The forage ratio and Ivlev's electivity index are common measures to quantify food selection but the values of both indices depend not only on the extent of selection but also on the relative abundances of the food types in the environment. They are therefore useless when food types with different relative abundances are compared, or when the relation between selection and relative abundance is studied. Modified versions of both indices are proposed which are based directly on the rates of decrement (mortality) of the food due to feeding, and are independent of the relative abundance.

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The study addresses the problem of gene pools and units of microevolution in mainly apomictic species. The genetic variation of Daphnia cucullata at the phosphoglucose-isomerase locus was studied in 6 lakes for 3-6 consecutive years. The two most distant lakes were 12 km apart. All lakes were sampled once a year; in one of the lakes (Klostersee), the main basin (WT) and the largest bay (WB, volume l/7 of WT) were sampled every month for 2; years.Three alleles and all six expected genotypes were found. Spatial differentiation was substantial, indicating lake-specific selection. There was a geographic gradient of allele frequencies without an apparent environmental gradient, suggesting exchange processes. Genotype frequencies in WB and WT showed marked fluctuations. They differed consistently between the two parts of the lake yet were highly correlated. The correlation was best if the WB frequencies were allowed to lag about one month behind the WT frequencies. This is to be expected if bay and basin experience different selection pressures but exchange parts of their populations, which requires some time. No parallel time lags were observed in environmental or demographic parameters. There was a significant correlation between fecundity (eggs/female) and genotype frequency, suggesting clone-specific differences in fitness.Biparental reproduction was very rare and superimposed on a continuum of parthenogenesis. Deviations from Hardy-Weinberg proportions were common and significant unless one allele was predominant. The direction of deviation of a given genotype frequency was usually the same in all lakes, suggesting overall differences in fitness between genotypes apart from lake-specific conditions of selection.

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A modification of the logistic model is proposed which incorporates the principles of optimal density and cooperation. Three components have been added defining effectivity, "saturation" and exponentiality of cooperation. By adequate choice of constants a wide array of population characteristics may be represented including threshholds of growth. The main application of the model concerns, low density problems such as colonization and extinction.