Studies on differences in the genetic architecture of related species of Drosophila.

ERY little is known about the nature and extent of genetic changes which take place during the process of speciation. DEVRIES believed that new species arose by single mutational events. This is certainly trae in the special case of the origin of species by allopolyploidy. In general, however, species differ from one another in more or less numerous gene substitutions and the origin of species must therefore involve combinations of several mutational steps. The classical view of speciation tends to assume that closely related species differ from one another in relatively few loci, with most of the loci remaining unaltered, even in rather remote forms. On the other hand, the formation of new species, and even of subspecies, may involve (at least in higher organisms) genotypic reconstructions affecting numerous loci. The study of so-called sibling species may yield critical data for discriminating between the above points of view. Sibling species are externally very similar to one another, and yet are biologically true reproductively isolated species. The obvious question is whether the morphological similarity or near identity of the sibling species reflects a genetic near identity. It is possible that similar phenotypes may be achieved on the basis of rather different genotypes. The genus Drosophila contains many examples of pairs and of groups of sibling species. The study reported in the present paper is an attempt to investigate the differences in the genetic architecture (composition and organization) between pairs of sibling species, namely Drosophila melanogaster and Drosophila simulans on the one hand, and Drosophila pseudoobscura and Drosophila persimilis on the other. The genetic material within the gene pool of a species permits individuals sharing in the gene pool to produce fertile offspring successfully. When this organization is modified in the process of speciation so that a new gene pool arises which is reproductively isolated from the first, two different genetic systems are formed. It is the nature and extent of the differences in the organization of genetic materials which is being investigated. D. melanogaster crossed with D. simulans does not produce fertile offspring. D. pseudoobscura and D. persimilis produce fertile female hybrids and sterile males when crossed to each other. Fertility can be restored by continuous backcrossing of the hybrids to either parental species. In nature, however, hybrids have never been found, even where