Data from: Incipient habitat race formation in an amphibian

Theory defines conditions under which sympatric speciation may occur, and
several possible examples of the process in action have been identified.
In most cases, organisms specialize onto habitats that fall into discrete
categories, such as host species used by herbivores and parasites.
Ecological specialization within a continuous habitat gradient is
theoretically possible, but becomes less likely with increasing gene flow
among clinal habitat types. Here, I show that habitat race formation is
underway in a frog, Rana temporaria, along a continuous and spatially
mosaic habitat gradient. Tadpoles from 23 populations raised in an outdoor
mesocosm experiment showed adaptive phenotypic variation correlated with
the predator density in their pond of origin. A survey of microsatellite
markers in 48 populations found that neutral genetic divergence was
enhanced between ponds with very different densities of predators. This
represents a new example of habitat specialization along a continuous
habitat gradient with no spatial autocorrelation in habitat.