Salinity and subspeciation in Natrix sipedon.
Natrix sipedon confluens and N.s. clarki are interbreeding races of water snakes which retain their racial distinctions through the operation of environmental barriers to complete genetic migration. An analysis of specimens collected along a transect perpendicular to the zone on intergradation between the two races shows a gradient in morphological types corresponding to a similar gradient in environments. Laboratory experiments demonstrated that individuals of clarki, the brackish-water race, possess a higher resistance to salinity than confluens, the freshwater race. This difference is due to the fact that clarki will not drink seawater, but confluens will succumb to its effects. The existence of well-differentiated, salt-marsh inhabiting races of the common water snake is a deomonstration of the effectiveness of selection in maintaining special adaption in the subunits of a species population.