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“Reconciling Phylogenetic Systems and HGT: Perils and Prospects” (12/8/17)

January 24, 2018

Phillip Honenberger

Abstract: Phylogenetic systematics, in the sense articulated by Hennig (1966) and since defended and developed by many others (e.g. Wiley 1980, Wiley and Leiberman 2011), proposes to classify organisms on the basis of nested separations of lineages in the course of evolutionary history. Yet the increasingly appreciated frequency of horizontal gene transfer – that is, cases wherein an organism receives genetic material from the environment or other organisms by non-reproductive processes, such as viral insertion – plausibly challenges key features of the phylogenetic perspective, as Doolittle (1999, 2010) and others have argued. These challenges raise the question of whether and to what extent phylogenetic systematics is an appropriate approach to classifying organisms, species, and lineages wherein HGT has played a role. My aim in this talk is twofold. First, I seek to articulate the tension between HGT and phylogenetic systematics as clearly as possible. Second, I describe and evaluate the most promising strategies for resolution, based on existing literature. These strategies include denying the applicability of phylogenetic systematics to introgressed lineages; founding phylogenetic systematics on “trees of cells” rather than “trees of characters” or “trees of genes”; and incorporating “speciation by hybridization” into phylogenetic models. These strategies of resolution are not entirely unfeasible. However, each has significantly troubling implications for long-standing amibitions of phylogenetic systematics, such as to provide a universal and non-ambiguous hierarchical classification of organic life (Hennig 1966). If time permits, I’ll offer a few remarks about what I take this tension and the limited prospects for its resolution to mean for biological classification and the reconstruction of evolutionary history more generally.

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