Differential survival leads to differential reproduction, allowing the operational definition of natural selection to be focused on the latter process. Fitness Fitness also may be called adaptive value or selective value . It is the contribution of offspring by a genotype to the next generation. Less fit genotypes are less able to contribute toward the next generation than are others. We have said that individuals with alternative, genetically-determined traits leave differing numbers of offspring to reproduce in subsequent generations due to differential survival and reproduction owing to the ability of those organisms to compete, find mates, avoid predators, forage, survive harsh weather conditions, or otherwise survive and reproduce such that they are able to leave relatively greater numbers of offspring to reproduce in the subsequent generation.
Fitness refers to the relative ability of an organism to contribute to subsequent generations.
It is an integral aspect of the process of natural selection , so we had better be able to come up with something. Fitness also may be defined or measured by the proportion of its genes which end up in the population gene pool in subsequent generations. Changes in the frequency of phenotypes in a population result from differential survival. These changes result in changes in the frequencies of alleles in the population of gametes produced by the surviving adults. In turn, these changes lead to changes in the gene and genotype frequencies of the population, which determine the phenotypes of the next generation.