Darwinian fitness is the reproductive advantage that an individual obtains from its genotype. It is also known as Selective Value or Adaptive Value and is a composite of many advantageous as well as disadvantageous characteristics of the animal. Fittest is not the individual that can survive but the one that can leave the largest number of offspring reaching reproductive age. Reproductive success is therefore the key element of fitness.
Relative fitness is symbolised by the letter w, and is inversely related to selection coefficient, s.
w = 1- s or it can be w = 1+ s
The frequency of the recessive gene a will decrease in the next generation owing to lower fitness and that of dominant gene A will increase because of higher fitness. As the frequency of recessive gene goes down, selection against the recessive gene a becomes progressively slower. For instance, if q = 0.5 in the beginning, it will take 8 generations to reduce the frequency of gene a to 0.1, 100 generations to reduce it to 0.01 and 1000 generations to reduce the value of q to 0.001.