PHYLOGENETIC TESTS OF DIRECTIONAL BIAS IN HIERARCHICAL EVOLUTION

The history of life is punctuated by several major transitions in hierarchy, understood 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 the maximum hierarchical structure. However, the underlying mechanism is unknown. The 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.

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 using minor transitions in hierarchy, or, in other words, changes in the degree of individuation of the upper level. For example, both Gonium (a green alga in which all cells are identical) and Volvox (with two cell types) are multicellular, but Volvox is more individuated at the multicellular level.

This study tests the null hypothesis that the proportion (or rate) of increases and decreases in hierarchical structure are equal. Clades that span major or minor transitions were identified, and those with previously published phylogenetic estimates were selected for analysis. Character states representing minor transitions were determined for as many taxa included in the previous phylogenetic analyses as possible. Parsimony- and maximum-likelihood-based methods of testing for asymmetrical rates of character evolution were employed. In general, most analyses failed to reject equal rates of hierarchical increase and decrease. In fact, a bias towards decreasing complexity was observed for several clades. These results suggest that evolutionary increases in hierarchical complexity are not more common than decreases, and do not support a biased mechanism for the observed trend of increasing hierarchical structure.