technologies to follow expression and sequence variation at the population level. The genetic dissection of other adaptive traits in Aquilegia should also be possible soon as genomic resources such as whole-genome sequencing become available.
Adaptive radiations have played, and continue to play, a central role in the development of evolutionary theory. After his exploration of the Galápagos Islands, Darwin recognized that variants of species were confined to specific islands. In particular, Darwin (1963) noted that mockingbirds he collected on different islands represented 3 distinct forms and that all of the forms were similar to mockingbirds from the mainland of South America. He further noted that the different islands were within sight of one another, all quite similar in their habitats, and apparently geologically young. Based on these observations, Darwin (1876) began to suspect that species gradually became modified. Thus began his search for a process that could account for such observations, leading to his formulation of the theory of natural selection.
Key to Darwin’s suspicion of common ancestry was the close proximity of environmentally similar islands comprising the Galápagos archipelago. In fact, many classic examples of adaptive radiations involve islands or lakes; notable examples include Darwin’s finches of the Galápagos, honeycreeper birds and silversword plants of Hawaii, and cichlid fish of lakes Malawi and Victoria in Africa. The power of these examples for inferring the force of natural selection in evolution stems from the marked diversity in form and function among a group of clearly closely related taxa. The isolation of islands and lakes makes plausible the inference that the taxa are descended from a single ancestor, and the link between variation in form and function makes credible the notion that natural selection and evolution have played key roles.
A great deal of research effort has gone into substantiating the assumptions behind examples of adaptive radiation. In an influential book, Schluter (2000) laid out the definition of adaptive radiation as having 4 features: (i) common ancestry, (ii) a phenotype–environment correlation, (iii) trait utility, and (iv) rapid speciation. Monophyly and rapid speciation for many of the classic examples of adaptive radiation have been established by using molecular techniques [e.g., cichlids (Meyer et al., 1990), Galápagos finches (Petren et al., 1999; Sato et al., 1999), and Hawaiian silverswords (Baldwin and Sanderson, 1998)]. Ecological and manipulative experiments are used to identify and test phenotype–environmental correlations and trait utility. Ultimately, such studies have pointed to the link between divergent natural selection and reproductive isolation and, thus, speciation (Schluter, 2000).