Evidence that seeds are the critical resource providing an ecological opportunity for these finches is severalfold. Finally, the most telling evidence that seed resources are critical to the birds comes from reduced survival of smaller-sized birds during extended droughts in which small seeds become rare. If the recolonizing population is genetically distinctive enough from the ancestral population, due to a combination of natural selection, mutation, genetic drift, etc., then the two species maintain their separate identities as incipient species. Evidence for reproductive isolation in such coexisting pairs of finch species comes from several sources. The expanse of ocean between islands apparently isolates populations to such an extent, and for long enough time periods, to allow genetic differentiation on the different islands.
Few birds undertake a flight out over open ocean, and incentives to do so are probably rare.
Thus populations come to differ over time in allopatry in response to natural selection, but mutation, founder effects, and other forms of genetic drift must also contribute to this differentiation. The completion of the speciation process requires that newly sympatric populations must not only remain genetically distinct, but they must also be sufficiently different ecologically to coexist. If they are genetically distinct but ecologically very similar upon recontact, then one will outcompete the other, driving it to extinction. At the other extreme, if the two populations diverge ecologically to a very large extent whle allopatric, then they may coexist without much if any interspecific competition when they become sympatric. The most likely scenario, however, is that the two populations will overlap each other ecologically upon becoming sympatric simply because no force operates systematically to cause their divergence when they are allopatric, in which case they must diverge ecologically when they first come into contact with each other.