All types of interactions have the potential to influence the population sizes of interacting species. By contributing to the differential survival or reproduction of individuals with different traits, they can also alter genotype frequencies within the interacting populations over time. Thus these interactions have both ecological consequences, as when they affect the distribution and abundance of a species, and evolutionary consequences, as when they lead to evolutionary change. Darwin observed that evolutionary change occurs not only in response to physical conditions, as described in Chapter 53, but also in response to interactions among species. In his introduction to On the Origin of Species, Darwin pointed out that woodpeckers have feet, tails, beaks, and tongues “admirably adapted to catch insects under the bark of trees” as a result of their long-standing interactions with their insect prey.

While abiotic factors also act as agents of selection, they differ in a fundamental way from biotic agents of selection in that they do not themselves undergo change as a result of the interaction. Snow and ice do not become more deadly as a result of encountering cold-resistant organisms, but predators can, over evolutionary time, become swifter, more powerful, or more efficient at capturing their prey. In response, prey species may become swifter, tougher, less conspicuous, or more poisonous, all of which decrease their likelihood of being consumed. This back and forth evolutionary response between interacting species is known as coevolution.

The types of interactions most likely to lead to coevolution are those that occur predictably and with high frequency over time and that have a strong effect on the fitness of species. Thus species involved in amensal and commensal interactions are less likely to experience coevolutionary change as a result of the interaction than are species involved in predatory, competitive, and mutualistic interactions.