Student Theses and Dissertations
Date of Award
1966
Document Type
Thesis
Degree Name
Doctor of Philosophy (PhD)
Thesis Advisor
Theodosius Dobzhansky
Keywords
genetic mechanisms, Drosophila pseudoobscura, chromosomal inversions, temperature selection, coadaptation, genetic divergence
Abstract
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. Eleven experimental populations were derived from the samples of natural populations ranging from Canada to Mexico. Each population was begun, as far as practicable, with the same chromosomal constitution as had the sample from the locality in nature. These populations were crossed to yield F1 and F2 hybrids. The F1's varied irregularly, while the F2's showed a consistent "breakdown" of size, the F2's being significantly smaller than their F1 parents. The natural populations have coadapted, or internally balanced, genetic systems, with genes mutually adjusted by selection for favorable interactions. Recombination disrupted the balanced genic complexes to give the F2 breakdown. The frequencies of the inversions were followed in these same experimental populations for two years. The equilibrium frequencies established in the laboratory populations were quite different from those in the natural populations which were the ancestors of the laboratory ones. These results with Drosophila pseudoobscura. stand in interesting contrast wi th those obtained by European workers who studied Drosophila subobscura. They lend additional support to the hypothesis that Drosophila pseudoobscura is genetically flexible, while Drosophila subobscura is genetically rigid. The contrast between the mode of genetic adjustment in the two species illustrates the very different pathways evolution may take in adapting similar organisms to similar environmental stresses.
License and Reuse Information
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Recommended Citation
Anderson, Wyatt W., "Studies on Selection in Natural and Experimental Populations of Drosophila Pseudoobscura" (1966). Student Theses and Dissertations. 579.
https://digitalcommons.rockefeller.edu/student_theses_and_dissertations/579
Comments
A thesis presented to the faculty of The Rockefeller University in partial fulfillment of the requirements for the degree of Doctor of Philosophy