Microorganisms in a Siberian hot spring have been caught in the act of diverging to form two species.

Rachel Whitaker at the University of Illinois in Urbana-Champaign and her team sequenced the genomes of 12 strains of Sulfolobus islandicus — an archaeon living in hot springs around a volcano on Russia's Kamchatka Peninsula. The sequences show that the microbes belong to two groups — dubbed red and blue — and that genes are exchanged more readily within than between the groups. This means that each group of microbes meets the classic definition of a biological species.

The authors think that physiological differences between the two populations could be preventing gene flow. For instance, strains of the red group replicate faster and to higher densities than do those of the blue group.

PLoS Biol. 10, e1001265 (2012)