Integration of the Gene Pool as Demonstrated by Resistance to DDT

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Abstract
Two independent lines of D. melanogaster were selected for resistance to DDT by exposing the adults to an aerosol and using as the parents of succeeding generations the survivors of doses killing approximately fifty per cent of the treated flies. At generations seventeen and eighteen the LD50''s of the two lines were approximately three times that of the control and at generations twenty-two and twenty-three they had increased to about four times. Crosses made reciprocally between the two lines at these two points produced F1 populations with LD50''s equal to those of the lines crossed, but when F1 flies were inbred to produce F2''s, the LD50''s of the latter were significantly lower and the variances of the mortality distributions were higher. These phenomena indicate that the two lines had achieved resistance by consolidating different genetic factors which segregated in the F2. The significance of this finding is discussed in the light of other recent evidence that the gene pool of a population is an integrated whole.