantage. However, neutral mutations fluctuate in frequency as a result of genetic drift. Many are lost, but some become fixed.
So, at the DNA level, variation can exist that has no effect on protein sequences, because multiple codons encode the same amino acid, and therefore neutral alleles do exist.
The neutral theory of molecular evolution is extraordinarily useful and serves as a null hypothesis that allows researchers to identify examples of positive selection on nucleotide sequences.