While discussing the Darwinian premise of natural selection we observed that the term selection is synonymous with non-random reproduction, and that the success of the survivors is related to the number of offsprings they leave behind. Is it possible to quantify such success? In other words can natural selection be quantified? The answer to these questions is yes and is related to the notion of 'fitness'. The term fitness variously known as Darwidan fitness or selective value or adaptive value is a measure of the reproductive efficiency of one genotype relative to another genotype within a given population of species Let us now look into the details of the concept of fitness.

Let us .take an example to explain this concept.

Let alleles A and a form three genotype AA, Aa and aa. Let genotypes AA and Aa produce four progeny each and genotype aa produce only two progeny. The fitness or the adaptive value of genotypes AA and Aa would then be 414 = 1 and that of aa would be 214 = 0.5. The two genotypes AA and Aa have similar fitness values and aa has 50% uf the fitness value of the other two. The genotype with highest reproductive efficiency is given the fitness value of 1 and the fitness of the other genotypes are calculated relative to the one with maxinlum fitness value. Fitness is denoted by the letter w. 'Table explains to you the method of calculating the fitness value for the various genotypes.