# Fitness (biology)

Fitness ( Engl. Fitness "appropriateness", "suitability") is a technical term from the population genetics . In contrast to physical fitness , the term reproductive fitness is occasionally chosen.

The biological term “fitness” has nothing to do with the colloquial German term “sportiness” or “well trained”; Herbert Spencer's famous quotation, Survival of the Fittest , is repeatedly incorrectly translated as “survival of the fittest”; in fact, it means “survival of the most conformist”.

Frequently used synonyms for fitness are adjustment or adaptation value , relative survival rate or suitability .

## Darwin fitness

Despite its central role in evolutionary theory and the intuitive clarity of the term, its precise definition is difficult and it is used in slightly different meanings to this day. Fitness is a measure of the adaptation (the technical term for this is adaptation) of an individual or a genotype to its environment. The adaptive value of a trait (or in the case of a gene: the coding of this trait) is measured according to how it affects the number of its offspring; an adaptation is better if it increases the number of offspring, i. H. Fitness measures the sum of adjustments based on the number of fertile offspring. An individual with higher fitness has, under exactly the same environmental conditions, more offspring than one with lower fitness. Ideally, the characteristics that cause the higher number of offspring can be determined. The overall fitness can be explained by the effect of individual characteristics of the individual, for example his higher resistance to environmental factors such as drought or cold, his higher resistance to parasites or simply his higher current reproductive rate. Since the offspring, through inheritance, also have the favorable adaptations, they can prevail in the course of evolution until the environmental conditions change. Reproductive fitness is always related to the current, respective environment of the life being examined. Therefore, it cannot be measured away from its living space (e.g. in the laboratory).

The measured variable “fitness” is useful in evolutionary theory, but not central. It is easily possible to define and justify the theory entirely without this term. Indeed, in his original version of the theory of evolution , Charles Darwin managed without him, nor does the definition go back to himself.

When examining fitness, it often makes sense to consider sub-processes such as survival rate, reproduction rate, mating success, lifespan, etc., as these are often easier to measure. If the sub-processes essential for overall fitness have been selected, the fitness thus determined is a good approximation of overall fitness. Since the sum of the offspring over the entire lifespan of an individual would have to be determined for this, measuring them is often too time-consuming in practice, especially for species with long-lived or hidden individuals that are difficult to observe.

## Fitness as a population genetic term

The population geneticists have adopted the concept of fitness defined above largely unchanged. In order to be able to measure fitness in a meaningful way, the term was also made more precise and a description in mathematical language was justified. This was particularly necessary in order to be able to grasp fitness not only as a property of individual individuals but also of populations . The simplest form, “individual fitness”, simply corresponds to the definition for the individual. Its arithmetic mean and variance can be calculated for the population. However, individual fitness depends on genetic predispositions and the respective environmental factors in a way that is difficult to understand in detail. For this reason, another variable is introduced, which is referred to as “absolute fitness”, which is exclusively related to the genotype. According to this definition, absolute fitness is something like the mean expected value of fitness for an individual of a certain genotype . This can be determined for homozygous individuals by examining larger series (it should be noted that the definition of fitness includes the living space, because fitness is dependent on the living space). The mean absolute fitness corresponds exactly to the mean individual fitness, since in the second case the same individuals have only been distributed to two or more subgroups.

Absolute fitness values ​​are usually not relevant for processing, as they do not allow comparisons. Therefore, the absolute fitness value is usually normalized to a comparison value. The result is then referred to as "relative fitness". Usually the fitness of the fittest genotype is used as a reference (so that it receives the value 1). The result is a relative fitness between 0 and 1. The relative fitness, inserted in equations, can be used to predict allele frequencies with differently strong selection.

## Overall fitness

The concept of fitness was expanded by also taking into account the reproductive success of closely related individuals in the calculation, as their genes are largely identical. The term “ overall fitness” has established itself for this consideration (also: “inclusive fitness”). By looking at overall fitness, the emergence of altruistic behavior can be explained in particular .

## Calculation of fitness

The following equation is used to calculate the fitness or the growth rate of a genotype i (the formula applies to a population of asexually reproducing genotypes, with the generations overlapping):

${\ displaystyle \ sum _ {x = 1} ^ {L} l_ {x} m_ {x} e ^ {- rx} = 1}$

• ${\ displaystyle l_ {x}}$ is the probability of survival up to age x
• ${\ displaystyle m_ {x}}$ is the average fertility at age x
• L is the maximum lifetime

If the generations overlap, the fitness of an individual is measured as the growth rate r of the genotype i. In the event that the generations do not overlap, the fitness of a genotype is measured using the replacement rate R ( ). R is the product of the mean fertility of a genotype and the probability of survival up to reproductive age. ${\ displaystyle \ approx e ^ {r}}$

It is more difficult to calculate fitness for sexually reproducing individuals because the frequency of a genotype in each generation depends on the survivability and fertility of all those genotypes which, through crossing, can contribute to its creation. The fitness of a sexual genotype can be estimated by measuring its and values ​​and calculating the rate of growth or replacement rate. ${\ displaystyle l_ {x}}$${\ displaystyle m_ {x}}$