Release of Genetic Variability through Recombination. I. Drosophila Pseudoobscura.

ENDEL discovered that genes segregate, rather than blend during the process M of germ cell formation in heterozygotes. This makes sexual and asexual reproduction profoundly different in their genetic consequences. Asexual reproduction yields a progeny which, barring mutation, is genotypically uniform and identical with the mother. Sexual reproduction engenders genetic variability in the offspring. A variety of genotypes appear in the progeny of heterozygous parents, and these genotypes become available for the sorting-out action of natural or artificial selection. Herein lies the biological function of sexual reproduction; it creates a great variety of genotypes, perhaps so great that no genotype is likely to become embodied in more than a single individual. Under asexual reproduction genotypic diversity can arise only through occurrence of different mutations in different lines of descent. Compared to sexuality, this is a most inefficient method of producing genetic diversity. All of the above is a matter of elementary genetic theory. But it is very little known just how much genetic variability can be released by recombination of genes available in a small sample of a sexual population. This problem bears directly on the disputed ground of the classical and balance theories of population structure (DOBZHANSKY 1955, 1957). If the classical theory is correct, then most individuals in sexual populations are homozygous for most genes, and a small group of individuals will carry only a few unfixed loci. On the other hand, under the balance theory, every individual is expected to be highly heterozygous; a small population sample will, then, contain many unfixed loci, and their recombination may yield considerable genetic diversity. According to the classical theory, the occurrence of an evolutionary change will usually require new mutations; according to the balance theory the recombination of the unfixed loci already available in a sexual population will be important and perhaps will eclipse the contribution of the new mutants. This is the first of a series of articles describing experiments designed to produce information concerning the release of genetic variability through recombination. For this purpose we have chosen three species of Drosophila-D. pseudoobscura, D. persimilis and D. prosaltans, and have studied the recombination in their “second” chromosomes.