Bias in the introduction of variation as an orienting factor in evolution

According to New Synthesis doctrine, the direction of evolution is determin ed by selection and not by "internal causes" that act by way of propensitie s of variation. This doctrine rests on the theoretical claim that because m utation rates are small in comparison to selection coefficients, mutation i s powerless to overcome opposing selection. Using a simple population-genet ic model, this claim is shown to depend on assuming the prior availability of variation, so that mutation may act only as a "pressure" on the frequenc ies of existing alleles, and not as the evolutionary process that introduce s novelty. As shown here, mutational bias in the introduction of novelty ca n strongly influence the course of evolution, even when mutation rates are small in comparison to selection coefficients. Recognizing this mode of cau sation provides a distinct mechanistic basis for an "internalist" approach to determining the contribution of mutational and developmental factors to evolutionary phenomena such as homoplasy, parallelism, and directionality.