Phenotypic and genotypic structure of a natural Drosophila population with respect to the reaction of individuals to an increase in population density and its seasonal change

: In spring and summer, the natural population of Drosophila studied exhibited a high level of phenotypic and genotypic heterogeneity with regard to change of individual fertility with an increase of density of adult flies. From spring to autumn, the level of genotypic variation decreased. In autumn, all phenotypic variation resulted from environmental factors. In spring, individuals sensitive to overcrowding, adapted to low density, and highly fertile in low-density conditions prevailed in the population. In summer and autumn, the population was mostly composed of individuals with opposite characteristics. On the basis of the obtained results, a conclusion on operation of cyclic density-dependent selection in the population examined was made. Selection's functional role in control of seasonal population size dynamics is discussed.