Population Genetics in the American Tropics. IX. Rhythmic Genetic Changes That Prove the Adaptive Nature of the Detrimental Load in Drosophila melanogaster from Caracolisito, Colombia
Our comparative genetic study with the second chromosome of Drosophila melanogaster from Hungary and Colombia (25) produced a quantitative appraisal of the detrimental, normal, subvital, and supervital loads. Results such as these lend support to the theory that the genetic characteristics of a gene pool reflect the ecological situation of the population. The most prominent and significant data in this direction were reported for populations of D. pseudoobscura on Mount San Jacinto in California (6,9,16), and of D. pseudoobscura and D. persimilis in Yosemite Park, California (7,8). These pioneering works and our present one detected changes in time. However, while Dobzhansky’s shifts were described in terms of the relative frequencies of various karyotypes for which they were found to be polymorphic, our present report deals with genetic modifications in viability similar to those found in Fusagasugá (26). The profound changes in the organization of the genetic architecture found in Fusagasugá were caused by recurrent cycles of rainfall-bound seasons in the area. Our analysis revealed surprising shifts in the mean viability of homozygotes. Furthermore, a suggestive correlation appeared between rain cycles and the genetic load observed. If maximum and minimum temperatures had been used to describe the climatogical situation, as was done in the past, we would not have detected anything consistent with the cyclic shifts in lethals and semilethals. Other work (27) revealed that even within 7 months significant shifts in the sterility content in the second chromosome of D. melanogaster from Fusagasugá could be effected by the cyclic rainfall-bound seasons. Testing for heterogeneity among homozygotes of the different temporal populations showed the coexistence of a high- and a low-sterility group. Further partitioning of X 2 in contingency tables revealed that through female sterility in the dry season and male sterility in the wet season the local demographic unit is altered. Still other works (21,22) have shown how even reforestation can produce profound genetic changes within a single area.
KeywordsPopulation Genetic Genetic Load Temporal Population Cotton Growth Original Wood
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