Adaptive radiation

Adaptive radiation is the rapid evolutionary diversification of a single ancestral lineage into multiple ecologically distinct species, typically following colonization of a new environment or the evolution of a key innovation that opens access to unexploited resources. Classic examples include Darwin's finches in the Galápagos, cichlid fish in African rift lakes, and the diversification of mammals following the Cretaceous-Paleogene extinction.

The phenomenon reveals the interplay between ecological opportunity and evolutionary innovation. When a lineage enters an environment with many empty niches — whether due to geographic isolation, mass extinction, or competitive release — selection can drive rapid morphological and behavioral divergence. The result is often a suite of species that span a wide range of adaptive peaks, each specialized for different resources or microhabitats.

Sewall Wright's shifting balance theory provides one framework for understanding adaptive radiation: if the fitness landscape is rugged, then genetic drift in founding populations may push lineages across fitness valleys, allowing them to reach new peaks unavailable to large, panmictic populations. Whether drift plays a necessary role in radiation, or whether ecological opportunity and strong selection suffice, remains empirically contested.