The evolution of complementary adaptations in two species caused by the selection pressures that each exerts on the other. It is common in symbiotic associations (see symbiosis). For example, many insect-pollinated plants have evolved flowers whose shapes, colours, etc., make them attractive to particular insects; at the same time the pollinating insects have evolved sense organs and mouthparts specialized for quickly locating, and extracting nectar from, particular species of plants.