Genetics of Natural Populations. Xix. Origin of Lations of Drosophila Pseudoobscura

N MOST parts of the geographic distribution of Drosophila psezcdoobscura, I chromosomal types which differ in gene arrangement in the third, the X, and, less frequently, in other chromosomes occur in the populations. The diversity of the gene orders is due to inversion of chromosome sections. Since the chromosomal types interbreed a t random, inversion heterozygotes and homozygotes are found in nature. The observation that seasonal changes in relative frequencies of third-chromosome gene arrangements take place in populations of Piiion Flats, on Mount San Jacinto, in California, has suggested that these gene arrangements affect the adaptive properties of their carriers (DOBZHANSKY 1943). Indeed, experimental populations kept in so-called “population cages’’ have shown that the inversion heterozygotes which carry two third chromosomes with different gene arrangements derived from the same population are superior in fitness to the corresponding homozygotes (WRIGHT and DOBZHANSKY 1946; DOBZHANSKY 1947 a, b). The chromosomal polymorphism is, accordingly, adaptive and it is balanced. Natural selection preserves all chromosomal types in a population a t equilibrium frequency levels determined by their relative adaptive values. Intrapopulational heterosis is thus maintained. Inversion heterozygotes which carry two third chromosomes with different gene arrangements derived from the population of Keen Camp on Mount San Jacinto (some 13 miles from Pifion Flats) or from the population of Mather (300 miles away from Piiion Flats) also possess, with one exception, higher fitness than do the homozygotes. Populations of these three localities consist mostly of the same chromosomal types. Experiments with population cages showed, however. that the adaptive values of the heterozygotes and homozygotes for the same inversions are not the same in the Pifion, Keen, and Mather populations (DOBZHANSKY 1948a; b). The structural differences b e tween the chromosomes evidently do not account for the heterosis observed in the inversion heterozygotes. It is more likely that the heterosis is an outcome of interaction of polygene complexes carried in the chromosomes with different gene arrangements. These polygene complexes are fitted together, or “coadapted,” by natural selection in the course of the evolutionary process. Heterosis is a result of natural selection (DOBZHANSKY 1949). The process of mutual adjustment of gene contents may, however, have taken place only between chromosomes which occur in the same population. Chromosomes of different populations need not be coadapted, unless these HETEROSIS THROUGH NATURAL SELECTION I N POPU-