Coevolution and Species Interactions

When two species reciprocally exert selection on each other, they coevolve, producing the arms races of predators and prey, the intimate fit of mutualists, and the perpetual chase of hosts and parasites.

Definition

Coevolution is reciprocal evolutionary change in interacting species, in which each acts as a selective force on the other, and species interactions are the ecological relationships through which that selection operates.

Scope

This topic covers reciprocal evolutionary change between interacting species: antagonistic coevolution and arms races in predator-prey and host-parasite systems, mutualistic coevolution in pollination and seed dispersal, the Red Queen dynamic, diffuse versus pairwise coevolution, and the geographic mosaic in which coevolution varies across populations. It treats how interactions drive diversification and adaptation.

Core questions

• How do interacting species reciprocally drive each other's evolution?
• What produces evolutionary arms races between antagonists?
• How does coevolution shape mutualisms such as pollination?
• Why does coevolution vary geographically across populations?

Key theories

Red Queen dynamics

In antagonistic interactions, each species must keep evolving simply to maintain its relative fitness as its partner counter-adapts, so that continual change yields no lasting advantage, a perpetual race captured by Van Valen's Red Queen.

Geographic mosaic of coevolution

Because the strength and outcome of selection between interacting species differ among localities, coevolution proceeds as a shifting mosaic of hotspots and coldspots whose traits are mixed by gene flow.

Mechanisms

Coevolution occurs when a trait in one species imposes selection on a trait in another, whose evolutionary response in turn alters selection back on the first. In antagonistic systems this drives escalating defences and counter-defences, as when plants evolve toxins and herbivores evolve detoxification. In mutualisms it can refine the match between partners, such as flower and pollinator morphology. Because populations interact in different ecological and genetic contexts, coevolutionary outcomes vary across space, and gene flow among them blends locally adapted traits into a geographic mosaic.

Clinical relevance

Coevolutionary thinking informs the management of crop pests and pathogens, the durability of pest-resistant crops, the evolution of drug and pesticide resistance, and the conservation of coevolved mutualisms such as pollination. This is educational context, not management prescription.

History

Ehrlich and Raven framed plant-herbivore coevolution in 1964, and Van Valen proposed the Red Queen hypothesis in 1973. Thompson developed the geographic mosaic theory in the 1990s and 2000s, embedding coevolution in spatially variable selection across interacting populations.

Debates

Pairwise versus diffuse coevolution

Whether coevolution typically involves tight reciprocal adaptation between species pairs or diffuse selection among many interacting species is debated, with most natural cases lying between the extremes.

Key figures

• Paul Ehrlich
• Peter Raven
• Leigh Van Valen
• John N. Thompson

Seminal works

• thompson2005
• ehrlich1964
• vanvalen1973

Frequently asked questions

What is the Red Queen hypothesis?

The Red Queen hypothesis holds that species in antagonistic interactions must continually evolve just to maintain their fitness as their enemies also evolve, named for the character who has to keep running to stay in the same place.

What is the geographic mosaic of coevolution?

It is the idea that coevolution between species plays out differently across their ranges, with some places where selection is strong and reciprocal and others where it is weak, producing a patchwork of locally coevolved traits.