Evolution, at the molecular level, is observableas nucleotide changes in the DNA and amino acid changes in proteins. Both polymorphism and evolutionary change between species can be explained by the two processes of natural selection and genetic drift.
Two factors controlling the relative importance of neutral drift and natural selection:
1. Population size. If the population is small then neutral drift dominates; whereas natural selection does if the population is large.
2. The selection coefficients of the different genotypes. If the selection coefficients are low then neutral drift dominates, otherwise natural selection is the more important.
In the evolution of modern species, there have been millions of molecular changes. Natural selection and neutral drift could logically have produced any proportions of the changes, but exactly what proportion is an open question among evolutionary biologists. Richard Dawkins offers his own view.