The series,the network,and the tree: changing metaphors of order in nature |
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Authors: | Olivier Rieppel |
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Institution: | (1) Department of Geology, The Field Museum, 1400 S. Lake Shore Drive, Chicago, IL 606605-2496, USA |
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Abstract: | The history of biological systematics documents a continuing tension between classifications in terms of nested hierarchies
congruent with branching diagrams (the ‘Tree of Life’) versus reticulated relations. The recognition of conflicting character
distribution led to the dissolution of the scala naturae into reticulated systems, which were then transformed into phylogenetic trees by the addition of a vertical axis. The cladistic
revolution in systematics resulted in a representation of phylogeny as a strictly bifurcating pattern (cladogram). Due to
the ubiquity of character conflict—at the genetic or morphological level, or at any level in between—some characters will
necessarily have to be discarded (qua noise) in favor of others in support of a strictly bifurcating phylogenetic tree. Pattern analysts will seek maximal congruence
in the distribution of characters (ultimately of any kind) relative to a branching tree-topology; process explainers will
call such tree-topologies into question by reference to incompatible evolutionary processes. Pattern analysts will argue that
process explanations must not be brought to bear on pattern reconstruction; process explainers will insist that the reconstructed
pattern requires a process explanation to become scientifically relevant, i.e., relevant to evolutionary theory. The core
question driving the current debate about the adequacy of the ‘Tree of Life’ metaphor seems to be whether the systematic dichotomization
of the living world is an adequate representation of the complex evolutionary history of global biodiversity. In ‘Questioning
the Tree of Life’, it seems beneficial to draw at least four conceptual distinctions: pattern reconstruction versus process explanation as different epistemological approaches to the study of phylogeny; open versus closed systems as expressions of different kinds of population (species) structures; phylogenetic trees versus cladograms as representations of evolutionary processes versus patterns of relationships; and genes versus species as expressions of different levels of causal integration and evolutionary transformation. |
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