Table 1 Glossary definitions of “natural selection” and “fitness” from leading evolutionary textbooks

Ridley (2004)

The process by which the forms of organisms in a population that are best adapted to the environment increase in frequency relative to less well-adapted forms over a number of generations

The average number of offspring produced by individuals with a certain genotype, relative to the number produced by individuals with other genotypes. When genotypes differ in fitness because of their effects on survival, fitness can be measured as the ratio of a genotype's frequency among the adults divided by its frequency among individuals at birth

Futuyma (2005)

The differential survival and/or reproduction of classes of entities that differ in one or more characteristics. To constitute natural selection, the difference in survival and/or reproduction cannot be due to chance, and it must have the potential consequence of altering the proportions of the different entities. Thus, natural selection is also definable as a deterministic difference in the contribution of different classes of entities to subsequent generations. Usually, the differences are inherited. The entities may be alleles, genotypes or subsets of genotypes, populations, or, in the broadest sense, species. A complex concept

The success of an entity in reproducing; hence, the average contribution of an allele or genotype to the next generation or to succeeding generations

Stearns and Hoekstra (2005)

The correlation of a trait with variation in reproductive success

Relative lifetime reproductive success, which includes the probability of surviving to reproduce. In certain situations, other measures are more appropriate. The most important modifications to this definition include the inclusion of the effects of age-specific reproduction and of density dependence

Rose and Mueller (2006)

The differential net reproduction of genetically distinct entities, whether mobile genetic elements, organisms, demes, or entire species

The average reproduction of an individual or genotype, calibrated over a complete life cycle

Barton et al. (2007)

The process by which genotypes with higher fitness increase in frequency in a population

The number of offspring left by an individual after one generation. The fitness of an allele is the average fitness of individuals carrying that allele

Freeman and Herron (2007)

A difference, on average, between the survival or fecundity of individuals with certain phenotypes compared with individuals with other phenotypes

The extent to which an individual contributes genes to future generations or an individual's score on a measure of performance expected to correlate with genetic contribution to future generations (such as lifetime reproductive success)

Hall and Hallgrimsson (2008)

Differential reproduction or survival of replicating organisms caused by agencies other than humans^a. Because such differential selective effects are widely prevalent and often act on hereditary (genetic) variations, natural selection is a common major cause for a change in the gene frequencies of a population that leads to a new distinctive genetic constitution (evolution)

Central to evolutionary theory evaluating genotypes and populations, fitness has many definitions, ranging from comparing growth rates to comparing long-term survival rates. The basic fitness concept that population geneticists commonly use is relative reproductive success, as governed by selection in a particular environment

Kardong (2008)

The culling process by which individuals with beneficial traits survive and reproduce more frequently, on average, than individuals with less favorable traits

The relative reproductive success of individuals, within a population, in leaving offspring in the next generation. At the genetic level, fitness is measured by the relative success of one genotype (or allele) compared to other genotypes (or alleles)