Neutral mutation

It can be shown that the neutral evolution rate exactly equals the neutral mutation rate, and is independent of population size.

In a population of size N there are a total of 2N genes at each locus. On average, each gene contributes one copy of itself to the next generation; but because of random sampling, some genes will contribute more than one copy and others will contribute none. Those genes that contribute no copies to the first generation cannot contribute copies to later generations: once a gene fails to be copied, it is lost forever. Every generation some genes will 'drop out', and be unable to contribute to future generations. Each generation, some of the 2N original genes are lost in this way.

If we look far enough forwards we always come to a time when all the 2N genes are descended from just one of the 2N genes now: all but one of the original genes have dropped out. That one gene will have hit a long enough run of lucky increases and will have spread through the whole population. It will have been fixed by genetic drift.