58 ¢ differentiation that there has been a significant gene flow between , Villages so that the villages can by no means be regarded as isolated _ populations. The question of "dilution" of genetic effects deserves to be considered in the light of population genetics. Human populations generally carry a number of mutated genetic loci which have accrued from spontaneous mutations in preceding generations. These mutations are generally recessive in their etfects, and while they are usually deleterious in their indtvidual effects, they are not al.) intrinsically bad since they provide the necessary variability in a population to allow it to respond to changing environments, > eee and thus to permit the species to evolve. Although evolution depends on the continued presence of genetic variation, one of its most important immediate consequences in a population is the inevitable production of ill- adapted individuals. This cost, in terms of reduced fitness associated with. the production of less than optimally fit indivduals, is called the genetic load of the population. In this sense, genetic load is the cost to the species of the opportunity to engage in evolution. Most ot these continually arising spontaneous mutations are harmful “in various degrees, and, by failing sooner or later to be transmitted to the following generations, they are removed from the population at a rate proportional to their harmfulness. A cell carrying the mutation may die, or, being . germ cell, it may fuil to be fertilized, or the fertilized egg moy fail to be implanted, or being implanted, may die. Loss may also occur