GENETICS OF NATURAL POPULATIONS. XXXIV. ADAPTIVE NORM, GENETIC LOAD AND GENETIC ELITE IN DROSOPHILA PSEUDOOBSCURA

Abstract

ATURAL populations of Drosophila, man, and presumably of all sexual, diploid, and outbreeding organisms contain a multitude of genotypes. A majority of these genotypes make their carriers tolerably well adapted to survive and to reproduce in the environments which the population frequently encounters in its natural habitats. The array of such genotypes constitutes the adaptive norm of the species or population. Some genotypes yield, however, low fitness in the habitual environments; these compose the genetic load of the population. And finally, some genotypes confer a fitness distinctly above the mean of the adaptive norm; these are the genetic elite of the population. The ill-adapted genotypes which arise in the population when it follows its normal breeding system are the expressed genetic load; deviations from the normal system toward greater inbreeding bring to the surface a part of the normally concealed genetic load consisting of recessive genes and gene combinations. Populations that are normally outbred have concealed loads which are much greater than the expressed loads. The problem not thus far resolved to the satisfaction of all geneticists is how well concealed is the concealed load. The chief bone of contention is whether recessive genes and gene combinations which are deleterious when homozygous are likewise deleterious, however slightly, also when heterozygous. In other words, are the effects of a gene on fitness qualitatively similar, however different they may be quantitatively, in homozygotes and heterozygotes? How often do genes unfavorable to homozygotes confer hybrid vigor, heterosis, on heterozy-gotes? The present investigation is a sequel to those of SPASSKY, SPASSKY, PAVLOV-SKY, KRIMBAS, KRIMBAS, and DOBZHANSKY, (1960), DOBZHANSKY, KRIMBAS and KRIMBAS (1960) and WALLACE and DOBZHANSKY (1962). These studies utilized chromosomes extracted from natural populations of Drosophila pseudoobscura; the chromosomes which were lethal, semilethal, subvital, or normally viable in double dose gave uniform average viabilities in the heterozygotes; however, the chromosomes giving the most viable homozygotes gave also the most viable heterozygotes. These results are puzzling: although even the superior homozy-gotes proved to be below the corresponding heterozygotes in viability, why are the supervital chromosomes rare in the populations? Why has natural selection 1The work reported here has been carried out under Contracts AT-(30-1)-1151 and AT-(30-1)-3096, U. S. Atomic Energy Commission, with Columbia University and the Rockefeller Institute respectively.