A Genetic Analysis of the Adaptive Values of Populations

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Abstract
Using a stock of flies carrying a Cy L second chromosome, samples of second chromosomes were isolated from populations and tested for viability in over 2700 homozygous combinations and over 6000 heterozygous combinations. Viabilities were detd. in relation to the expected 2 Cy 1/+1 +/+ ratio and ranged from complete lethality (0% wild type) to the normal ratio. Adaptive values of 5 populations were estimated from the avg. frequencies of wild flies in all and in "normal" (non-lethal, non-semilethal) heterozygous tests for each population. The highest avg. frequency of wild flies was that of normal individuals of population 1 and this was considered as a standard. Adaptive values of other populations were detd. by comparing the deviations of the avg. of all cultures, and of normal cultures, of each population from the standard. These tests indicated that the adaptive value of each population was detd. largely by the normal heterozygotes and in a minor degree by heterozygotes carrying lethals. Second chromosomes, which had a known behavior in homozygous condition, were termed "drastic" or "normal" depending on whether fewer or more than half of homozygous flies survived in test cultures, and each was combined with two other randomly chosen second chromosomes. The various combinations were termed "2-drastic," "1-drastic," and " "no-drastic." The data included only combinations giving rise to heterozygotes of normal viability and indicated a tendency for no-drastic combinations to yield higher frequencies of wild flies than 1-drastic and for 1-drastic combns. to exceed 2-drastic combns. The avg. decreases in viability attributable to the 2-drastic and the 1-drastic combinations were 3.2% and 1.4% respectively. By comparing the decrease in viability attributable to no-drastic combinations (no lethal or semilethal chromosomes present) and to the average "normal" combination (including lethal and semilethal chromosomes), it was found that, while the normal individuals of a population have viabilities accounting for 29 to 99% of the reductions in adaptive values of different populations, a substantial amt. of the effect of the normal individuals is detd. by the lethal and semilethal chromosomes which they carry. Flies heterozygous for different second chromosomes differed in viability as shown by extensive crosses and tests. A comparison of viabilities between pairs of heterozygous combinations carrying a common second chromosome indicated that the viability of a combination of two chromosomes was characteristic of the combination, not of either one of the two chromosomes.