Student Theses and Dissertations

Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Two genetic characters in Drosophila pseudoobscura were utilized in an investigation of some of the genetic mechanisms in evolution. The characters are (1) body size, which is a continuously-varying, polygenic trait, and (2) the arrangement of genes along the third chromosome, which is a Mendelizing, discrete trait. Collections of Drosophila pseudoobscura were taken in many localities in the American West. The two characters vary' regularly with the physiographic division of the West. This variation is evidence that the frequencies of the genes controlling each character are strongly regulated by selection; such variation is the first stage in the genetic divergence which leads to the formation of new species. The frequencies of the gene arrangements on the third chromosomes are contrasted with those obtained in previous samples dating back as far as thirty years. A consistent pattern of change is apparent. The agent of selection responsible for these changes cannot be decided at present, although several possibilities are discussed. The system of inversions on the third chromosome is shown to be independent of that on the X-chromosome. One of the commonest geographic variations of insects is that of body size with temperature, the genetically larger strains coming from the cooler regions. Body size was studied in six experimental populations of Drosophila pseudoobscura which had been exposed to different temperatures. These populations were genetically identical at their inception but were maintained thereafter at different temperatures. After six years a striking genetic divergence in body size was found. The populations kept at the lower temperature had genetically larger flies than those kept at the higher temperatures. Crosses between the populations showed that the genes for larger size are partially dominant. The temperature-directed selection tor body size in these experimental populations may well be similar to that which has produced the temperature-oriented gradients for body size in natural populations of several species of Drosophila.


A thesis submitted to the Faculty of The Rockefeller University in partial fulfillment of the requirements for the degree of Doctor of Philosophy

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