Selection and the effect of temperature on scutellar bristle number in Drosophila.

ELECTION of a genotype which gives rise to similar phenotypes in different environments is one evolutionary strategy which may result in high adaptedness of a population to the environments in which it lives. Presumably, the stability of certain traits is the outcome of a long history of natural selection tending towards an adaptive phenotype. Several investigators have attempted to select strains of Drosophila for phenotypic stability of variable traits (FALCONER 1957; THODAY 1959,1960; RENDEL and SHELDON 1960; PROUT 1962; SCHARLOO 1964). Interestingly, the stability of a trait with respect to small (microenvironmental) and to more drastic (macroenvironmental) variations generally seemed to be independent. When selection reduced phenotypic variance in a given environment, responsiveness to macroenvironmental variables was not affected. Likewise, selection for decreased responsiveness to major environmental differences did not increase microenvironmental buffering ( WADDINGTON 1960; KINDRED 1965). RENDEL and SHELDON (1960) obtained different results. They succeeded in reducing the phenotypic variance of scutellar bristle number around a mean of two bristles in scute Drosophila. This reduction in variance at 25°C was accompanied by lowered sensitivity of bristle number to temperature changes. The experiments reported in the present article involved selection for altered temperature responsiveness of scutellar bristle number and were designed to study the relation between microenvironmental and macroenvironmental buffering. These experiments are in some ways similar to those of KINDRED (1965) although they were done independently.