The solution to the paradox appears to be that such
global traits as symmetry are affected by mutations to
many, if not most, genes. Most mutations that arise are to
some extent deleterious, so deviation from physical symmetry
becomes an index of the load of mutations an individual
is carrying. Selection, particularly that operating via
mate choice, favors symmetry, and thus individual deleterious
mutations are winnowed from the population. However,
so many genes are involved that there is a constant
stream of new mutations maintaining population diversity.
Thus, symmetry is a fitness indicator trait in that it is a
reliable signal of genetic quality.