A Study of Reaction Norms in Natural Populations of Drosophila pseudoobscura

Gupta, Anand; Lewontin, Richard

A study of the reaction norms on the number of bristles, viability and development time is presented for 32 strains isochromosomal for second chromosome and their heterozygotes from three different natural populations of Drosophila pseudoobscura, at two egg densities and three temperatures. Although there were average effects of genotype, temperature, and density, there were very large geno-type-environment interaction effects. That is, the genotype with the highest bristle number or fastest development at one temperature did not maintain its difference from other lines at other temperatures. About 30-45% of pairwise comparisons between genotypes showed actual reversals in relative position when temperature was changed. Thus, it is not possible to characterize a genotype as having a higher bristle number or faster development than another, since this can only be relative to a given environment. Naturally occurring genotypes differ in this respect from classical mutants which may overlap wild types in some environments but always vary at least in one direction from normal. It is, therefore, a mistake to infer the developmental behavior from laboratory mutants. Evidence is also presented that without knowing the norms of reaction, the present distribution of environments, the present distribution of genotypes, and without then specifying which environments and which genotypes are to be fixed or eliminated, it is impossible to predict whether the total variation would be increased, decreased, or remain unchanged by environmental or genetic changes, or what the outcome of natural selection would be.

A Study of Reaction Norms in Natural Populations of Drosophila pseudoobscura

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