Evolution

Running Out of New States

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Science  23 Jun 2000:
Vol. 288, Issue 5474, pp. 2099
DOI: 10.1126/science.288.5474.2099b

Phylogenetic analyses seek to identify the hierarchical order of organisms from the distribution of character states—molecular or morphologic—shared amongst them. Homoplasy, the presence of identical states because of evolutionary convergence rather than common ancestry, is the bane of such studies. For molecular data, the small number of bases introduces the possibility that character matrices will be saturated by chance homoplasies, especially when few taxa are sampled relative to the amounts of change. With no obvious limits on the numbers of states for morphologic characters, this problem of exhaustion of states has seemed less important for morphological data such as the fossil record provides.

Wagner uses the frequencies of derivations of new morphologic states in cladistic studies of a wide range of animal fossil taxa to demonstrate that limits to the number of states might exist after all. New states are generally not added continuously throughout the histories of clades; instead, the proportion of incompatible character pairs (which necessarily imply homoplasy) increases as clades age. The patterns suggest that morphologic evolution is restricted, perhaps by intrinsic constraints or persistent selective trends. As younger taxa introduce homoplasy, the hierarchical structure of the character data deteriorates and maximum congruence among states becomes less likely to reflect homology. Exhaustion indicates that sampling high proportions of taxa and considering stratigraphic occurrence data may be required for accurate phylogenetic reconstructions.—SJS; AMS

Evolution54, 365 (2000).

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