Introduction

What Is Coevolution?

All organismal populations experience multiple selective pressures deriving from varied aspects of their environment. In addition to abiotic features (e.g., climate), this 'environment' is often comprised of many other organisms. Thus, most populations evolve in response to interactions with other species. While the abiotic components of the environment cannot evolve in response to organisms, the biotic components can - and this phenomenon has played an integral role in the evolution of phenotypic diversity. Coevolution is reciprocal evolutionary change between interacting species driven by natural selection. That is, each player in a coevolutionary relationship evolves adaptations in response to its interaction with the other player(s). Although this general concept has been around since Darwin, the term 'coevolution' was coined by Paul Ehrlich and Peter Raven in a classic article in 1964, ''Butterflies and plants: A study in coevolution.'' Since then, the field of research examining coevolution has blossomed into a large-scale research program.

The Broad Importance of Coevolution

Coevolution is undisputed as one of the most important processes shaping biodiversity. The importance of coevolution goes far beyond the classic examples, such as predator-prey coevolutionary arms races, figs and fig wasps, yuccas and yucca moths, ants and acacias, and fungal farming by several taxa. Coevolution's influence spans all subdisciplines within ecology and evolutionary biology. Indeed, a large extent of the historical and ongoing patterns of phenotypic evolution and species diversification is the product of coevolution. Coevolution can stem from numerous types of species interactions that are commonplace on this planet, such as interspecific competition for resources, predator-prey interactions, host-parasite interactions, plant-herbivore interactions, and flower-pollinator interactions. Even the eukaryotic cell originated from a symbiotic relationship where one of the species evolved into the organelles we now call mitochondria. A similar scenario is responsible for the formation of chloroplasts, and thus the origin of plants. Most vertebrate and invertebrate species rely heavily on coevolved symbionts residing within their digestive system or other special organs to allow proper digestion and growth. Coral reefs, and the communities they support, depend largely on coevolved symbioses between corals and zooanthellae, as well as interactions with other corals and algae-feeding fish. The symbiotic organisms, lichens, are critically important during primary succession in terrestrial ecosystems. Even the colonization of land by plants was facilitated by mutualistic interactions with mycorrhizal fungi. Coevolution's influence is so far reaching that the history of life on Earth would be unrecognizable in the absence of it.

Empirical Evidence for Coevolution

Despite the widespread importance of coevolutionary interactions, empirical demonstration of coevolution is a difficult task. This derives from the inherent difficulties of demonstrating adaptation - much less reciprocal adaptation - that has plagued evolutionary biology for many decades (see Adaptation) Nevertheless, a great deal of supportive evidence has been gathered for coe-volution, coming from an assortment of tests for an array of hypotheses stemming from coevolutionary theory. As is discussed below, coevolution comes in many forms, and thus there are different manners in which researchers approach the question of coevolutionary association. As a general approach, researchers often examine phe-notypic, ecological, and genetic evidence to test the hypothesis that organisms are evolving (or have evolved) in response to one another.

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