Journal of Heredity (1992)Abstract When an environmental change imposes strong directional selection, there are two advantages of sexual reproduction. First, an asexual population is limited to the most extreme individual in the population, and progress under directional selection can go no farther without mutation; no such limitation applies to a sexual population. Second, more quantitatively, directional selection in an asexual population mono-tonically decreases the variance, whereas the variance of a sexual population quickly reaches a steady value; this difference remains even if the direction of selection occasionally changes. With realistic environmental changes small alterations in any particular measurement or trait are usually sufficient to keep up with the changes, but fitness, since it depends on a large number of traits, will be selected with greater intensity, which may be enough to confer a distinct advantage on sexual reproduction. This applies particularly to a large or rapid environmental change. Eventually mutation will enhance the variance, but by then it may be too late to prevent extinction of asexual strains..