Simple sequence repeats as a source of quantitative genetic variation

Mast traits in biological populations appear to be under stabilizing selection which acts to eliminate quantitative genetic variation Yet, virtually all measured traits in biological populations continue to show significant quantitative genetic variation. The paradox can be resolved by postulating the existence of an abundant, though unspecified, source of mutations that has quantitative effects on phenotype, bid noes not reduce fitness. Does such a source actually exist? We propose that it noes, In the form of repeat-number variation in SSRs (simple sequence repeats, of which the triplet repeats of human neurodegenerative diseases are a special case). Viewing SSRs as a major source of quantitative mutation has broad implications for understanding molecular processes of evolutionary adaptation, including the evolutionary control of the mutation process itself.