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Enzyme gene variability has been studied by starch gel electrophoresis in 20 Finnish vendace (Coregonus albula) populations. Eighteen populations originate from fresh water and two from brackish water. Fourteen populations spawned in the time typical for the species, i.e. in the autumn, four in the winter and two in the spring. Genotype-environment interactions were studied with regard to several hydrological and spatial variables. Genetic polymorphism in vendace is relatively high for a fish. About half of the enzyme loci studied were polymorphic and the mean heterozygosity per locus per individual is 0.08. The degree of polymorphism is related to the metabolic function of enzymes. The average genetic distance between populations, based on five polymorphic loci, is 0.036 (ranging from 0.001 to 0.245) and 0.016 based on all loci studied. The genetic distances indicate that the winter- and spring-spawning populations are the most differentiated. They differ also very much from each other. This indicates that each one of them has arisen independently from populations spawning in autumn. The change in spawning time can be attributed to selection operating on zygotes and juveniles. This selection is due to adverse winter oxygen conditions associated with dystrophy and warm water. The genetic differentiation observed between populations may be due to natural selection or genetic drift or both.

The genic variability at twenty loci in marginal, submarginal and central populations of Drosophila subobscura has been studied by starch gel electrophoresis. The samples from marginal populations orginate from Finland, the submarginal ones from Sweden and Denmark, whereas flies collected from France and northeastern Italy are taken to represent central populations. The average degrees of genic polymorphism are undifferentiated over the distribution area of D. subobscura. Most loci have fairly uniform allele frequencies in all populations. Local differences have been found at six loci. These fluctuations represent adaptations to local environmental differences.

The time measuring system ofDrosophila littoralis strains originating between 40–70° N was found to be highly variable and latitude dependent. The critical daylength for photoperiodic adult diapause varied from 12 h or no diapause response in the south to 20 h in north. The median timing of pupal eclosion rhythm varied correspondingly from 21 h to 12 h from lights off in LD 3∶21, and the period of free-running rhythm of eclosion from 24 h to 19 h. The phase of the free-running rhythm was also variable, and correlated with the phase of the entrained rhythm. Latitudinal variation in the entrained rhythm of eclosion and in diapause is adaptive, leading to eclosion early in the morning and to overwintering at the adult stage. In some strains with a late phase of eclosion, strong transient cycles were seen following the transition from LL to DD. A total damping of the free-running eclosion rhythm within 2–7 days was common to all strains. This damping was more pronounced in the northern strains. The phase and period of eclosion rhythms were statistically independent. Diapause was not correlated with any parameters of the eclosion rhythm in the analysis. Diapause may still be influenced by the period of the eclosion rhythm, even though its minor contribution may be masked by a more variable, eclosion rhythm independent system in the determination of diapause.