CORRELATED RATES OF MOLECULAR AND MORPHOLOGICAL EVOLUTION.

Since Zuckerkandl and Pauling (1962, 1965) proposed the molecular clock, many studies seem to have supported their prediction that rates of molecular and morphological evolution generally will be decoupled. Most of these studies were aimed at taxa in which rates of morphological evolution were thought to vary greatly a priori. For the current survey eight diverse taxa were systematically chosen from published studies without regard to prior expectations about rates. Two approaches showed that rates of molecular and morphological evolution may usually be coupled. First, correlations in the total number of changes accumulated in terminal taxa suggest that some mechanism alters the rates of both morphological and molecular evolution in concert. Second, node-density effects were removed statistically, and average corrected base-to-tip totals were compared among sister clades. Across all taxa 50 of 72 of these corrected contrasts support the hypothesis that rates of molecular and morphological evolution are correlated; this finding is highly significant by a binomial test. Furthermore, there were positive correlations between inferred molecular and morphological branch lengths in seven of eight cases, which is also significant. These branch length correlations are consistent with the rate correlations, and suggest that amounts of molecular and morphological evolution often are correlated also. This study supports the assumptions of several phylogenetic methods, and highlights a need for new inquiries into many aspects of both molecular and morphological evolution.