Introduction

Adaptive radiation is defined as the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage (Simpson 1953; Schluter 2000). Examples include the diversification of Darwin's finches on the Galápagos islands, Anolis lizards on Caribbean islands, Hawaiian silverswords, a mainland radiation of columbines, and cichlids of the East African Great Lakes, among many others (Simpson 1953; Givnish & Sytsma 1997; Losos 1998; Schluter 2000; Gillespie 2004; Salzburger & Meyer 2004; Seehausen 2006). Adaptive radiation typically follows the colonization of a new environment or the establishment of a ‘key innovation’ (e.g. nectar spurs in columbines, Hodges 1997) which opens new ecological niches and/or new paths for evolution.

Adaptive radiation is both spectacular and a remarkably complex process, which is affected by many different factors (genetical, ecological, developmental, environmental, etc.) interweaving in non-linear ways. Different, sometimes contradictory scenarios explaining adaptive radiation have been advanced (Simpson 1953; Mayr 1963; Schluter 2000). Some authors emphasize random genetic drift in small founder populations (Mayr 1963), while others focus on strong directional selection in small founder populations (Eldredge 2003; Eldredge et al. 2005), strong diversifying selection (Schluter 2000), or relaxed selection (Mayr 1963). Identifying the more plausible and general scenarios is a highly controversial endeavour. The large timescale involved and the lack of precise data on its initial and intermediate stages even make identifying general patterns of adaptive radiation very difficult (Simpson 1953; Losos 1998; Schluter 2000; Gillespie 2004; Salzburger & Meyer 2004; Seehausen 2006).