Applies to allelic variation at a single gene locus, under particular environmental conditions. There is a direct correlation between amount of genetic variation in a population and the rate of evolutionary change by natural selection. Evolution will proceed more rapidly in variable populations than in uniform ones. The value of L ranges from 0 to 1. Any factor resulting in differences in fitness among genotypes can cause genetic load.
Genetic load measures the average individual deviation from the optimal phenotype.
In recognition of the sources of the less fit alleles, loads are associated with mutation, gene flow, recombination, genetic drift, and segregation which increase the frequency of inferior genotypes that selection tends to eliminate. For example, the mutational load is the fraction of individuals in a population that dies each generation because of a deleterious mutation at one locus. It is an indication of the reduction in mean fitness due to introduction of new, potentially harmful genotypes as a result of mutation. It also may be expressed through sterility, inability to find mate, or any condition reducing reproductive ability of less fit genotypes. Estimates of genetic deaths are, therefore, calculated as the number of individuals eliminated from a population as a result of selection against them.