D. Darwinian Fitness 1. Although Darwin's deduction of natural selection is phrased in terms of differential success among genotypes in the competition to obtain resources, which suggests that the driving force behind natural selection is differential survival, Darwin clearly recongized that natural selection could also arise due to differential reproductive success among individuals. Individual fitness may be defined, at this point, as the number of offspring produced by an individual. Average fitness of individuals of a given genotype is influenced both by the probability that such an individual will survive to reproduce and by the number of offspring that it produces if it survives. E. Natural Selection 1.
Natural selection occurs when genotypes differ in average fitness.
Natural selection IS NOT the same thing as Evolution. Natural selection is one cause of Evolutionary change, as we shall soon see. Processes other than natural selection can cause Evolutionary change, as we shall also soon see. Natural Selection in the Wild. This example not only demonstrates that genotypes differ in fitness, but also illustrates how fitnesses are calculated experimentally.