An interesting indeterminacy of outcomes is observed in experiments with certain laboratory populations of Drosophila. Natural populations of Drosophila pseudoobscura and of many other species are polymorphic with respect to the gene arrangements in their chromosomes. The polymorphism is balanced; the heterokaryotypes with two chromosomes of a pair differing in gene arrangement are, in most environments, superior in Darwinian fitness to the corresponding homokaryotypes. The relative frequencies of the different karyotypes in experimental populations change from generation to generation until quasistable equilibria are attained. The results of the experiments depend, however, on whether the populations are uniform or mixed in geographic origins, i.e., on the chromosomes having been derived from the same natural population or from populations of different localities. In the former populations, the results are generally determinate; if the environment is well controlled, the selection rates and the equilibrium points are constant and reproducible. Not so in populations of geographically mixed origins; replicate experiments often give disparate results, and the selection rates and the equilibrium points are unpredictable for a given population.
HIS is the third in a series of articles on release of genetic variability through T recombination in three species of Drosophila. The previous articles have dealt with Drosophila pseudoobscura (SPASSKY et al. 1958) and D. persimilis (SPIESS 1959). The present article is concerned with D. prosaltans. The ecological peculiarities of this species are pertinent. It is native exclusively in the tropics, while the other two species live mainly in the temperate zone. D. pseudoobscura is widely distributed, very common, ubiquitous, and ecologically versatile; D. persimilis occurs in a much less extensive geographic area, and although it builds very dense populations in favorable habitats it is specialized to live chiefly in cool and humid locations; D. prosaltans is a rare form, which reaches considerable population densities only ephemerally in few scattered neighborhoods (DOBZHANSKY and PAVAN 1950). The spontaneous mutation rates for autosomal lethals are, at similar temperatures and in homologous chromosomes, about twice as high in D. persimilis and D. prosaltans as they are in D. pseudoobscura SPASSKY 1952, 1954). The genetic loads carried in the populations (accumulated recessive lethal, semilethal, and subvital gene complexes in the chromosomes) seem nevertheless to be higher in D. pseudoobscura than in D. persimilis or in D. prosaltans SPAS-SKY 1953, 1954). In accordance with this, the loss of fitness produced by inbreeding and homozygosis for naturally occurring gene complexes is greater in D. pseudoobscura than in D. persimilis or in D. prosaltans. It is tempting to speculate that the adaptive norm of D. pseudoobscura depends upon balanced heterozygosis to a greater extent than is the case in D. persimilis and in D. prosaltans. In other words, the genetic architecture of D. pseudoobscura approaches that postulated by the "balance" hypothesis, while the genetic architectures of the other two species incline relatively more towards the situation envisaged by the "classical" hypothesis ( DOBZHANSKY 1955).
Material and technique in Drosophila prosaltansThe experiments have followed the same plan as those with D. pseudoobscura and D. persimilis (SPASSKY et al. 1958, SPIESS 1959. Ten second chromosomes, permitting normal or subnormal viability in homozygotes, were extracted from The work reported in this article has been carried out under Contract Number AT-(30-1)-
ENDEL discovered that genes segregate, rather than blend during the process 1 This research was sponsored by the Office of Naval Research under Contract Number Nonr-2 The work reported in this article has been carried out under Contract Number AT-(30-1)-
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