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Evolutionary Changes in Laboratory Cultures of Drosophila pseudoobscura

Th. Dobzhansky and B. Spassky
Evolution
Vol. 1, No. 3 (Sep., 1947), pp. 191-216
DOI: 10.2307/2405495
Stable URL: http://www.jstor.org/stable/2405495
Page Count: 26
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Evolutionary Changes in Laboratory Cultures of Drosophila pseudoobscura
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Abstract

The present investigation is designed to test the hypothesis that mutations beneficial to their carriers may be observed if, instead of normal or wild-type strains, one uses, as initial materials, strains the vitality of which is reduced by some deleterious genetic factors. The wild-type is presumably always close to the highest adaptive level attainable in the normal environment in which the species lives; favorable mutations that might counteract the effects of defective heredity are, however, easily conceivable. Seven strains of Drosophila pseudo-obscura were made homozygous for second or for fourth chromosomes which were known to diminish the viability of homozygotes. Some of them also slowed down the development rate, or reduced the fertility, or made the homozygotes structurally abnormal. Each strain was subdivided into four lines, two of which were kept homozygous, and two were 'balanced' in such a way, that the second or fourth chromosomes were always in heterozygous condition. The homozygous, but not the balanced, lines were kept in strongly overpopulated and crowded cultures. In one homozygous and in one balanced line of each strain, the males were given a treatment of 1000 r-units of X-rays in every generation. All lines were kept for twenty-five consecutive generations at 21⚬, and for the next twenty-five generations at 251/2⚬. The whole experiment lasted, thus, for fifty generations, or from November 1942 till January 1947. The viability, development rate, and other characteristics of the twenty-eight lines were tested at intervals of several generations. Testing in every generation would be too laborious, since even as it was the work required classification and recording of at least 410,784 flies. Improvements of viability were observed in five out of seven homozygous untreated strains after fifty generations (figure 1). Two strains did not change significantly. Among the X-ray treated homozygous strains, six out of the seven showed appreciable gains in viability, and one was unchanged. In at least seven of these fourteen homozygous strains, the improvements were quite striking. Among the balanced untreated strains, three developed lethals in the balanced chromosomes, three were unchanged and one improved to a slight extent (figure 2). Recessive lethals or semilethals appeared in the balanced chromosomes of five X-rayed strains, while two strains were either unchanged or slightly improved. The contrast between the homozygous and the balanced lines is striking : most of the former improve and most of the latter degenerate. Several of the homozygous strains which were initially characterized by reductions of the development rates lost these characteristics in the process of breeding. Speeding up of the development rate was observed also in two of the balanced strains. Similarly, such disabilities of the balanced strains, as semisterility and visible structural abnormalities, were in part eliminated.

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