Fitness in the sense of biological fitness is a key term in evolutionary biology. The fitness of individuals under specific conditions and in specific situations can be measured with greater or lesser ease; however, it is not a simple matter to define it in general terms. The fitness of two organisms can be evaluated only in retrospect, in relative values, on the basis of the number of progeny that each of them leaves in the population after a sufficiently large number of generations. If one of them leaves twice as many descendants, it is assumed that it probably has twice the fitness. However, it is not possible, for example, to measure the physical parameters of an individual and to determine his fitness on the basis of the data obtained. Fitness depends not only on the qualities of the particular individual, but also on the fitness of the other individuals in the population. In addition, it is closely related to the external conditions. Individuals with a certain phenotype can have greater fitness under certain conditions in certain habitats, while different individuals can have greater fitness in the same population under different circumstances.
The fact that the fitness of an individual can be estimated on the basis of the number of his progeny could lead to the erroneous impression that fitness is equivalent to fertility or the rate of reproduction. However, this is an entirely erroneous impression. Under certain conditions, organisms that reproduce more slowly have a longer generation period or a smaller number of progeny can have greater fitness. For example, if the blood of a host simultaneously contains two populations of the parasitic protozoa Trypanosoma, which differ in a surface antigen and the rate of growth, then the more rapidly reproducing protozoan variant will cause a stronger and more frequent immunity reaction of the host and will generally be more rapidly liquidated. The more slowly reproducing variant survives longer in the blood and thus has a greater chance of being transferred to a new host by blood-sucking insects. A different but, in its consequences, identical situation occurs if the immune system is not capable of eliminating the parasite and the host is killed by the parasite. The more rapidly reproducing parasite will kill its host faster, so that it has a lesser chance of being transferred to a new host. Thus, it has lower fitness.
Classical population genetics employs the term fitness (adaptive/selection value, w) in a more exact and simultaneously narrower sense. Here, fitness characterizes the degree to which a certain genotype contributes to the gene pool of the next generation through its progeny, compared to the genotype of the fittest individuals against whom no selection is acting. The genotype of individuals against which no selection pressure is active (population genetics studies ideal models, such a genotype doesn’t exist in real populations) has a selection value of w = 1, while other i genotypes have selection values w = 1 – si, where si is the selection pressure against individuals with the i-th genotype. Thus, in population genetics, fitness, as a relative quantity, can assume values from 0 to 1; in evolutionary biology it is legitimate to consider things in terms of absolute fitness, i.e. in the entire range of positive numbers.