Th. Dobzhansky

Comparative genetics of Drosophila willistoni

Drosophila willistoni is one of the species which were used in the pioneer studies on comparative genetics of Drosophila. Lancefield and Metz (1922) and Ferry, Lancefield and Metz (1923) reported a number of mutants, published a linkage map of the X-chromosome, and suggested that only about half of the X of this species corresponds to the X-chromosome of D. melanogaster, while the other half of the X of D. willistoni carries genes which are autosomal in D. melanogaster. The work on D. willistoni was then abandoned. Sturtevant and Novitski (1941) re-analysed the published information and concluded that one of the two limbs of the X-chromosome of D. willistoni corresponds to the X, and the other limb to the left part of the third chromosome of D. melanogaster. The homologies of the other chromosomes remained obscure. The mutant strains of Metz and his collaborators are no longer in existence. Because D. willistoni is very favourable for investigations on the population genetics of tropical species, we initiated iii 1944 a new collection of mutants and the construction of linkage maps. The results are reported in the present article.

Comparative genetics of Drosophila prosaltans

THE architecture of the germ plasm undergoes changes in the process of evolution. The genes become altered by mutation, and their arrangement in the chromosomes is modified by structural change. Comparison of the genetic maps of chromosomes in related species is one of the methods of studying the evolution of the germ plasm. Species of Drosophila are favourable for such studies, and about ten of them have been investigated in more or less detail. It appears that the degree of divergence in systematic characters is not very strongly correlated either with the amount of gene differentiation, or with divergence in the gene arrangement. The information now available is, however, insufficient to give a clear idea about the mutual relationships of the various classes of changes that are observed. More data is obviously necessary. No species of the saltans group of the subgenus Sophophora has been studied. Since 1944, we have collected mutants and examined the linkage relationships in Drosophila prosaltans Duda, which is a member of this group. The results are reported in the present article.

ADAPTATION IN MAN AND ANIMALS: A SYNTHESIS

Unless a synthesizer restricts himself to giving a rehash of the works he is supposed to synthesize, he must try to find a significant common denominator that is otherwise in danger of escaping the attention of the audience. The line papers by Osborne, Lee, and Newman, however thought-provoking when taken separately, may seem to deal with scarcely related topics; yet they have conveyed in three different ways one basic message. I might attempt to restate this message as follows : certain classical dichotomies are no longer valid. Dichotomies are persuasive, and they are dangerous because of their persuasiveness. We often speak and write that certain traits are hereditary and others environmental. The former come from the throws of the dice of destiny, and one cannot change one’s heredity; the latter result from how people live and, a t least in theory, people could live differently. It would greatly simplify matters if we could classify traits and determine which are hereditary and which environmental. This is impossible, because without exception all traits are hereditary and all are environmental. The color of skin is environmental, not only because one can tan himself or bleach himself by outdoor or indoor life, but also because to have any skin color one must have a skin, and the skin is a product of a process of development in which both genes and environment participate. Personality and social adjustment are influenced by heredity, not because a person is born with them and would have them regardless of how he lived but rather because other persons with other heredities, whose living experiences were approximately the same, developed different personalities and are better, or less well, adjusted to generally the same environment. How little these simple considerations are understood in some circles may be seen in an article published in a recent issue of a wellknown periodical by two American physicians. In that article the authors argue that a certain disease cannot be hereditary because people sometimes recover from its attacks! I do not mean to say that the distinction between hereditary and environmental diseases or between hereditary and environmental behavior traits is meaningless but, to be meaningful, the problem must be stated correctly. What we must ask is: What part of the observed variance in a given characteristic is due to the variety of genotypes existing in the individuals under study and what part to the variety of environments to which these individuals are exposed? This is the manner in which the question is asked by Osborne and I)e George’ in their studies on the characteristics of twins. The answer obtained will be different for different populations and for different environments. The more uniform the environments the greater will be the observed heritability; the more genetically uniform a population under study, the greater will be the estimated environmental component. The apparent discrepancy be-