Excerpt from Science Now "Gut Microbes Can Split a Species"; see link above to read the Science article
Here's how to create a new species. Put animals—say finches—from the same species on separate islands and let them do their thing for many, many generations. Over time, each group will adapt to its new environment, and the genomes of the two populations will become so different that if you reintroduce the animals to the same habitat, they can no longer breed successfully. Voilà, one species has become two. But a new study suggests that DNA isn't the only thing that separates species: Some populations diverge because of the microbes in their guts.
The paper is "important and potentially groundbreaking," says John Werren, a biologist at the University of Rochester in New York. "Scientists have studied speciation … for many years, and this opens up a whole new aspect to it."