Hybrids form when 2 related species mate in a contact zone where their distribution ranges overlap. The closer the 2 species are genetically, the more likely it is that they will produce hybrid individuals. There is a vast number of publications out there on almost every aspect of hybrids and hybridization. A meager contribution to the literature from yours truly and Francisco Welter-Schultes just got published in Schriften zur Malakozoologie1.
We did this study to determine if 2 land snail species, Albinaria munda and A. lerosiensis, were hybridizing in a contact zone in the Bodrum Peninsula in western Turkey. The 2 species can be distinguished from each other by several characteristics of their shells. So we reasoned that if there were hybrids their shells would be intermediate in at least some of those characteristics and, therefore, detectable using various conchological analyses.
Albinaria munda (left) and Albinaria lerosiensis.
There is a good review by Jiggins & Mallet (2000) explaining what sorts of genotype and phenotype distributions one could expect in potential hybrid zones. Basically, if there are no hybrids, a trait being examined will form a bimodal distribution, one for each species, if the distributions are sufficiently separated from each other. If there are mostly hybrids, then the distribution for that trait will be unimodal; but if there is a mixture of 2 parental and 1 hybrid populations, then the distribution will be flat-topped.
The individual frequency distributions for all the conchological traits we examined were normal giving rise to bimodal distributions when combined. These results indicated that hybrids were either absent or rare. The results of one such analysis is in the figure below.
This is a plot of the number of ribs in the ultimate whorl (Ru) versus the ratio of the height of the 1st 2 whorls above the aperture to the shell diameter for 59 shells of A. lerosiensis (triangles) and 96 shells of Albinaria munda (circles). The arrow points at the only possible hybrid.
Our conclusion is that the 2 species are reproductively isolated and, therefore, may have diverged from each other during the Miocene, at least about 5 million years ago.
The details are in the paper. Incidentally, the pdf version that I have uploaded is a twice-scanned copy and expectedly, not a good one. If and when I get a better one from the journal I will substitute it.
Örstan A. & Welter-Schultes, F. 2007. Reproductive isolation between two Albinaria species in a contact zone (Gastropoda: Clauisliidae). Schriften zur Malakozoologie, 23:33-38. pdf
Jiggins, C.D. & Mallet, J. 2000. Bimodal hybrid zones & speciation. TREE 15:250-255.