How Do the Genetic Loads Affect the Fitness of Their Carriers in Drosophila Populations?

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Abstract
Most individuals in natural populations of Drosophila carry one or more chromosomes which are lethal, semilethal, subvital, or otherwise injurious in double dose (in homozygous condition). Attempts to find out what effects such chromosomes have in heterozygous carriers have given, as a review of the literature shows, apparently conflicting results. The conflicts are mostly resolved if one realizes that the genetic variants which persist in natural populations are not a fair sample of those arising by mutation. Natural selection tends to eliminate the variants that reduce the fitness of heterozygous carriers, and to conserve the quasi-neutral and useful ones. It is suggested that the genetic diversity in populations can be divided in the categories of adaptive norm, genetic load, and the genetic elite. The adaptive norm lies within 2 standard deviations on either side of the mean fitness of individuals observed in a population under panmixia or whatever other breeding system the population has followed in its evolution. The genetic load and the genetic elite lie respectively below and above 2 standard deviations from the mean fitness.