This observation supports the idea that selection can and does produce reinforcement. Correlation of genetic distance with prezygotic isolation and with postzygotic isolation, for comparisons of many sympatric and allopatric species of Drosophila , shows that prezygotic isolation is higher among species with lower genetic distance. Since genetic distance may be roughly proportional to divergence time, correlaton of genetic distance with prezygotic isolation can be used to estimate the time needed for speciation. In the case of these Drosophila species, it is between 1.5 and 3.5 million years. Chromosomal changes could potentially lead to speciation.
Chromosomal rearrangements can establish postzygote isolation in a single step.
If chromosomal rearrangements could become established, they could provide a basis for rapid speciation. A rare rearrangement would be unlikely to be paired with a matching chromosome and would therefore be selected against. However, rearrangements could become common enough to yield a successfully interbreeding population, either by something like molecular drive or by inbreeding in which siblings would be likely to share the rearrangement. Chromosomal differences between species are fairly common, but they also occur within species. The importance of chromosomal changes for speciation remains unknown.