COMPARATIVE-STUDY OF ADAPTIVE RADIATIONS WITH AN EXAMPLE USING PARASITIC FLATWORMS (PLATYHELMINTHES, CERCOMERIA)

Studies of adaptive radiations require robust phylogenies, estimates o f species numbers for monophyletic groups within clades, assessments o f the adaptive value of putative key innovations, and estimates of the frequency of speciation modes. Four criteria are necessary to identif y an adaptive radiation within the parasitic platyhelminths: (1) a gro up contains significantly more species than its sister group, (2) spec ies richness is apomorphic, (3) apomorphic traits enhance the potentia l for adaptively driven modes of speciation (sympatric speciation and speciation by peripheral isolation via host switching), and (4) the fr equency of adaptively driven speciation modes is high within the group when compared with data from free-living groups. Only the species-ric h Monogenea fulfill all four criteria. The Digenea and Eucestoda also are more species rich than their sister groups, their species richness is derived, and they possess unique characters that increase the pote ntial for host switching to occur. However, because there is not enoug h information to determine whether the frequency of adaptive modes of speciation is high for those groups, we cannot yet assert that their r adiations have been adaptive.