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Gene Frequency in Small Populations of Mutant Drosophila

Peter Buri
Evolution
Vol. 10, No. 4 (Dec., 1956), pp. 367-402
DOI: 10.2307/2406998
Stable URL: http://www.jstor.org/stable/2406998
Page Count: 36
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Gene Frequency in Small Populations of Mutant Drosophila
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Abstract

1) Competition between the alleles bw75 and bw in population cages gave no evidence of a trend in gene frequency change in any of seven cages during the period of observation, although the mean frequency over all cages at the time of the first sampling was significantly below 0.50, the frequency in the foundation flies. 2) Experiments designed to ascertain the relative viabilities and productivities of the several genotypes (bw75/bw75, bw75/ bw & bw/bw), male and female, showed significant departures from the expected segregation ratios in 3 of 21 population types. These differences could not be systematically interpreted and were not reproduced in a subsidiary study. 3) Study was made of the changes in the frequency of bw75 between generations in cultures perpetuated for 19 generations with random samples of 16 individuals. The sample size was sufficiently small to permit considerable random drift. Two series of about 100 cultures each were set up at an initial frequency of 0.50. Series I was conducted in 35 cc. homeopathic vials, series II in 60 cc. specimen jars. The total gene frequency in series I did not change systematically during the course of the experiment. In series II the gene frequency rose for four generations after which no consistent changes occurred. 4) The fixation rate after generation 11 in series I was not significantly different from uniformity by chi-square. Series II was not carried for a number of generations sufficient to permit a satisfactory estimate of fixation rate. 5) No evidence consistent with an hypothesis of selection appeared in either series when the sample frequencies were grouped according to donor frequency classes. From this standpoint gene frequency changes could safely be attributed wholly to accidents of sampling. 6) The distribution of gene frequency by generation among the bottles of series I and II approached stability of form in generations 11 and 17 respectively. The grouped frequency distribution over the last 9 generations in series I was in poor accord with an hypothesis of uniformity, based on the apparent absence of selection, showing both an excessive irregularity and a negative slope of probable significance. The grouped distribution over the last 3 generations of series II was in good accord with the expected uniformity. 7) Estimates of the average effective population size made from the observed recipient class variances gave values of Ne equal to 9.0 and 11.5 for series I and II respectively, 56% and 72% of the sample size of 16 individuals. 8) A highly significant regression of 1/2Ne on donor class frequency was observed in series I indicating higher variances at higher frequencies than could be accounted for on the assumption of a common estimate of effective population size. This effect explains the slope of the frequency distributions of series I after stability of form was approximated. The theoretical distribution on the assumption of the observed regression is in agreement with respect to general slope but indicates that stability of form had not been fully reached as early as preliminary considerations suggested. The lack of accord between the observed and theoretical distri butions of series I is discussed from several standpoints, 9) The observed fixation rate in series I was higher than expected assuming an effective population size of 9.0 but the difference was not significant. 10) Curves representing the expected increase in the total variance of gene frequency and decrease in the percentage heterozygosis over the period of observation were calculated for each series using the mean estimates of effective population size based upon observed recipient class variances. The agreement between the observed rates and rates indicated by the calculated curves was good in the case of series II but showed some inconsistency in series I. 11) The differences between series I and II with respect to effective population size estimates, dependence of 1/2Ne on q, agreement with expectation of the form of the distribution with expectation of the form of the distribution of gene frequencies after stability had been approached, the homogeneity of the observed recipient class variances, rates of increase in total variance and decrease in heterozygosis are related to the size of the culture container. The data of series I are more heterogeneous in all respects.

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