This article provides an outline of our current understanding of the phylogeny of the kingdom Metazoa, which comprises all multicellular animals (most authors equate Metazoa with Animalia, but a few apply the latter name to a more inclusive group of Metazoa plus Choanoflagellata, the most likely sister taxon of animals). Metazoan phylogenetics has a long pedigree. The first generation of evolutionary biologists in the period immediately following the publication of Charles Darwin's On the Origin of Species enthusiastically embarked upon the challenging task of reconstructing the genealogy of life (Bowler, 1996). The influential research program of evolutionary morphology that was established by Ernst Haeckel and Carl Gegenbaur at the University of Jena in Germany had an especially prominent place at the cradle of metazoan phylogenetics as they and their followers brought both anatomical and developmental evidence to bear on the problems of animal phylogeny.
However, after a strong start in the last decades of the nineteenth century during which phylogenetic research virtually came to define evolutionary biology, interests in reconstructing the phylogeny of life diminished concomitant with a surge to embrace a more experimental approach to biological questions (see Relevance of Understanding Brain Evolution, Evolutionary Neuroethology - A Case Study: Origins and Evolution of Novel Forms of Locomotion in Hippid Sand Crabs (Malacostraca, Decapoda, Anomala).
Yet, although metazoan phylogenetics became a much less conspicuous discipline in biology at the beginning of the twentieth century, research into the deep history of the Metazoa certainly did not disappear altogether. Now, in the twenty-first century, the study of metazoan phylogeny has reached its zenith of popularity. Several factors are responsible for this situation. First, in the late 1980s and early 1990s the first analyses of metazoan phylogeny based on molecular sequence evidence were published, and these provided a huge stimulus for other workers to start analyzing metazoan phylogeny (Bergstrom, 1986; Field et al., 1988; Lake, 1990). Importantly, the continued collection and analysis of new molecular evidence have now established a widely accepted so-called new view of animal evolution (Halanych, 2004).
Second, the early 1990s witnessed the first computerized cladistic analyses of morphological evidence (Schram, 1991; Eernisse etal., 199 ). The compilation and analysis of explicit morphological data matrices made the practice of morphological phylogenetics much more transparent than heretofore. Current phy-logenetic analyses of the Metazoa habitually analyze both molecular and morphological data either separately or combined into a single large data matrix.
Third, the last quarter of the twentieth century also saw the gestation and birth of evolutionary developmental biology - evo-devo in short. The origins of this increasingly mature discipline similarly extend back to the evolutionary morphology of the late nineteenth century. Understanding the large-scale pattern of animal relationships is central to evo-devo because at the core of its research agenda is the desire to understand the evolution of animal body plans in terms of changes in developmental patterns and processes (Hall, 1999).
This article will outline our current understanding of metazoan phylogeny on the basis of available molecular and morphological evidence. The reader should note, however, that no definitive picture of animal relationships can be presented at this time. Metazoan phylogenetics is currently a highly active discipline characterized by a great flux of ideas. Many questions still remain unresolved, and the hope for an overarching consensus should at this time be tempered by the realization that different data sources and methods of analysis may yield conflicting phylogenies, as will be discussed below. Therefore the picture of metazoan phylogeny presented in this article is necessarily provisional. However, one should not underestimate the significant progress that has been made in our understanding of animal relationships in many different parts of the metazoan phylogeny in the last 15 years or so.
It should be noted that the subject area covered in this article is enormous, covering a huge literature. In view of space limitations and in the interest of readability I have included only a minimum of intext citations, principally only a small sample of important recent and comprehensive works that are expressly focused on high-level metazoan phylo-genetics, and review papers. These key references should help guide the interested reader into the vast literature on animal phylogeny. I especially recommend Halanych (2004) and selected chapters in Cracraft and Donoghue (2004) as optimal starting points for a more indepth study of metazoan phylogeny.
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