Characteristics I N Inbred Mouse Populations Plateaued by Directional Selection1

is no doubt that intensity of natural selection varies with characters, T z " , a t long-term unidirectional selection counteracts natural selection, which in the long run favors intermediate and physiologically harmonious genotypes. As a joint product of these two pressures, genetic variation in populations plateaued by selection is often complex (CLAYTON and ROBERTSON 1957). REEVE and ROBERTSON (1953) selected for long wing in Drosophila for 68 generations, then inbred with and without selection. In nine generations the lines inbred without selection lost half their wing length deviation and most of their variance, while those inbred with selection decreased in variance but retained the original level in (wing length. Mass-mating without selection, applied for five generations, reduced the wing length of the selected to that of the unselected lines. Using the marked-inversion technic for all major chromosomes, BROWN and BELL (1961) found that lethal, sterility and subvital factors in a plateaued Drosophila population produced by selection for fecundity were of little importance, but that there was a significant amount of nonadditive genetic variation. The study of variations in inbred strains originating from plateaued populations may offer solutions to some problems relating to the genetics of artificial selection, such as how to uncover hidden genetic variations, and to determine whether inbreeding and natural selection interact. In ow study of the quantitative inheritance of body size two populations of mice were plateaued by long-term directional selection, one for large (LGJCkc) and the other for small (SM/Ckc) body size, and maintained by brother-sister matings for more than 20 generations. Data on body weight were used as a criterion of change in body size within each strain, and between strains differences in response to inbreeding. We also analyzed the reproduction records and the incidence of "careener," and compared leukocyte levels in early and late generations of inbreeding in the SM strain. These data provide supporting evidence for genetic changes following inbreeding.