GENETIC DIVERGENCE IN BODY SIZE AMONG EXPERIMENTAL POPULATIONS OF DROSOPHILA PSEUDOOBSCURA KEPT AT DIFFERENT TEMPERATURES

The body dimensions of many animals vary in a regular way with environmental temperature, and the several "ecogeographical rules" have been formulated to describe these situations (see Mayr, 1963, for a review). The rules are most often discussed for homeotherms, since a possible connection with conservation of body heat arises, but many organisms with no internal regulation of body temperature vary in a similar way (Ray, 1960; Lindsey, 1966). The morphological variation may be simply a phenotypic response to temperature, reflecting developmental plasticity, or it may be partly or wholly genetic. Genetic differences in morphology are established from comparisons among geographic strains reared under the same, or nearly the same, conditions. The probable role of selection in producing the genetic differences is inferred from a correlation of genetic variation in morphology with environmental variation in temperature. Particularly for poikilotherms such as insects, a physiological basis for selection by environmental temperature or its correlates is often not apparent. Genetic variation in morphology, as measured by such characters as wing and thorax length, has been recorded for four Drosophila species. Stalker and Carson (1947) found that wing length in D. robusta decreased with mean annual temperature of the localities at which collections were taken, while thorax length varied in the opposite direction. Prevosti (1955) and Misra and Reeve (1964) showed a decrease in both wing length and thorax length with mean annual temperature for populations of D. subobscura. Tantawy and Mallah (1961) found that wing length and thorax length were higher in strains of D. melanogaster from cooler areas within the Middle East. Wing length is an excellent index of body size in D. pseudoobscura (Sokoloff, 1966); although it varies among populations, there is no correlation with latitude or mean annual temperature (Sokoloff, 1965; Anderson, 1968). In nature, of course, temperature is only one of many factors which influence selection for body size, and the confounding effects of other selective factors may obscure the individual effect of temperature. That discernible clines exist at all is a clear indication of the importance of temperature. Laboratory populations maintained at different temperatures provide a means of assessing the selective effects of temperature on body size with less complication from other environmental factors. Anderson (1966) studied experimental populations of D. pseudoobscura which were kept for six years at 16, 25, and 27 C, two replicate populations being maintained at each temperature. After one and a half years there was no evidence for divergence in body size, as measured by wing length, but at six years a striking divergence was evident, the populations kept at 16 C having become genetically determined for much larger size than those kept at 25 and 27 C. Temperature was clearly a major environmental difference among these populations, and the similarity of results for the replicate populations at each temperature argues against a chance divergence to larger size at the lower temperature and smaller size at the higher temperatures. Thus the morphological divergence among the experimental populations could well result from a selection similar to that which acts in