Are polymorphism and evolutionary rate of allozyme proteins limited by mutation or selection?

It is well established that some allozyme proteins are more polymorphic and evolve at a faster rate than others, and that allozyme protein heterozygosity and genetic distance are correlated. These empirical observations could be explained either by differences in mutation rate or by differences in the type or intensity of selection between allozyme proteins. Computer simulation is used here to study a variety of models of allozyme evolution to gauge the potential impact of mutation and selection. The results suggest that the range of models that can account for variation in allozyme heterozygosity and genetic distance through variation in the type or intensity of adaptive directional or overdominant selection is rather narrow, and that many selection models can be rejected as an explanation. However, the variation is easily explained by variation in mutation rate in a wide range of models. The simulation results thus provide support for the view that evolution of allozyme proteins is mutation-limited.