The evolution of dominance: a physiological consequence moulded by naturalselection

Dominance relationships have been sub ect to a long debate since Fisher fir st argued that modifiers would be selected for if they made wild type allel es more dominant over mutant alleles. An alternative explanation, put forwa rd by Wright and developed by Kacser and Burns, is that the general recessi vity of novel deleterious mutations is simply a physiological consequence o f metabolic pathways. This explanation has gained support over the years to become a truly paradigm. Nevertheless there are reasons to believe that do minance relationships have been moulded by natural selection to some extent . First, the metabolic pathways are themselves products of evolutionary pro cesses that may have led them to be more stable to perturbations, including mutations. Secondly, empirical data from natural populations and laborator y selections indicate that dominance modifiers can be effectively be select ed. Finally, theoretical models show that the selection of modifiers can be substantial during the spread of adaptive alleles or when a polymorphism i s maintained either by overdominant selection or by migration-selection bal ance.