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The Monophyly of the Ecdysozoa

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The monophyly of the Ecdysozoa has raised questions based off of evaluations of datasets from whole genomes. Interconnections within the group itself are also fairly disputed. Both morphological and molecular evidence are utilized when determining whether this superphylum is really monophyletic, and much of the data yields two potential hypotheses: the Ecdysozoa and Lophotrochozoa hypothesis, dividing protostomes into Ecdysozoa and Lophotrochozoa groups, and the Articulata hypothesis, of which arthropods and annelids are interrelated as Articulata. Though morphological evidence has been used to support of both hypotheses in terms of being monophylic, various zoologists stay devoted to the Articulata concept while most computer-assisted cladistic analyses appear to support Ecdysozoa. This phylogenetic debate can be taken into the perspective as the resultant of biased interpretation – that since the differences in historical epistemology are adopted in different ways by different researchers, it inevitably leads to much disagreement.

The solution to this problem is by performing a set of morphological sensitivity experiments on assorted published morphological datasets. After gathering data from random samples, the bonding between different phylogenetic epistemologies is going to be revealed, thus to establish the truthfulness of the Articulata and Ecdysozoa hypotheses. On the genetics side, the publication of the seminal paper by Aguinaldo et al. (1997) illustrates one of the most impactful molecular evidence between the Articulata and Ecdysozoa hypotheses. Not only does it present perhaps the clearest example of the surprising depth at which molecular evidence forces us to reconceptualize the evolution, but it also reassess how hypotheses should be generated. However, there does appear to be several methodological reasons for the discrepancies found later in phylogenetic reconstruction only using molecular data. Because of these discrepancies, genetics by itself is not enough to prove either hypothesis. By analyzing and comparing morphological characters, genetic sequences, and reasons of separation between both Ecodysozoa and Articulata, it can be concluded whether either group is justified to what they called to be.

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During the 20th century, research and publications about animal phylogeny have not generally been regarded as the epitome of exciting reporting in the world of scientific advancements. However, all of this changed in the late 1980s when the scientific community was alerted by the emergence of rather unexpected molecular metazoan phylogenies that significantly contradicts with many of the previous discoveries already engraved in textbooks. The molecular and morphological evidence found so far suggests multiple noticeable phylogenetic discrepancies that are in need of explanation. The Articulata-Ecdysozoa debate creates numerous questions that draws attention to many major topics on a variety of levels of universality in comparative zoology, ranging from the use of morphology verses molecules, adoption of different epistemologies in phylogenetic research, use of evidence from fossil record in morphological pattern reconstruction, and the possible likelihood of convergence of superficially convincing homologies such as segmentation and cuticle molting. The Articulata-Ecdysozoa debate also entails a renewed look at some of the everlasting problems, such as how we can determine whether morphologies with only a limited amount of similarities are potentially true homologies that have become modified during evolution or independently evolved features. Needless to say, the study of these elements provides an intellectual escapade that has the power to embellish many aspects of metazoan phylogenetics as a science, however, without the hope of a quick and easy resolution.

There are two main approaches in terms of organism radiation. The first is depended on traditional, morphology-based phylogenetics and the second ‘new’ phylogeny is based on molecular data, especially 18S rRNA and Hox gene. Similar techniques have also been implemented in studies on metazoan propagation. The morphology based studies emphasize the strong similarities in the relationship between annelids and arthropods (showing support for the Articulata hypothesis), especially segmentation, with details such as the addition of new segments from para-segments, existence of a posterior growth zone, coelomic/mesodermal compartments, and metanephridia. Opposingly, nearly all molecular studies based on Hox gene structure and 18S rRNA sequencing emphasize that Ecdysozoa consists a monophyletic group, whereas Lophotrochozoa, comprised of Trochozoa and Lophozoa, is less strongly supported and phyla such as annelids and molluscs are often seen as polyphyletic.

On the other hand, some scientists have investigated the morphological out-turn involved in the Articulata−Ecdysozoa inconsistency by comparing similarities/differences of nematodes and arthropods versus annelids and arthropods, but without reaching any determined resolution. Organisms within Ecdysozoa are classified by the character of moulting, which is apomorphic. However, most of the lophotrochozoan characters, such as ciliated larvae and epithelia, has to be plesiomorphic since these characters are found present in both deuterostomes and cnidarians. This discrepancy alone indicates that Lophotrochozoa may possibly be paraphyletic. Not a lot of papers try to integrate these two datasets, but when they do, it is usually with the use of cladistic analyses for the combined datasets. However, these analyses are always performed without thorough examinations of morphological implications, which is clearly unsatisfactory and produces unpersuasive results.

History of the Articulata-Ecdysozoa Debate based on Morphology

It is quite interesting that with the current Articulata-Ecdysozoa debate we have for the first time in metazoan phylogenetics history, most people in the community of invertebrate morphologists side with respect to the Arthropoda in terms of phylogenetic position. The system of comparative zoology that existed before were very different in various parts of the globe before the introduction of molecular systematics. And among these older traditions, the German and Anglo-Saxon traditions have sustained varying views on animal phylogeny that were influenced strongly by only a relatively small population of zoologists. Until fairly recently, Libbie Hyman’s ideologies were widely accepted as the model for the Anglo-Saxon way of animal phylogeny, ranging from invertebrate zoology chapters in American textbooks to current review papers written on metazoan phylogenetics. On the other hand, German literature on animal phylogeny is based off of views of widely influential zoologists such as Werner Ulrich, Adolf Remane, and Rolf Siewing.

Surprisingly, taking into consideration that these different comparative invertebrate zoology traditions have existed for a long time, it is striking to see only until very recently is the validity of Articulata questioned. Ever since Georges Cuvier proposed the pre-Darwinian existence of the Articulata, the close relationship between arthropods and annelids has been on a consensus view among various zoologists throughout time, from the first generation of evolutionary morphologists of the 19th century in Britain and Germany, to the most contemporary zoologists in the 1990s. Surely, nowadays arthropods have been said to be closely related to what we now consider members of the Ecdysozoa at various points throughout history in both the German and English literature, for example Kristensen, Rauther, and Bütschli, but these views have never established the status of being accepted as textbook knowledge. However, this consensus seems to now have dissipated, and the Articulata-Ecdysozoa debate is now officially confirmed to be included within morphological phylogenetics.


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