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The Effects of Population Density and Composition on Viability in Drosophila melanogaster

Richard C. Lewontin
Evolution
Vol. 9, No. 1 (Mar., 1955), pp. 27-41
DOI: 10.2307/2405355
Stable URL: http://www.jstor.org/stable/2405355
Page Count: 15
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The Effects of Population Density and Composition on Viability in Drosophila melanogaster
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Abstract

Twenty-two strains of Drosophila melanogaster were compared with respect to their larval viabilities. These strains included Swedish-B wild type, the mutant white, nineteen strains homozygous for second chromosomes isolated from flies maintained in population cages, and one strain of random heterozygotes from crosses among the homozygous strains. The larval viabilities were tested at population densities of 1, 2, 4, 8, 20, and 40 larvae per shell vial containing a synthetic medium. Pure culture experiments contained only larvae of the tested genotype, while mixed cultures contained equal numbers of the tested genotype and the mutant white. Most strains showed an optimum viability at intermediate densities although a few showed no change over the range used. The strains differed from each other in viability at all densities. They also differed from each other when compared at their respective optimal densities. In addition there was a change in viability at all densities from mixed to pure culture. Some strains showed an increased viability in mixed culture over that in pure culture while others showed a decrease. This lack of regularity resulted in a change in the relative positions of the homozygotes on the viability scale from pure to mixed culture. Finally a negative correlation between the numbers of wild type and white larvae surviving was observed at the highest and lowest densities. The correlation was positive at intermediate optimal densities. The existence of an optimal intermediate density indicates some facilitation between individuals of like genotypes. The significant differences between strains when compared at their respective optimal densities point to a component of viability independent of density. This would allow natural selection to work in an expanding population. The increase in viability in mixed culture of some strains shows that a strong facilitation may exist between genotypes. On the other hand the negative correlations at the extreme densities and the lowering of viability in mixed culture for some genotypes show that genotypes may actively interfere with or inhibit one another. The general conclusion is that the viability of a genotype is a function of the other genotypes which coexist with it, the result of any particular combination not being predictable on the basis of the viability of the coexisting genotypes when tested in isolation. That this is general is indicated by the differences in the viability of one genotype (white) when tested with all other genotypes. It has been shown that facilitation may lead to a stable polymorphism of genotypes.

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