Like mathematician Terence Tao hasn't heard that one before, hyuk. But he gives a nice account of the Grants' work on introgressive hybridization of ground finches:
This was all very reasonable and predictable, but it led to an interesting puzzle - given the modest genetic pool of the Geospiza scandens population, how was it that both the small-beak genes and large-beak genes survived for millions of years, given that selective pressures tended to strongly favour one over the other every decade or so?
The answer, hypothesised and then confirmed by Grant and her collaborators, was introgressive hybridisation - the occasional sharing of genes between Geospiza scandens and Geospiza fortis due to interbreeding.
We didn't include the finches in our paper on introgression,, but it's a well-documented example. For the finches, the clear importance of reinforcement selection on the species barrier between the forms means that their species difference is greater than it would be in the absence of such selection -- and in my thinking probably greater than species barriers in early hominids.
(via Gene Expression)