Carl Zimmer has an article in the NYTimes Science section on how humans can interfere with diverging populations, increasing the frequency of hybrids and preventing speciation. He give two examples: three-spine sticklebacks in British Columbia, Canada and ground finches in the Galapagos. The sticklebacks colonized lakes on Victoria Island from the Pacific Ocean and became specialized populations in the new environment. Hybrids between different ecomorphs were selected against because of their intermediate phenotype. Zimmer reports that there are an excess of hybrids in recent samples taken from the lakes, and that human introduction of crayfish may be to blame. The original study can be found here.
The other example Zimmer presents are the ground finches Darwin described in the Galapagos. We all know the story about the different sized beaks and how each is adapted to a specific diet. If you don’t know the story, read this book. On Santa Cruz Island, the birds come in two different flavors (large beaks and small beaks), but a population near a human settlement has way too many hybrids. This may be because humans feed the birds rice, removing the selection pressure for beak specialization. The original article can be found here.
Zimmer has more at the Loom.