Fitness (biology)

In biology, fitness is a quantitative way to measure reproductive success. It describes how much an individual contributes to the gene pool of the next generation. This measurement can apply to a specific genotype, which is a set of genes. It can also apply to a phenotype, which is the physical expression of those genes. Scientists often view fitness as a propensity or a probability. It is not a guarantee of how many offspring an individual will have. Instead, it is the expected average for a class of individuals. For example, a single unlucky event cannot lower the fitness of a genotype. If a specific infant dies unexpectedly, that is bad luck, not low fitness. Fitness is a property of a group, not just one single organism.

Fitness works through the relationship between genes and the environment. A genotype's fitness is expressed through its phenotype. However, the developmental environment can also affect how a phenotype appears. Because environments change, the fitness of a phenotype can change too. In one selective environment, a trait might be very helpful. In a different environment, that same trait might offer no advantage. For organisms that use asexual reproduction, scientists can assign fitness directly to genotypes. For organisms using sexual reproduction, the process is more complex. Recombination scrambles alleles into new combinations every generation. In these cases, scientists assign fitness to alleles by averaging them across different genetic backgrounds.

There are two primary ways to measure fitness: absolute and relative. Absolute fitness measures the proportional change in a genotype's abundance over one generation. If the absolute fitness is greater than 1, the genotype is increasing in number. If it is less than 1, the genotype is declining. Relative fitness is used to see how different genotypes compare to one another. It determines the change in genotype frequency within the total population. Scientists often set one genotype as a reference point with a relative fitness of 1. Relative fitness can be any non-negative number, including zero. While absolute fitness tells us about population growth, relative fitness tells us about the prevalence of traits.

Natural selection uses these differences to drive Darwinian evolution. When a genotype has a higher relative fitness than the mean, its frequency increases. If its fitness is lower than the mean, its frequency declines. In cases where only two genotypes exist, the change follows a specific pattern. If one genotype is fitter than the other, its frequency grows approximately logistically. This process is often described using a selection coefficient. This coefficient helps scientists calculate the rate of change between two competing genotypes.

Beyond individual success, scientists study inclusive fitness. This concept includes the ability of an allele to help others survive. Specifically, it looks at how an allele helps other individuals who share that same allele. This mechanism is often called kin selection. To avoid double counting, inclusive fitness excludes the contribution of others to the focal individual. It focuses on the net benefit the allele provides to the shared gene pool. This explains why certain social behaviors evolve in the animal kingdom.

Several important thinkers shaped these biological concepts. Herbert Spencer coined the phrase "survival of the fittest" in his 1864 work, *Principles of Biology*. He used this term to describe Charles Darwin's idea of natural selection. Later, J.B.S. Haldane became the first to quantify fitness mathematically. He did this in his 1924 paper regarding natural and artificial selection. In 1964, W.D. Hamilton introduced the concept of inclusive fitness. His work helped explain the evolution of social behavior through genetics.

Finally, scientists use the term genetic load to measure population health. Genetic load is the average fitness of a population relative to an ideal genotype. It can also be measured against the most fit genotype actually present. Genetic load can increase due to harmful mutations or inbreeding. It can also increase through outcrossing or migration. Another type is substitutional load, which is the cost of selection. This happens when beneficial mutations increase the maximum fitness possible. This creates a new, higher standard for all other mutations to meet.