154 ADRIANO A. BUZZATI-TRAVERSO 



We come then to the third level, that of gene effects. Here it is well known 

 that heterozygotes for a certain locus sometimes show a higher viability or a 

 better adaptation to the environment than either homozygote. The most 

 extreme examples are those of the widespread occurrence of lethals in wild 

 populations of Drosophila, noted in the next section. 



Every population of plants and animals that has been studied from 

 the genetic viewpoint has proved to be heterozygous for several loci. We 

 have now at our disposal a large series of data showing that the phenomenon 

 of genetic polymorphism is frequent in plants, animals, and man. These offer 

 to the student of evolutionary mechanisms the best opportunities to test his 

 hypotheses concerning the relative importance of selection, mutation pres- 

 sure, migration, and genetic drift as factors of evolution. Wherever we find 

 a well established example of balanced polymorphism, such as that of blood 

 groups and taste sensitivity in man, it seems safe to assume that this is due to 

 selection in favor of the heterozygote. How this selection actually may pro- 

 duce an increase in the chances of survival of the heterozygote, as compared 

 to both homozygotes, is an open question. When the characters favored by 

 natural or artificial selection are the result of several genes in heterozygous 

 condition, the analysis becomes very difficult indeed, as the experience of 

 plant and animal breeders clearly shows. 



EXPERIMENTS WITH HETEROSIS 



The importance of the problem of heterosis forpopulation-genetical 

 studies is clearly shown, not only by such general considerations and by the 

 few examples mentioned, but also by the everyday experience of people 

 interested in such lines of work. I have come across problems involving 

 heterosis several times and shall describe some results we have obtained 

 which may be of interest for the problem under discussion, especially at the 

 level of single gene differences. 



Several Drosophila workers have been able to show the occurrence of 

 heterosis in the fruit flies. L'Heritier and Teissier (1933), Kalmus (1945), and 

 Teissier (1947a, b) have shown that some visible recessive mutants of Dro- 

 sophila melanogaster such as ehony and sepia have a higher selective value in 

 heterozygous condition than either of the corresponding homozygotes under 

 laboratory conditions. Dobzhansky and collaborators in Drosophila pseudo- 

 obscura, Plough, Ives, and Child, as well as other American and Russian 

 workers in Drosophila melanogaster, have shown that recessive lethals are 

 widely spread in natural populations. It is generally accepted that such genes 

 are being maintained in the population because the heterozygotes are being 

 selected. Teissier (1942, 1944) has brought similar evidence under labora- 

 tory conditions for Drosophila melanogaster. 



It has been shown in several populations of species of the genus Drosophila 

 that heterozygous inversions are being selected, under natural and ex- 



