Racial divergence in fin ray variation patterns inGasterosteus aculeatus

Abstract

1. Dorsal, anal and pectoral fin ray numbers are shown to be modifiable by temperature and salinity conditions during embryonic development, in both western European population groups ofGasterosteus aculeatus. This modification follows a characteristic pattern in relation to both factors mentioned. Temperature lability varies from low to high according to the salinity. 2. The temperature variation pattern is different for the two population groups. High temperature lability in dorsal and anal fin rays is shown to exist at low concentrations for the fresh-water populations (A-type), at high salinities for the salt-water populations (B-type). Modification of dorsal and anal fin rays is almost absent at high concentrations for theA-type, at low concentrations for theB-type. Pectoral fin rays show a reversed pattern in each of the groups. 3. The results are discussed with respect to the facts reported from natural populations, and as to their bearings on the methods employed in the study of evolutionary divergence.