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 degree of a population's departure from the ideal genotype. It provides a measure of the difference between the average fitness of individuals in a population and the optimal genotypic fitness for the particular environment under consideration. The value of L ranges from 0 to 1.
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. 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. The cost of selection to fix some alleles and to eliminate others can be high in terms of the number of genetic deaths. One calculation has it that a population would have to lose 90X its size each generation for selection to substitute one allele for another.