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Genetic Structure of Populations. II. Viabilities and Variances of Heterozygotes in Constant and Fluctuating Environments

H. T. Band
Evolution
Vol. 17, No. 3 (Sep., 1963), pp. 307-319
DOI: 10.2307/2406160
Stable URL: http://www.jstor.org/stable/2406160
Page Count: 13
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Genetic Structure of Populations. II. Viabilities and Variances of Heterozygotes in Constant and Fluctuating Environments
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Abstract

1. One hundred heterozygous combinations of second chromosomes from the September, 1960, collection of Drosophila melanogaster from South Amherst, Massachusetts were tested in three environments. The three environments represent a constant temperature, C77; a fluctuating environment with a wide range between daily maximum and minimum temperatures, F64/90; and a fluctuating environment with a narrow range between daily maximum and minimum temperatures, F69/84. All three environments have a mean at or near 77⚬F. The two fluctuating environments were achieved by use of a 24-hour, cam-controlled incubator, with minimum temperature at 7 A.M. and maximum temperature at 3 P.M. 2. The experimental wide range of 26⚬ (F64/90) and the experimental narrow range of 15⚬ (F69/84) approximate the wide- and narrow-range temperatures observed in the natural environment during precollecting periods. The frequency of drastic variants (lethal and semilethals) carried by the D. melanogaster population shows a significant negative correlation with the average range during the precollecting period. 3. Matings between second chromosome lines were Cy/i × Bl/j such that the i/j wild-type progeny belong to one of three heterozygous classes: drastic/drastic (d/d), drastic/nondrastic (d/nd), or nondrastic/nondrastic (nd/nd). In terms of the wild-type progeny produced, the 100 heterozygous combinations can be subdivided into 30 d/d, 40 d/nd, and 30 nd/nd combinations. The same combinations were tested in each environment. Percentage wild-type viability is the measured variable. 4. Both percentage viability and ratio results indicate that heterozygotes are neither alike in the same environment nor constant over all environments. Results agree with predictions from the correlation data and with findings from the random heterozygote series. Heterozygotes containing drastic chromosomes, d/d and d/nd, have higher viability in the narrow-range environment; heterozygotes containing only nondrastic chromosomes, nd/nd, have the highest viability in the wide-range environment. Differences in viability among the heterozygous classes in these environments are significant. In the C77 environment, the order of viabilities among the heterozygous classes is the same as that for the random heterozygote series, the d/d class having the highest viability. 5. Variances among the classes also change in the fluctuating environments. In C77 all classes show comparable environmental variances, though the d/d class continues to show the highest genetic variance. These results agree with the random heterozygote series. In both fluctuating environments the total variance of viability increases. This is due largely to increased environmental variances and suggests that the chromosomes are not adapted to the warmer temperatures. Genetic variances also change. 6. The results indicate that different heterozygotes have different adaptive values and adaptive potentials, by which the population can continually readjust its genetic composition in relation to the changing environment. 7. The results also agree with the hypothesis that greater genetic diversity is associated with favorable environments while unfavorable environments lead to the reduction of genetic diversity.

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