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.