Daniel R. Matute of the department of ecology and evolution at the University of Chicago has published a study of two species of Drosophila, that show, for as far as I am aware the first time, the presence of postmating prezygotic isolation. Specifically, that means a barrier or barriers that act after mating but before fertilization.
The paper appeared on March 23, in the online journal Public Library of Science (PLoS) Biology. It is important from an evolutionary standpoint as a demonstration of the diversity of ways that reinforcement by gametic isolation can bolster isolation between species or proto-species.
Following is the abstract from the paper. For the complete paper, please see the following URL: http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1000341.
Reinforcement, a process by which natural selection increases reproductive isolation between populations, has been suggested to be an important force in the formation of new species. However, all existing cases of reinforcement involve an increase in mate discrimination between species. Here, I report the first case of reinforcement of postmating prezygotic isolation (i.e., barriers that act after mating but before fertilization) in animals. On the slopes of the African island of São Tomé, Drosophila yakuba and its endemic sister species D. santomea hybridize within a well-demarcated hybrid zone. I find that D. yakuba females from within this zone, but not from outside it, show an increase in gametic isolation from males of D. santomea, an apparent result of natural selection acting to reduce maladaptive hybridization between species. To determine whether such a barrier could evolve under laboratory conditions, I exposed D. yakuba lines derived from allopatric populations to experimental sympatry with D. santomea, and found that both behavioral and gametic isolation become stronger after only four generations. Reinforcement thus appears to be the best explanation for the heightened gametic isolation seen in sympatry. This appears to be the first example in animals in which natural selection has promoted the evolution of stronger interspecific genetic barriers that act after mating but before fertilization. This suggests that many other genetic barriers between species have been increased by natural selection but have been overlooked because they are difficult to study.