Coevolution is the process by which two or more species reciprocally shape each other’s evolution through sustained selective pressure — a change in one lineage creates a new selective environment for the other, which adapts in turn, which reshapes the pressure back on the first. It ranges from tightly paired relationships (a single predator and its prey) to diffuse coevolution across a whole guild of interacting species.
Coevolution
Coevolution is usually split into two broad modes, depending on whether the interacting species’ interests align or conflict:
- Antagonistic coevolution — between species with opposing interests: predator/prey, host/parasite, plant/herbivore. Each side is selected to outcompete the other (better defense vs. better attack), producing an evolutionary arms race. This is the basis of the Red Queen Hypothesis (Van Valen, 1973): because part of each species’ environment is the ongoing adaptation of its rivals, organisms must keep evolving just to hold their relative fitness steady, not to improve it.
- Mutualistic coevolution — between species that benefit each other, such as flowering plants and their pollinators, or hosts and their microbial symbionts. Selection favors traits that make the exchange more efficient for both sides, which can drive extreme reciprocal specialization — Darwin’s prediction of a moth with a foot-long tongue to pollinate Madagascar’s star orchid, confirmed decades later, is the textbook case.
The term was formalized by Paul Ehrlich and Peter Raven in their 1964 paper on butterflies and their host plants, which proposed coevolution between insects and plants as a major driver of terrestrial biodiversity. Later work — notably John Thompson’s geographic mosaic theory of coevolution — showed that the strength and even the direction of coevolutionary selection between the same two species can vary considerably across different populations and localities, rather than being one fixed, uniform relationship.