removing them. For a trait influenced by a single gene,
selection does not have to be very strong to keep the level
of standing variation close to zero, because mutations arise
relatively infrequently. However, the rate of appearance of
mutations affecting a trait rises directly with the number of
genes involved in building it, and selection does not remove
them instantaneously. Thus, for a trait affected by
many genes, even if selection is strong, there will be a
significant amount of standing genetic variation (Houle,
1998).