Researchers have found the first evidence in plants of a 'ring' species — one that arises when a population expands its range along two fronts in a ring around unsuitable habitat. When organisms from the two groups meet, they behave as separate species.

Credit: I. CACHO

Ivalú Cacho and David Baum at the University of Wisconsin-Madison sequenced two genes from 40 populations of Caribbean slipper spurge (Euphorbia tithymaloides). The sequences revealed that the plant spread from Mexico along two trajectories: one population (pictured right) headed through South America and then north to the Lesser Antilles, and the other (left) spread by hopping east across islands such as Jamaica and Puerto Rico.

The two populations converged on the Virgin Islands, where they coexist as distinct subspecies on the island of St Croix. Despite being in close proximity to one another, there has not been significant gene flow between the subspecies.

Proc. R. Soc. B (2012)