In biology, coevolution is "the change of a biological object triggered by the change of a related object." Coevolution can occur at many biological levels: it can be as microscopic as correlated mutations between amino acids in a protein, or as macroscopic as covarying traits between different species in an environment. Each party in a coevolutionary relationship exerts selective pressures on the other, thereby affecting each other's evolution. Coevolution of different species includes the evolution of a host species and its parasites (host–parasite coevolution), and examples of mutualism evolving through time. Evolution in response to abiotic factors, such as climate change, is not coevolution (since climate is not alive and does not undergo biological evolution). Coevolution between pairs of entities exists, such as that between predator and prey, host and symbiont or host and parasite, but many cases are less clearcut: a species may evolve in response to a number of other species, each of which is also evolving in response to a set of species. This situation has been referred to as "diffuse coevolution."
There is little evidence of coevolution driving large-scale changes in Earth's history, since abiotic factors such as mass extinction and expansion into ecospace seem to guide the shifts in the abundance of major groups. However, there is evidence for coevolution at the level of populations and species. For example, the concept of coevolution was briefly described by Charles Darwin in On the Origin of Species, and developed in detail in Fertilisation of Orchids. It is likely that viruses and their hosts may have coevolved in various scenarios.
Coevolution is primarily a biological concept, but has been applied by analogy to fields such as computer science and astronomy.