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There are several Colombian populations of Drosophila pseudoobscura which have begun to show divergence. The temporal samples of Torobarroso, one of the local populations identified, demonstrate genetic "resourcefulness" and selective opportunism by fluctuating mortality. The data suggest that both balanced selection and mutational origin are necessary to account for the frequencies of drastic genes encountered. In genetic isolates there seems to exist alternatively a mutational component and a balanced - selection component. These components overcome the damage in the first instance and in the second instance, profit from the recessive load.

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We have attempted to make quantitative comparisons between the genetic systems provoked by some neo-tropical environments and a European one. These comparisons permit certain clarifications regarding the comprehension of tile genetic load and of the genetic "norm" in reference to particular ecological characteristics.