Studies of adaptive radiations have exploded during the last 20 years. In a search of the ISI Web of Science with “adaptive radiation” (limited to the subject area of evolutionary biology) we found 80 articles published in 2008 compared with only 1 in 1990. Furthermore, citations of these articles have grown exponentially, from 34 citations in 1990 to >3,000 in 2008. The growing interest in adaptive radiations may be ascribed to several factors. First, molecular phylogenetics has allowed the identification of monophyly and rapid speciation in taxa outside of the traditionally recognized examples from isolated islands and lakes [e.g., Hodges and Arnold (1994)] and thus has expanded the repertoire of study systems. Second, there has been a resurgence in the study of speciation (Coyne and Orr, 2004). Speciation research has often focused on the development of genetic incompatibilities [e.g., Mihola et al. (2009) and Phadnis and Orr (2009)], whereas studies of adaptive radiations have focused attention on the role of divergent natural selection to alternate environments as a primary cause of reproductive isolation (Schluter, 1996, 2000, 2001; Rundle et al., 2000; Via, 2001). Thus, studies of adaptive radiations focus on both the evolution of adaptations and reproductive isolation.
Understanding the processes of adaptation and speciation requires considering taxa that have not yet fully attained reproductive isolation. Fundamentally, the ability to make hybrids allows the dissection of the genetic basis of traits and permits tests of how individual genetic elements could affect individual fitness. For example, Bradshaw and Schemske (2003) made near-isogenic lines of Mimulus cardinalis and Mimulus lewisii, each containing the alternate allele from the other species for a flower color locus. They found that pollinator visitation patterns radically changed and that, in the proper ecological setting, an adaptive shift in pollinator preference could possibly occur with a single mutation (Bradshaw and Schemske, 2003). In addition, studying speciation and adaptation early in the process has significant advantages because subsequent genetic changes can obscure the actual causal mutations [Via and West (2008); see also Via, Chapter 1, this volume]. For example, loss-of-function mutations that confer an adaptive advantage may be followed by additional mutations that would, on their own, cause loss of function but were not involved with the evolution of the trait itself (Zufall and Rausher, 2004). Thus, recent adaptive radiations are especially fruitful resources for dissecting the genetic basis of adaptations and speciation.
Adaptive radiations have also played a major role in identifying evolutionary trends. This area of study is important because trends imply predictable patterns in evolutionary history. There has been, for example, much debate about whether body size and complexity increase through time (Damuth, 1993; Jablonski, 1997; Hibbett and Binder, 2002; Hibbett, 2004; Van Valkenburgh et al., 2004). One way to test for repeatable trends