J. Crow. 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..