When two populations of a species become geographically isolated from each other, their genes diverge from one another over time.
Eventually, when a male from one group mates with a female from the other group, the offspring will die or be born sterile, as crosses between horses and donkeys produce sterile mules. At this point, they have become two distinct species.
Now, Cornell researchers report in the October issue of Public Library of Science Biology (Vol. 7, No. 10) that rapidly evolving “junk” DNA may create incompatibilities between two related species, preventing them from reproducing.
In this case, the researchers studied crosses between closely related fruit flies, Drosophila melanogaster and D. simulans. Nearly 100 years ago, scientists discovered that when male D. melanogasters mate with female D. simulans, normal males survive, but the female embryos die.