I.10 The set of characteristics affecting an individual’s chance to transfer his genes to the genetic pool of the next generations is called biological fitness
Fitnessin 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.
Fitness does not exist as an objectively defined general property; it is only possible to measure its external manifestations, i.e. the number of progeny of individual organisms. However, in each case, there are quite specific properties of the organisms that contribute to the fitness of the individual organisms to a greater or lesser degree. Under certain conditions, the success of an individual depends on the rate of reproduction and, in this case, the fitness of the organisms is determined by the rate of reproduction. In another species or under other conditions, this factor could consist in the resistance against a certain pathogen, the ability to utilize the available nutrients as well as possible, or the fastest legs. For some purposes, it could be advantageous to differentiate the individual components of the fitness. Fertility, viability andsexual attractiveness, i.e. the ability to be successful in the process of sexual selection, are mentioned most frequently. These categories can be further elaborated until a quite specific quality is found, such as the number of eggs laid, the speed of reaching sexual maturity, resistance to malaria, running or flying speed, size of antlers or colour of feathers in the season of reproduction. The only common denominator of all these properties is the ability to be preferred in natural selection and, thus, to increase their frequency in the gene pool of the relevant population.
The term fitness is, in a certain sense, superfluous in evolutionary biology. Even Darwin originally managed to get by without it and began to use this term (introduced by Herbert Spencer) on the recommendation of his friends only in the later editions of his book “On the Origin of the Species by Means of Natural Selection”. However, the new term was shown to be very useful from the standpoint of simplified expression and communication, as it allowed avoiding awkward phrases of the type “difference in the ability to be preferred by natural selection”.
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.