ular change often vary among plant families and orders in a manner that violates the assumption of a simple molecular clock. Finally, protein-coding genes exhibit patterns of amino acid change that appear to depend on protein structure, and these patterns may reveal subtle aspects of structure/function relationships. Only comparative studies of molecular sequences have the resolution to reveal this underlying complexity. A complete description of the complexity of molecular change is essential to a full understanding of the mechanisms of evolutionary change and in the formulation of realistic models of mutational processes.
Support from National Institutes of Health Grants GM 45144 (to M.T.C.), GM 15528 (to B.S.G.) and GM 45344 (to North Carolina State University) is gratefully acknowledged. The order of authorship is listed alphabetically. All authors made an equal contribution to this article.
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