The empirical motivation of the neutral theory In the pre-molecular days , the common view was that there wasn't all that much genetic variation, because we couldn't see very much Within species. In Drosophila several hundred variant genes had been patiently studies, but usually there was an overwhelmingly common 'wild type' and a very rare mutant type Between species. But if you can't cross individuals of different species, you can study the genetic variation.
Selection requires deaths.
So there is, in a sense, a cost when a new mutation replaces another. Many individuals of the disfavored type must die to change the frequency Kimura calculated that for the number of allozyme differences between species, for example in mammals, required too many genetic deaths to have occurred purely by selection. Similarly, there was too much variation within species for it to be maintained by selection alone. Kimura and the neutralists argued that selection was not the answer to this problem of molecular variation, drift was the answer. The neutral theory Most of the genetic variation that we see , both within and between species, is explained by mutation and drift of selectively neutral alternatives, not selection Some things this statement does NOT mean Most genes are functionless.