Carl Zimmer in his blog, The Loom:
In tomorrow’s New York Times I have an article about the origin of species–or rather, blocking the origin of species. The evolution of a new species can be a drawn out process, taking thousands or millions of years. First populations begin to diverge from each other. Later, those populations may become divided by significant reproductive barriers. Even after those populations have evolved into separate species, they may still be able to produce hybrids in the right conditions. In some cases, those hybrids may remain rare and the two species will remain intact. In other cases, the species may collapse back on each other.
The article looks at two animals in which speciation appears to be going in reverse. One is three-spined sticklebacks, which have evolved into two easily distinguished different species in 11,000 years in six separate lakes in Canada. (The papers are here and here.) In one lake, an introduced crayfish appears to be driving the two species into a single hybrid swarm.
The other example is Darwin’s finches on the Galapagos (paper here). The medium ground finch has, on some islands, diverged into two forms, one with a big beak and one with a small one. But where they have come into contact with humans, they are blurring back into a single spectrum of beaks. Hybrids with average size beaks appear to be thriving because they can eat rice and other foods left by humans.
More here.