Adaptive Changes Induced by Natural Selection in Wild Populations of Drosophila

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Abstract
The gene arrangement in the 3d chromosome of D. pseudoobscura is variable. Each gene arrangement occurs in populations of a definite geographic area. However, 2 or more gene arrangements may occur together in many populations . Inversion homozygotes and heterozygotes occur frequently in natural populations. The relative frequencies of various gene arrangements in some populations undergo seasonal cyclic changes. These changes are produced by natural selection, and represent adaptive reconstructions of the population genotype, thus facilitating survival in different seasonal environments. Some of the changes taking place in nature can be reproduced experimentally in "populations cages," Populations containing desired proportions of chromosomes with different gene arrangements are introduced into the cages, samples of the eggs deposited by the flies in the cages are taken from time to time, and the incidence of the chromosomes of different types determined in these samples. The relative proportions of chromosomes with different gene arrangements remain constant in population cages kept at 161/2[degree]C. Changes are frequently observed in cages kept at temp. above 20 [degree]C: the incidence of some gene arrangements increases and of others decreases. However, the final outcome of the selective process is rarely a complete replacement of one gene arrangement by another. Instead, an equilibrium is usually reached at which all gene arrangements present in the initial population of an experimental cage are retained, but often with frequencies very different from the initial ones. The establishment of equilibria in the populations indicates that individuals heterozygous for different gene arrangements (inversion heterozygotes) are characterized by the highest adaptive values, while homo-zygotes are relatively inferior in survival and reproduction. The correctness of this interpretation is demonstrated by means of observations on deviations from the Hardy-Wein-berg proportions of heterozygotes and homozygotes among the adult flies developed in the population cages.