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| - While few would contest the need for classification to reflect genealogy via use of monophyletic groupings, it is when cladistic methods are carried to their logical end result and a system of complete hierarchy is established that bitter debate erupts. A taxon which is united by common descent and yet which excludes all the groups derived from that shared ancestry, is not at all reflective of evolutionary reality—it is instead a relict of a pre-evolutionary system of typological classification (Patterson 1981, 1982). In other words it is a taxonomic artifact, which cuts evolution off in midstream by implying disparity where there is none. Thus, proponents of cladistic methodology have long argued for the need to discourage the use of paraphyletic taxa and delimit in their stead, holophyle
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| abstract
| - While few would contest the need for classification to reflect genealogy via use of monophyletic groupings, it is when cladistic methods are carried to their logical end result and a system of complete hierarchy is established that bitter debate erupts. A taxon which is united by common descent and yet which excludes all the groups derived from that shared ancestry, is not at all reflective of evolutionary reality—it is instead a relict of a pre-evolutionary system of typological classification (Patterson 1981, 1982). In other words it is a taxonomic artifact, which cuts evolution off in midstream by implying disparity where there is none. Thus, proponents of cladistic methodology have long argued for the need to discourage the use of paraphyletic taxa and delimit in their stead, holophyletic clades. The implications of this reasoning are profound—and to some systematists, unsettling. If paraphyletic grades are not indicative of evolution, then pre-evolutionary taxa established using Linnean methods are by default artificial constructs. No better example can be found than that of Reptilia sensu Linne. Linne defined Reptilia such that it excluded birds, and later systematists employed his definition with little modification for the next two hundred years. And yet, it has been evident since 1861 that birds are derived from ancestral stock somewhere within Archosauromorpha, and are therefore nested within Reptilia. Under a typological system such as Linne used, however, a form like Archaeopteryx cannot be both reptile and bird; it can only be one or the other. Yet it is both. And this is the fundamental dilemma: to extricate birds from Reptilia denudes Reptilia of any phylogenetic reality—it makes the taxon paraphyletic. Including birds and defining the avian lineage such that it is nested within Archosauromorpha and Reptilia as a whole can resurrect Reptilia as a valid taxonomic category. Thus emended, Reptilia can be defined as: “the common ancestor of extant turtles and saurians, and all its descendants” (Gauthier et al 1988). Some authors have protested that this sort of phylogenetic “semantics” is confusing both in its counter-intuitiveness and its rejection of taxonomic convention (Paul 2002). Yet neither of these reasons are satisfactory reasons to distort the phylogeny of reptiles The principal arguments, which have been advanced against a strict application of holophyly to phylogenetic reconstruction, do not dispute that holophyletic clades are preferable to paraphyletic groupings—in that the former is superior in modeling phylogeny than the latter. Rather, arguments from convention and clarity (at the expense of accuracy) have been made, but moreover, some systematists insist that there are practical difficulties in redefining taxa (particularly extinct taxa) such that they meet the requirements of holophyly (Carroll 1988). Carroll, for example, uses a cascade analogy whereby the problem of delimiting a clade is continuously offset to the next earliest representative of the lineage, and so on. Carroll uses the phylogenetic affinities of birds and dinosaurs as his primary example of this cascade in action, arguing that if one makes theropods birds, it renders Saurischia paraphyletic, and so on. However, in this case, it is by including both the sister clade of birds—Deinonychosauria—and Avialae proper (Aves sensu Linne, emended by Chiappe) within a holophyletic containing node, Eumaniraptora, that the paraphyly of more inclusive nodes is avoided. Similarly, delimiting containing clades, or nodes on a cladogram, to prevent paraphyly can be effected in systematic review of any given lineages. The concomitant drawback is the need to name each node on a cladogram, which necessarily results in an explosive proliferation of taxa—cluttering and obfuscating the phylogenetic map and rendering it largely esoteric. This undesirable result is avoided through assigning names only to major nodes. Critics of cladistics have derided this process as arbitrary, and in so doing forget that all phylogenetic reconstruction is to a degree arbitrary. Latent arbitrariness notwithstanding, it is a general convention to consider major nodes as those robustly underwritten by synapomorphies (Paul 2002). A remedy to this nomenclatural problem which has been suggested in some circles, namely amongst vertebrate paleontologists, is the use of apomorphy + clade names for taxa. Gauthier (1999) and Paul (2002) have outlined the methodology and protocol for employing apomorphy + clade names in phylogenetic reconstruction. Paul (2002) has extensively employed such a system in his review of the systematics of Dinosauria. Despite claims to the contrary, apomorphy + clade names are in no particular way more convenient than erecting new node-based names for containing clades, in that they do the same thing and while innovative, I can see no pressing argument for their wide-spread introduction in phylogenetic reconstruction. Ultimately, critiques such as those offered by Carroll (1988) and Carroll & Dong (1991) have asserted that paraphyly is largely inevitable in our systematic analyses, not the least of the reasons being the limit of our taxonomic nomenclature to articulate the dynamic processes of evolution. This objection is correct to an extent, but it is paramount to bear in mind that paraphyletic taxa do not accurately reflect phylogeny. Thus, it should remain a goal of systematists to avoid as much as possible, describing and naming such taxa.
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