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The six sibling species of Drosophila willistoni group are among the most favorable, and at the same time challenging materials for studies on the genetics of speciation processes. The species are D. willistoni Sturtevant, D. paulistorum Dobzhansky & Pavan, D. pavlovskiana Kastritsis & Dobzhansky, D. equinoxialis Dobzhansky, D. tropicalis Burla & da Cunha, and D. insularis Dobzhansky. Burla et al. (1949) found slight morphological differences that were insufficient for identification of single individuals, but Spassky (1957) noted differences in the male genitalia which do permit such identification. Unambiguous discrimination is also possible through examination of the gene arrangements in the chromosomes of larval salivary glands (Burla et al., 1949; Dobzhansky et al., 1950), and of the variant enzymes detected by electrophoresis (Ayala et al., 1970). Owing to ethological isolation, crossmating of the species occurs rarely, although under laboratory conditions some cross-inseminations can be obtained (Burla et al., 1949). Intercrosses of D. insularis females with D. tropicalis and D. willistoni males, and occasionally with D. paulistorum

Drosophila affinis Sturtevant, D. quinaria Loew, D. putrida Sturt., D. robusta Sturt., D. tripunctata Loew, and D. immigrans Sturt. are the six native species most commonly encountered in north central Indiana, while D. melanogaster Meigen, D. hydei Sturt., and D. busckii Coquillett are introduced species which frequently attain large summer populations. Summer populations of the native species originate in deciduous woodlands, and during periods of population maxima in June the more prevalent species are widely dispersed through all of the major types of terrestrial habitats. Summer populations of the introduced species originate in proximity to human settlements and likewise disperse through a variety of habitats during periods of population increase, occurring with the native species in diverse situations in July but not achieving peak numbers until August. The species which infest tomatoes are widely, if thinly, dispersed at the time the crop ripens, and there is little expectation that preseason control efforts would be practicable. Despite disparate climatic conditions during the summers of 1958 and 1959, population developments in tomato fields were strikingly similar, thus indicating little correlation between normal weather extremes and population development. D. melanogaster is recognized as the principal species causing contamination of tomato products. Experimental evidence on comparative fecundity, developmental rates, and ovipositional behavior were obtained to explain the consistent prevalence of this species. Studies of diurnal activity and spatial distribution suggest that these facets of behavior are controlled principally by responses to atmospheric moisture. Adult flies were found to inhabit a space, only a few inches in height, directly above the soil surface. The micro environment provided by the foliage and fruit of the tomato plant does not require the insects to follow the pattern of peak activity at dawn and dusk often ascribed to Drosophila .

The natural population of a site in Fusagasuga, Colombia, was sampled weekly from March 1964 to December 1965, and weights of the collected adult Drosophila were determined. D. melanogaster Meigen, D. immigrans Sturtevant, D. cardini Sturtevant, and D. capricorni Dobzhansky & Paven were the most abundant species. The total number collected of D. immigrans, D. bandcirantorum Dobzhansky& Pavan, the D, tripunctata Loew group, and 77. griscolineata Duda was higher after rainy months than after dry months. This correlation did not hold for D. mclanogaster or D. cardini . The average weight of female D. melanogaster and D. immigrans collected during rainy months was higher than that of females collected during dry months. However, the reverse was true for D. cardini . A greater variety of species of Drosophila was taken at Fusagasuga than in a previous study at a higher altitude in Colombia.

Sibling species are species morphologically so similar as to be practically indistinguishable from each other by inspection of their structural features. The Drosophila willistoni group consists of at least twelve species, six of which are siblings. Two of the sibling species, D. pavlovskiana Dobzhansky and Kastritis, and D. insularis Dobzhansky, are narrow endemics; the former in Guyana and the latter on some islands of the Lesser Antilles. The other four species, D. willistoni Sturtevant, D. equinoxialis Dobzhansky, D. paulistorum Dobzhansky and Pavan, and D. tropicalis Burla and de Canba, have wide and largely overlapping geographic distributions. The four species are sympatric through Central America and the northern half of continental South America; two or three of them are sympatric in the southern half of Mexico, the larger Caribbean islands, Peru, Bolivia, and the southern half of Brazil (Spassky et al., 1971). We have studied for several years genetic variation in the sibling species of the D. willistoni group, using techniques of gel electrophoresis and enzyme assay. D. equinoxialis consists of two subspecies, D. e. equinoxialis and D. e. caribbensis (Ayala, 1973). In this paper we report results of the study of reproductive isolation between these two subspecies, and we compare genetic variation within and between the subspecies. Ayala and Powell (1972) have