What is the term for two species evolving in response to each other?

The term for two species evolving in response to each other is coevolution. In biology, coevolution occurs when interacting species exert reciprocal natural selection, so a change in one species helps shape evolutionary changes in the other.

A classic example is an evolutionary arms race between a predator and its prey: improved defenses in the prey can favor more effective hunting adaptations in the predator, which can then favor still stronger defenses. Coevolution can also involve mutualism. Flowering plants and their pollinating insects may influence one another’s traits, such as floral shape, timing, or feeding structures.

Charles Darwin discussed reciprocal relationships between plants and insects in On the Origin of Species in 1859, using the related idea of coadaptation. The concept later expanded through studies of acacias and ants, plant defenses and herbivores, and hosts and pathogens. Coevolution does not mean every species living together evolves as a tightly matched pair; diffuse or guild coevolution can involve networks of several species. Parallel, divergent, and convergent evolution describe other patterns and do not specifically require reciprocal interaction.