Abstract
Phenotypic variations allow a lineage to move into new regions of the adaptive landscape. The purpose of this study is to analyse the life history of the pearlfishes (Carapinae) in a phylogenetic framework and particularly to highlight the evolution of parasite and commensal ways of life. Furthermore, we investigate the skull anatomy of parasites and commensals and discuss the developmental process that would explain the passage from one form to the other. The genus Carapus forms a paraphyletic grouping in contrast to the genus Encheliophis, which forms a monophyletic cluster. The combination of phylogenetic, morphologic and ontogenetic data clearly indicates that parasitic species derive from commensal species and do not constitute an iterative evolution from free-living forms. Although the head morphology of Carapus species differs completely from Encheliophis, C. homei is the sister group of the parasites. Interestingly, morphological characteristics allowing the establishment of the relation between Carapus homei and Encheliophis spp. concern the sound-producing mechanism, which can explain the diversification of the taxon but not the acquisition of the parasite morphotype. Carapus homei already has the sound-producing mechanism typically found in the parasite form but still has a commensal way of life and the corresponding head structure. Moreover, comparisons between the larval and adult Carapini highlight that the adult morphotype “Encheliophis” is obtained by going beyond the adult stage reached by Carapus. The entrance into the new adaptive landscape could have been realised by at least two processes: paedomorphosis and allometric repatterning.
Highlights
Adaptive radiation of a taxon results from morphological, physiological or behavioural modifications (Schluter, 2001), and the life histories of taxa are characterised by periodic introductions of novelties that have had significant effects on subsequent ecological and evolutionary diversity
Two main categories of innovations can be recognized in terms of how they impact the evolutionary dynamics and diversity of functional systems: those that directly influence the potential for phenotypic variation and those that allow the lineage to move into new regions of the adaptive landscape where new variants are favoured (Wainwright, 2007)
Each partition was analysed separately, but the resulting trees are not shown here as they were largely congruent; the parts that were not congruent compared to the analyses of the full data set are explained below
Summary
Adaptive radiation of a taxon results from morphological, physiological or behavioural modifications (Schluter, 2001), and the life histories of taxa are characterised by periodic introductions of novelties that have had significant effects on subsequent ecological and evolutionary diversity. These modifications can result from one or several changes of an ancestral body plan (Zelditch & Fink, 1996) or from the emergence of novelties. Two main categories of innovations can be recognized in terms of how they impact the evolutionary dynamics and diversity of functional systems: those that directly influence the potential for phenotypic variation and those that allow the lineage to move into new regions of the adaptive landscape where new variants are favoured (Wainwright, 2007). Heterochrony corresponds to changes in developmental rate or the relative time of appearance of features that link stages of ontogeny and phylogeny, for a given age, the descendant has a shape typical of the ancestor at a more juvenile age (paedomorphosis) or at a more mature age (peramorphosis) or retains ancestral shape but differs in size (Webster & Zelditch, 2005). Some evolutionary modifications to ontogeny can lie beyond the realm of changes in developmental rate or timing and various non-heterochronic modes of developmental reprogramming have been proposed (Webster & Zelditch, 2005)
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