Adaptive radiation is the evolution of diversity within a rapidly multiplying lineage. It can cause a single ancestral species to differentiate into an impressively vast array of species inhabiting a variety of environments. Much of life's diversity has arisen during adaptive radiations. Some of the most famous recent examples include the East African cichilid fishes, the Hawaiian silverswords, and Darwin's Galapagos finches. This book evaluates the causes of adaptive radiation. It focuses on the 'ecological' theory of adaptive radiation, a body of ideas that began with Darwin and were developed through the early part of the 20th century. This theory proposes that phenotypic divergence and speciation in adaptive radiation are caused ultimately by divergent natural selection arising from differences in environment and competition between species. The text re-evaluates the ecological theory, along with its most significant extensions and challenges, in the light of all the recent evidence. It is the first full exploration of the causes of adaptive radiation in decades.