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Are monophyly and synapomorphy the same or different? Revisiting the role of morphology in phylogenetics

by Leandro C S Assis, Olivier Rieppel
Cladistics ()

Abstract

Species are groups of organisms, marked out by reproductive (replicative) properties. Monophyletic taxa are groups of species,\nmarked out by synapomorphies. In Nelson⬚s analysis, monophyly and synapomorphy are identical relations. Monophyly and\nsynapomorphy, however, are not equivalent relations. Monophyly is epistemically not accessible, whereas synapomorphy is\nepistemically accessible through character analysis. Monophyly originates with speciation, the two sister-species that come into\nbeing through the splitting of the ancestral species lineage forming a monophyletic taxon at the lowest level of inclusiveness.\nSynapomorphy provides the empirical evidence for monophyly, inferred from character analysis in the context of a three-taxon\nstatement. If synapomorphy and monophyly were equivalent, phylogenetic systematists should find a single tree, instead of multiple\nequally parsimonious trees. Understanding synapomorphy as the relevant evidence for phylogenetic inference reveals a category\nmistake in contemporary phylogenetics: the treatment of morphological characters mapped onto molecular trees as synapomorphies\nand homoplasies. The mapping of morphological characters onto nodes of a molecular tree results in an empirically empty\nprocedure for synapomorphy discovery. Morphological synapomorphies and homoplasies can only be discovered by morphological\nand combined analyses. The use of morphology in phylogenetic inference in general is defended by examples from Laurales and\nSquamata in particular. To make empirical evidence scientifically relevant in order to search for concordance, or dis-concordance, of\nphylogenetic signal, is certainly more fruitful for phylogenetics than the uncritical mapping of morphological traits on a molecular\nscaffold.

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