SYNOPSIS. The contributions of modern genetics to the understanding of evolution have been threefold. First, it has documented the extent of genetic variation that exists in populations as a basis for future evolution. In particular it has shown that natural selection has not destroyed all variation, as might be predicted, so that there must be mechanisms for the maintenanceand origin of new selectable variation that balance the homogenizing forces of selection. Second, it has begun to provide a mechanism of the origin of genetic novelties which must be at the basis of the major features of the history of life. In doing so, it has shown how selection fornew features is at all times historically contingent and that evolution is at all times at riskof falling into genetic dead-ends. Third, modern genetics has greatly enriched the diversity ofmechanisms known to cause evolutionary change. All of these mechanisms involve the conversion of variation between individuals into variation between populations in time and space, but many are non-selective or even counter-selective. Natural selection is not the only mechanism of evolution.