EXPERIMENTAL STUDIES ON GENETICS OF FREE‐LIVING POPULATIONS OF DROSOPHILA

Summary 1. The gene mutations and the chromosomal changes obtained in Drosophila under laboratory conditions have their counterparts also in free-living populations. The fact that individuals of Drosophila found in nature are as a rule homogeneous and only rarely show striking variations is due to the majority of the mutants concealed in them being recessive to the normal or “wild-type” condition. 2. The concealed mutants can be detected either by inbreeding the offspring of wild individuals, or by more complex but more accurate genetic techniques that result in obtaining specimens homozygous for a given “wild” chromosome. As expected, the data obtained with the aid of either method are essentially similar. 3. In D. melanogaster the autosomes contain not only mutants producing visible external effects but also many recessive lethals. The kind and the frequency of the mutants found is variable from population to population and from year to year. 4. In free-living populations of D. pseudoobscuru the third chromosome, and probably other autosomes as well, are infested with lethals, semi-lethals, deleterious viability modifiers, modifiers of the development rate, visible mutants, and other genetic changes. Again, both the quality and the quantity of these changes vary from population to population. 5. Knowing the frequency of allelic lethals within a population, one can compute the rate at which they must be eliminated by natural selection. This rate can be compared to that of the origin of new lethals by mutation. Theoretically, the two rates must be alike in populations at equilibrium. Actually, the supposed elimination rate is smaller than the corresponding mutation rate. This may be due either to some of the lethals not being fully recessive, or else to the population size being subject to sharp seasonal fluctuations. 6. Inversions of chromosome segments are extremely common in free-living populations of all species of Drosophila so far studied in this respect. Certain kinds of multiple inversions make it possible to establish the phylogenetic relationships of the chromosome structures involved. 7. In D. pseudoobscura, D. azteca and probably in many other species each of the different structural types of the chromosomes occurs in a definite geographical region, thus giving rise to chromosomal races. 8. Aside from the above “macrogeographic” variability, there exist also differences, usually of a quantitative nature, between populations inhabiting neighbouring or even contiguous localities. Moreover, the genetic composition of a population does not remain constant from year to year. This “micro-geographic” variability is probably due to a restriction of the effective size of the breeding population in most species of Drosophila. 9. A species of Zhosophila does not represent a single panmictic population, but rather a mass of local colonies that are able to pursue, within limits, different evolutionary courses. The fate of a colony is, of course, controlled by natural selection, yet selection is not the sole determiner of the population dynamics. As predicted on theoretical grounds by Wright (1931, 1932), shifts in the genetic composition of a population may be due to the limitation of its genetically effective size.