Smith A B, Lafay B, Christen R
Department of Palaeontology, Natural History Museum, London, U.K.
Philos Trans R Soc Lond B Biol Sci. 1992 Dec 29;338(1286):365-82. doi: 10.1098/rstb.1992.0155.
The comparatively good fossil record of post-Palaeozoic echinoids allows rates of morphological change to be estimated over the past 260 million years and compared with rates of molecular evolution. Parsimony analysis of morphological data, based predominantly on skeletal characteristics, and parsimony, distance and maximum likelihood analyses of molecular data, from the first 380 bases from the 5' end of the 28S rRNA molecule, for 10 species of echinoid produce congruent phylogenies. The molecular sequence chosen is demonstrably far from saturation and sister groups have divergence times ranging from about 15 to 260 Ma. Parsimony analysis allows the great majority of molecular and morphological apomorphies to be placed in one of 18 independent geological time intervals, providing a direct measure of rates of evolution for periods in the geological past. Because most molecular fixed point mutations in our sequences cannot be polarized unambiguously by outgroup comparison (making the outgroup states effectively random), distance and parsimony analyses both tend spuriously to root the echinoid tree on the longest internal branch. A topology identical to that derived from morphological data is, however, obtained using Maximum Likelihood and also parsimony analysis where outgroup rooting is restricted to more conserved regions. This is taken as the correct topology for assessing rates of evolution. Overall, both morphological and molecular changes show a moderately strong correlation with time elapsed, but a weaker correlation with one another. Statistically significant differences in evolutionary rate are found between some, but not all, pair-wise comparisons of sister lineages for both molecular and morphological data. The molecular clock rate for echinaceans is three times faster than that for cidaroids and irregular echinoids. Spearman's rank correlation test, which requires only relative magnitude of changes to be known, suggests that morphological change has a slightly better correlation with time than does molecular change, averaged over all ten species. However, when just echinaceans are considered an extremely good correlation is found between the number of molecular changes and time elapsed, whereas morphological change remains poorly correlated. Thus, molecular rates approximate to a clocklike model within restricted echinoid clades, but vary significantly between clades. Averaging results over all echinoids produces a correlation that is no better than the correlation between morphological change and time elapsed.
后古生代海胆类相对良好的化石记录使得我们能够估算过去2.6亿年里的形态变化速率，并与分子进化速率进行比较。基于主要骨骼特征的形态学数据简约分析，以及对10种海胆类从28S rRNA分子5'端前380个碱基的分子数据进行的简约、距离和最大似然分析，得出了一致的系统发育树。所选的分子序列显然远未饱和，姐妹类群的分歧时间范围约为15至260百万年。简约分析使绝大多数分子和形态近裔性状能够被置于18个独立地质时间间隔中的一个，从而直接衡量地质历史时期的进化速率。由于我们序列中的大多数分子固定点突变无法通过外类群比较明确地极化（使得外类群状态实际上是随机的），距离分析和简约分析都倾向于错误地将海胆类树的根置于最长的内部分支上。然而，使用最大似然法以及将外类群生根限制在更保守区域的简约分析，得到了与从形态学数据得出的拓扑结构相同的结果。这被视为评估进化速率的正确拓扑结构。总体而言，形态和分子变化都与经过的时间呈现出中等强度的相关性，但彼此之间的相关性较弱。在分子和形态数据的一些但并非所有姐妹谱系的成对比较中，发现了进化速率的统计学显著差异。海胆类的分子钟速率比头帕海胆类和不规则海胆类快三倍。仅需知道变化的相对大小的斯皮尔曼等级相关检验表明，在所有十个物种中平均来看，形态变化与时间的相关性略优于分子变化与时间的相关性。然而，仅考虑海胆类时，发现分子变化的数量与经过的时间之间存在极强的相关性，而形态变化的相关性仍然很差。因此，在受限的海胆类分支内，分子速率近似于时钟模型，但在不同分支之间差异显著。对所有海胆类的结果进行平均，得到的相关性并不比形态变化与经过时间之间的相关性更好。
