It is important to stress that trade-offs and fluctuating
selection are not the only possible approaches to the maintenance
of heritable variation. Biologists have also observed
that there are a number of traits that are unidirectionally
correlated with fitness and yet in which substantial
heritable variation is maintained (Rowe & Houle, 1996).
An example would be physical symmetry. In general, the
more symmetrical an individual, the higher its fitness, and
yet heritable variation in symmetry persists. The maintenance
of variation in such cases appears paradoxical, because
directional selection might be expected to home in on
perfect symmetry and winnow out all variation.