Incipient Reproductive Isolation between Two Subspecies of Drosophila Pallidipennis.

HE process of transformation of races into species is in general slow. The T only approach to its study is through examination of its successive stages represented on our time level in different species and species groups. Nor is the process uniform in all organisms. Even though the essential feature of speciation in sexual cross-fertilizing forms is the development of reproductive isolation between the diverging races, the isolation is attained by different means in different cases. This necessitates careful study of diverse groups of organisms, as well as of different related species with contrasting reproductive biologies, modes of life, and distributional relationships. For many years it appeared that species of Drosophila, though they offer some of the best material for investigation of many genetic problems, are not favorable for studies on speciation; it looked as though the intraspecific variation among natural populations is very small, while the reproductive isolation between species of this genus is almost always complete. In other words, the critical stages of speciation, the intermediates between races and species, seemed to be absent, as if species formation has come to a standstill. Facts brought to light during the last decade have, however, shown that the above impressions were erroneous, based on a failure to realize that the speciation in Drosophila is frequently accompanied by very little differentiation in the visible characters. Undoubtedly separate species of this genus may be morphologically similar and even identical, while morphologically distinguishable subspecies are, compared to other organisms, rare. It is now clear that the genus Drosophila, contrary to the old idea, is quite rich in “borderline cases” between race and species. Owing to the inherent advantages of many Drosophila species as laboratory materials, they off er excellent opportunities for studies on the genetics of speciation. In the present article we wish to report on two subspecies of D. pallidipennis which have developed slight morphological differences and partial hybrid sterility, without either differentiation of the gene arrangements in their chromosomes or sexual isolation. We are obligated to MRS. N. P. SIVERTZEVDOBZHANSKY for her help in making the wing measurements, and to MISS IRENE MARKREICH for technical assistance.