POPULATION-STRUCTURE IN DAPHNIA-OBTUSA - QUANTITATIVE GENETIC AND ALLOZYMIC VARIATION

Quantitative genetic analyses for body size and for life history chara cters within and among populations of Daphnia obtusa reveal substantia l genetic variance at both hierarchical levels for all traits measured . Simultaneous allozymic analysis on the same population samples indic ate a moderate degree of differentiation: G(ST) = 0.28. No association s between electrophoretic genotype and phenotypic characters were foun d, providing support for the null hypothesis that the allozymic varian ts are effectively neutral. Therefore, G(ST) can be used as the null h ypothesis that neutral phenotypic evolution within populations led to the observed differentiation for the quantitative traits, which I call Q(ST). The results of this study provide evidence that natural select ion has promoted diversification for body size among populations, and has impeded diversification for relative fitness. Analyses of populati on differentiation for clutch size, age at reproduction, and growth ra te indicate that neutral phenotypic evolution cannot be excluded as th e cause.