Papers Published

1. Marcot, JD; McShea, DW, Increasing hierarchical complexity throughout the history of life: Phylogenetic tests of trend mechanisms, Paleobiology, vol. 33 no. 2 (March, 2007), pp. 182-200, Cambridge University Press (CUP) [doi].
   (last updated on 2024/04/16)

   Author's Comments:
   This paper -- now in press at Paleobiology -- is the culmination of about three years of work by my (now former) postdoc, Jon Marcot, and me. The project was funded by two major (for me) grants (NASA and NSF). The results undermine one aspect of the conventional wisdom about evolutionary trends in complexity, in particular, trends in hierarchical complexity, the integration of lower-level units (e.g., cells) to form higher-level wholes (e.g., multicellular individuals). The conventional wisdom holds that the trend is driven by natural selection, so that increases are, and ought to be, more frequent than decreases. Our study examined hierarchical transitions over four levels of organization, from bacterium to colony/society, using state-of-the-art phylogenetic methods. And no upward tendency was found. Decreases in complexity -- losses of hierarchy -- abound in evolution, and on average are as common as increases! The trend would appear to be a sort that is called passive. In one version of this mechanism, the trend arises as a result of passive diffusion away from a lower bound, a lower limit on hierarchy (perhaps the bacterial level).

   Abstract:
   The history of life is punctuated by a number of major transitions in hierarchy, defined here as the degree of nestedness of lower-level individuals within higher-level ones: the combination of single-celled prokaryotic cells to form the first eukaryotic cell, the aggregation of single eukaryotic cells to form complex multicellular organisms, and finally, the association of multicellular organisms to form complex colonial individuals. These transitions together constitute one of the most salient and certain trends in the history of life, in particular, a trend in maximum hierarchical structure, which can be understood as a trend in complexity. This trend could be produced by a biased mechanism, in which increases in hierarchy are more likely than decreases, or by an unbiased one, in which increases and decreases are about equally likely. At stake is whether or not natural selection or some other force acts powerfully over the history of life to drive complexity upward. Too few major transitions are known to permit rigorous statistical discrimination of trend mechanisms based on these transitions alone. However, the mechanism can be investigated by using "minor transitions" in hierarchy, or, in other words, changes in the degree of individuation of the upper level. This study tests the null hypothesis that the probability (or rate) of increase and decrease in individuation are equal in a phylogenetic context. We found published phylogenetic trees for clades spanning minor transitions across the tree of life and identified changes in character states associated with those minor transitions. We then used both parsimony- and maximum-likelihood-based methods to test for asymmetrical rates of character evolution. Most analyses failed to reject equal rates of hierarchical increase and decrease. In fact, a bias toward decreasing complexity was observed for several clades. These results suggest that no strong tendency exists for hierarchical complexity to increase. © 2007 The Paleontological Society. All rights reserved.