Genetic Drift and Natural Selection in Experimental Populations of Drosophila Pseudoobscura

Abstract

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.

Relative Fitness of a Third Chromosome Inversion in Drosophila Melanogaster

“…Evidence in accord with present findings was reported by Dobzhansky and Pavlovsky (1957), Spassky (1962), andSimmons (1966) in D.pseuiloobscura and by Spiess (1966) in D.persimilis. They have shown that in variable genetic backgrounds adaptive values ofthird chromosome gene arrangements (AR and PP in pseuiloobscura and WT, MD, and KL in persimilis) appeared to be different.…”

“…Wright and Dobzhansky 1946;Pavlosky and Dobzhansky 1966;Tobari and Kojima 1967;and many others). Evidence also exists to indicate that adaptive values of certain third chromosome gene arrangements in Drosophila pseudoobscura and D. persimilis appear to be conditioned by their genetic background (Dobzhansky and Pavlovsky 1957;Dobzhansky and Spassky 1962;Simmons 1966;Spiess 1966). …”

COMPARATIVE STUDIES OF ARRANGEMENTS WITHIN AND BETWEEN POPULATIONS OFDROSOPHILA PSEUDOOBSCURA

“…These observed frequencies are not significantly different from the values expected, assuming a Hardy-Weinberg equilibrium. As was found in Guatemala (1961 and1962), SC/TL heterokaryotype appears more often than expected. This supports the finding of Dobzhansky et al (1963) that in competitive laboratory cultures of D. pseudoobscura, SC/TL, is more viable than either SC/SC or TL/TL.…”

“…The third chromosome arrangement frequencies of males collected at our permanent station in Guatemala in 1960, 1961, and 1962, and in Colombia from August, 1961, to May, 1962, were determined. The Guatemala data are comparable to those published earlier for the years 1958 and 1959 (Epling et al, 1961 (Dobzhansky, 1943;Epling et al, 1957).…”

Estimation of components of phenotypic variance for developmental rate in Tribolium

“…In natural populations which are continually subject to gene flow in the form of migration a greater amount of epistasis would interact well in many combinations. In closed populations there is less need for such genes, and in fact, as Dobzhansky and Spassky (1962) Similar results were obtained with artificial selection for fast and slow development from the CS synthetic stock. Realised heritability estimates from 7 generations of selection, listed in table 7, were in accord with theoretical predictions.…”