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The viabilities between egg and adult life stages of Drosophila pseudoobscura karyotypes were studied at low, intermediate, and high frequencies. The viabilities of pairs of karyotypes were compared at each frequency and the viabilities of the three karyotypes, at one combination of frequencies. Eggs were counted into vials and samples taken of the adults emerging after viability selection. ST and CH gene arrangements of the third chromosome carrying different amylase alleles were used, and the karyotypes of adult flies were scored by gel electrophoresis. A statistical method related to the loglinear model was developed for estimating viabilities. This method takes account of the additional variability between replicates common in experiments of this kind and allows testing of nested hypotheses about the mechanism of selection. The viabilities of the homokaryotypes relative to the heterokaryotype were significantly higher at the low homokaryotypic frequency than at the higher ones. These viabilities do not show a consistent heterozygote advantage. This pattern of frequency-dependent viabilities will lead to a protected polymorphism for the gene arrangements, even in the absence of heterozygote advantage.

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The objective of this study is to examine the effects of developmental stage on amylase expression in the mosquitofish (Gambusia affinis holbrooki, Poeciliidae). Expression was characterized in terms of amylase activity and amount of amylase protein in five developmental stages: early, mid, and late embryos, 5-day-old juveniles, and adults. Two measures of activity were used: a starch-iodine assay, which measures the change in substrate, and the dinitrosalicyclic acid (DNSA) assay, which measures the change in product. To measure changes in the amount of amylase, the enzyme-linked immunosorbent assay (ELISA) was used. Highest activity and amount was observed in the adult stage. In general, both increased with development. While the two activity measures were positively correlated, these were only weakly correlated with the amount of amylase. Possible reasons for these weak correlations between amylase activity and amount are discussed.

The method used by Fisher and Ford (1947) to study the spread of a gene in a natural population has been modified to analyze the variation in allele frequencies from generation to generation in a common experimental procedure. A further analysis has been developed that is more sensitive to directional trends in the allele frequency over generations, and its use in detecting the action of directional selection on gene frequency at a locus is discussed. The power of each of these statistical tests is calculated for a number of cases, and the tests are applied to sets of isozyme data from Drosophila pseudoobscura and Zea mays.