Map of the geographic distribution of an idealized ring species. Two forms (red and blue; species A and B) have come into contact (perhaps with some overlap) but do not interbreed directly. They are connected by a long chain of populations encircling a geographic barrier, through which the traits of species A gradually change into the traits of species B. If the order of colonization can be inferred, then one can infer the location of the common ancestor (here, in yellow) and how range expansion around the barrier and the accumulation of small evolutionary changes led to the formation of two species.
Irwin BMC Biology 2012 10:21 doi:10.1186/1741-7007-10-21