COEXISTENCE OF ECOLOGICALLY SIMILAR COLONIZING SPECIES .2. POPULATIONDIFFERENTIATION IN DROSOPHILA-ALDRICHI AND D-BUZZATII FOR COMPETITIVEEFFECTS AND RESPONSES AT DIFFERENT TEMPERATURES AND ALLOZYME VARIATION IN D-ALDRICHI

Drosophila aldrichi and D. buzzatii are cactophilic species that colon ised Australia about 55-60 years ago. They are sympatric only in Austr alia. Thus they may be in the process of adapting to new environments and to each other, and diversifying among local, possibly isolated, po pulations. Larval competitive effects for three populations of each sp ecies (Roma, Planet Downs, and Binjour) were measured on semi-natural cactus rots at three temperatures, with preadult viability, developmen tal time and adult body weight scored for each sex and species. Popula tions of both species varied in their responses to the other species a s competitor, and one D. buzzatii population (Roma) reduced larval per formance of D. aldrichi significantly more than did other D. buzzatii populations. Geographic divergence for the three traits was similar in both species, with a relative performance index derived from these tr aits highest for Roma, second for Binjour, and least for the Planet Do wns population of each species. The Roma D. aldrichi population was th e most different from the other populations for the performance index and in terms of genetic distances derived from allozyme frequencies. A dditionally, comparisons of climatic variables among the population lo calities showed that the Roma environment was most different from the others. Differential natural selection in different areas of the cactu s distribution may be a major cause of population divergence in both s pecies. Drosophila aldrichi is superior for some fitness components at the highest temperature. Thus temperature variation throughout the ca ctus distribution may contribute to the different ranges of these two species, with competitive exclusion of D. aldrichi in the southern, co oler region of the cactus distribution, but coexistence in the norther n, warmer region.