Phylogenetic analysis of the Digenea (Platyhelminthes: Cercomeria) with comments on their adaptive radiation

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Abstract
Phylogenetic analysis of 63 digenean family groups, based on 113 adult characters and 90 larval characters, produced a phylogenetic tree comprising nine orders. Adult characters alone resolved 76% of the phylogenetic tree, whereas larval characters alone resolved 74%. There was no disagreement in phylogenetic inferences drawn from only larval or only adult characters, and yet the larval forms of digeneans do not seem to be recapitulations of ancestral adult forms. The consistency index for the tree is 74%, indicating a low degree of parallel evolution in digenean morphology. Diversification in six sets of ecological characteristics combined resolves 26% of the phylogenetic tree. The combination of (i) larval and adult congruence in the absence of recapitulation, (ii) low levels of parallel evolution in morphology, and (iii) ecological diversification lagging far behind morphological change discounts traditional notions of adaptive radiations. Digeneans have experienced great morphological diversification constrained by their developmental programs and history, and have maintained their functional intergrity with few changes caused by extrinsic, "adaptive," forces.