A feature is an adaptation for a particular function if it has evolved by natural selection for that function by enhancing the relative rate of increase—the fitness—of biological entities with that feature.
Because many characteristics are genetically variable in natural populations, they may evolve rapidly if selection pressures change. Especially because humans drastically alter environments and move species into new environments, many historical examples of rapid adaptive evolution have been documented, often within much less than a century.
Natural selection is a consistent difference in fitness among phenotypically different biological entities. It is the antithesis of chance. Natural selection may occur at different levels, such as genes, individual organisms, populations, and species.
Selection at the level of genes or organisms is likely to be the most important because the numbers and turnover rates of these entities are greater than those of populations or species. Therefore most features are unlikely to have evolved by group selection, the one form of selection that could in theory promote the evolution of features that benefit the species even though they are disadvantageous to the individual organism. Both genic and individual selection can be viewed as fitness differences among genes, with “selfish genes” being those that prevail.
Species selection is a correlation between a trait and the rate of speciation or extinction. It can result in variation among clades in diversity of species.
Not all features are adaptations. Methods for identifying and elucidating adaptations include studies of function and design, experimental studies of the correspondence between fitness and variation within species, and correlations between the traits of species and environmental or other features (the comparative method). Phylogenetic information may be necessary for proper use of the comparative method.
Organisms may not have perfect adaptations because of functional compromises or trade-offs, or because mutations enabling perfect adaptation have not been available.
As a consequence of adaptation of species to different environments and ways of life, natural selection is the basis of adaptive radiations and adaptive diversity. Competition for resources is one of many factors that can select for differences among species.
Natural selection need not promote harmony or balance in nature, and utterly lacking any moral content, it provides no foundation for morality or ethics in human behavior.