PELVIC GIRDLE POLYMORPHISM AND REPRODUCTIVE BARRIERS IN THE NINESPINESTICKLEBACK PUNGITIUS-PUNGITIUS (L) FROM NORTHWEST RUSSIA

Populations of the ninespine stickleback with the complete loss of the pelvic girdle are described from Russia. In some areas in northern Ka relia, these populations are reproductively isolated from parapatric p opulations of the 'wild type' with pelvic girdle, while in adjacent ar eas, populations are polymorphic and have intermediate girdle phenotyp es. This seems to reflect the initial stage of divergence. Experiments in large ponds show a selective advantage of the loss of the pelvic g irdle under pressure from insects predators and a selective disadvanta ge under predatory fish. Mate choice tests indicate reproductive isola tion between pelvic phenotypes.