Hybrids form when 2 closely related species mate in a contact zone where their distribution ranges overlap. Usually, hybrids are sterile and so the 2 species maintain their identities. My one and so far only (minor) contribution to the literature on hybridization was the subject of this post.
Today at the University of Maryland I had the pleasure to attend a seminar by a major player in the field, Jerry Coyne of the University of Chicago. Dr. Coyne's research is about 2 species of fruit flies, one of which, Drosophila santomea, lives on the island of São Tomé, while the other, D. yakuba, lives on the mainland Africa and also on the same island. Apparently, D. santomea descended from colonizing individuals of the mainland species cast on the island several tens of thousands years ago. Subsequently, when more flies arrived from the mainland, especially after the Portuguese came and cleared the low elevation forests, the allopatric speciation was more or less complete. Hybrids do form, but the male flies are sterile. The biological species concept, defined by Coyne as "a group of interbreeding organisms that shows substantial reproductive isolation from other such groups in nature", holds for each fly species.
All is well until we take a closer look at the hybrid zones up the mountain on São Tomé. It turns out that there is not one, but 2 hybrid zones. The 1st one is where it is supposed to be, between the ranges of the low elevation D. yakuba and the high elevation D. santomea. The 2nd one is, however, located where there should be no hybrids; actually no flies, because it's too cold: near the peak of the mountain. Moreover, in both hybrid zones only male hybrid flies have so far been found, although female hybrids exist in laboratory populations.
Mysteries remain and our need to unravel them is what makes doing and learning about science fun. My lack of understanding of most of the genetics behind Dr. Coyne's research did not prevent me from enjoying his talk.