Fitness Biology

In population genetics and evolutionary biology, fitness is a quantitative measure of reproductive success. It represents the expected contribution of an individual, genotype, or phenotype to the gene pool of subsequent generations. Fitness is a central concept in understanding natural selection and evolutionary change, as differences in fitness determine which genetic variants become more or less common over time.
Fitness is context-dependent: it is always defined relative to a particular environment and time period. A genotype or phenotype that is highly fit in one environment may have reduced fitness in another. To distinguish this evolutionary meaning from physical strength or health, the term Darwinian fitness is often used.

Concept and Definition

Fitness can be defined with respect to either a genotype or a phenotype. The fitness of a genotype is expressed through its phenotype, which itself is shaped by both genetic factors and the developmental environment. Conversely, the fitness of a phenotype may vary across selective environments, even when the underlying genotype remains unchanged.
In asexual populations, fitness can be assigned directly to genotypes because genetic transmission is straightforward. In sexually reproducing populations, however, recombination reshuffles alleles into new genotypes each generation. In such cases, fitness is often assigned to alleles by averaging their effects across different genetic backgrounds.
Natural selection operates by increasing the frequency of alleles associated with higher fitness. Over successive generations, this process leads to evolutionary change in accordance with the principles of Darwinian evolution.

Fitness and Reproductive Success

Fitness does not simply measure survival or lifespan. Rather, it reflects reproductive success, defined as the number of viable offspring or genetic copies contributed to future generations. Herbert Spencer’s phrase “survival of the fittest” is therefore best interpreted as the survival of those phenotypic or genotypic forms that leave the greatest number of copies of themselves over time.
An individual may survive for a long time but have low fitness if it produces few or no offspring, while another individual with a shorter lifespan may have high fitness if it reproduces successfully. Fitness is therefore fundamentally linked to reproduction rather than longevity alone.

Inclusive Fitness and Kin Selection

Inclusive fitness extends the concept of individual fitness by considering not only an individual’s own reproduction but also its effect on the reproduction of genetically related individuals. An allele can increase in frequency if it promotes the survival or reproduction of relatives who share that allele, even if this comes at a cost to the individual expressing it.
To avoid double counting, inclusive fitness excludes the contribution of others to the focal individual’s own reproduction. One of the main mechanisms through which inclusive fitness operates is kin selection, which provides an evolutionary explanation for altruistic behaviours observed in many social organisms.

Fitness as a Propensity

Fitness is often defined as a propensity or probability rather than as the actual number of offspring produced. According to this view, fitness is a property of a class of individuals sharing a genotype or phenotype, not of a single individual. The “expected number of offspring” refers to an average across many individuals, not the realised outcome for one organism.
Random events can affect individual outcomes without altering the underlying fitness of a genotype. For example, an individual carrying a highly advantageous genotype may fail to reproduce due to chance, but this does not imply that the genotype itself has low fitness. In probabilistic terms, fitness can be defined as the likelihood that an individual with a given set of traits will be included among the parents of the next generation.

Models of Fitness

To simplify analysis, fitness is often introduced in theoretical models using asexual populations without genetic recombination. In this setting, fitness can be assigned directly to genotypes. Two main operational definitions are commonly used: absolute fitness and relative fitness.

Absolute Fitness

Absolute fitness, usually denoted W, measures the proportional change in the abundance of a genotype from one generation to the next due to selection. If the number of individuals with a particular genotype increases, its absolute fitness is greater than one; if it decreases, absolute fitness is less than one.
Absolute fitness captures whether a genotype is growing or declining in the population. However, it depends on overall population growth and does not directly describe competition between genotypes.

Relative Fitness

Relative fitness, commonly denoted w, measures the fitness of a genotype relative to other genotypes in the population. It determines changes in genotype or allele frequencies rather than absolute numbers. Relative fitness is calculated by comparing a genotype’s absolute fitness to the mean fitness of the population.
Only relative values matter: relative fitness can take any non-negative value, and it is common to set the fitness of one reference genotype equal to one. Relative fitness plays a central role in standard population genetics models, such as the Wright–Fisher model and the Moran process.
While absolute fitness can be used to calculate relative fitness, the reverse is not possible. Relative fitness alone contains no information about changes in total population size.

Change in Genotype Frequencies Due to Selection

Changes in genotype frequencies follow directly from differences in relative fitness. A genotype with fitness greater than the population mean will increase in frequency, while one with lower fitness will decline.
In the simple case of two competing genotypes, the change in frequency of one genotype depends on the difference between its fitness and that of the other. This difference is often expressed using a selection coefficient, which measures the strength of selection favouring one genotype over another. When selection is weak, the increase in frequency of the fitter genotype approximately follows a logistic pattern over time.

Historical Development

The phrase “survival of the fittest” was coined by the British sociologist Herbert Spencer in 1864 to describe what Charles Darwin had called natural selection. Darwin later adopted the phrase, although it was sometimes misunderstood as emphasising physical strength rather than reproductive success.
The first rigorous quantification of fitness emerged in the early twentieth century. The British-Indian biologist J. B. S. Haldane played a key role in integrating fitness into the modern synthesis of Darwinian evolution and Mendelian genetics, beginning with his 1924 work on mathematical models of selection. A major conceptual advance followed in 1964 with W. D. Hamilton’s introduction of inclusive fitness, which transformed the study of social behaviour.

Genetic Load

Genetic load measures the reduction in a population’s mean fitness relative to a reference standard, such as the fitness of the optimal genotype or the most fit genotype present. It provides a way of quantifying how much a population’s fitness is lowered by factors such as deleterious mutations, migration, inbreeding depression, or maladaptation.
Genetic load can also increase when beneficial mutations raise the maximum attainable fitness, thereby increasing the gap between the best genotype and the population average. This phenomenon is sometimes referred to as Haldane’s dilemma, highlighting the cost of continual adaptation.