Say 7.5 million years since the last common ancestor, 25 years/generation in both species, that’s 300,000 generations.
Say 100 mutations/person/generation (mostly effectively neutral). Expect genetic drift to fix 100 mutations/generation. (That’s the standard assumption.) That’s 30 million mutations fixed by genetic drift per species.
3 billion base pairs in the genome (similar for both species) so that’s 1% in each species; 2% difference between species.
Please note, Richard Buggs is the expert, not me, so I’m accepting his figures.
The average is 88.9% so let’s round up to 90%. Neutral theory can account for about 1/5 of the genetic difference.
However since these are neutral mutation they probably have very little effect on the phenotypical differences between humans and chimps.
If we look at beneficial mutations ReMine has calculated that only 1670 beneficial mutations can have been fixed in 10 million years through natural selection. (ReMine’s paper was rejected by Nature not because of any error in his maths or conclusions but because the reviewers thought this was already well known.)
Perhaps some population geneticist can tell us how fixing of beneficial mutations would affect fixing of neutral ones. I assume some would ride on the coat-tails while others would be adversely affected.
I’m seeing major problems in this maths.