Hoyal Cuthill Jennifer F
Department of Earth Sciences , University of Cambridge , Downing Street, Cambridge CB2 3EQ , UK.
Interface Focus. 2015 Dec 6;5(6):20150049. doi: 10.1098/rsfs.2015.0049.
Biological variety and major evolutionary transitions suggest that the space of possible morphologies may have varied among lineages and through time. However, most models of phylogenetic character evolution assume that the potential state space is finite. Here, I explore what the morphological state space might be like, by analysing trends in homoplasy (repeated derivation of the same character state). Analyses of ten published character matrices are compared against computer simulations with different state space models: infinite states, finite states, ordered states and an 'inertial' model, simulating phylogenetic constraints. Of these, only the infinite states model results in evolution without homoplasy, a prediction which is not generally met by real phylogenies. Many authors have interpreted the ubiquity of homoplasy as evidence that the number of evolutionary alternatives is finite. However, homoplasy is also predicted by phylogenetic constraints on the morphological distance that can be traversed between ancestor and descendent. Phylogenetic rarefaction (sub-sampling) shows that finite and inertial state spaces do produce contrasting trends in the distribution of homoplasy. Two clades show trends characteristic of phylogenetic inertia, with decreasing homoplasy (increasing consistency index) as we sub-sample more distantly related taxa. One clade shows increasing homoplasy, suggesting exhaustion of finite states. Different clades may, therefore, show different patterns of character evolution. However, when parsimony uninformative characters are excluded (which may occur without documentation in cladistic studies), it may no longer be possible to distinguish inertial and finite state spaces. Interestingly, inertial models predict that homoplasy should be clustered among comparatively close relatives (parallel evolution), whereas finite state models do not. If morphological evolution is often inertial in nature, then homoplasy (false homology) may primarily occur between close relatives, perhaps being replaced by functional analogy at higher taxonomic scales.
生物多样性和主要的进化转变表明，可能的形态空间在不同谱系间以及随时间推移可能有所不同。然而，大多数系统发育特征进化模型假定潜在状态空间是有限的。在此，我通过分析同塑性（同一特征状态的重复衍生）趋势，来探究形态状态空间可能是什么样的。将已发表的十个特征矩阵分析结果与具有不同状态空间模型的计算机模拟结果进行比较：无限状态、有限状态、有序状态以及一个模拟系统发育限制的“惯性”模型。其中，只有无限状态模型能产生无同塑性的进化，这一预测在实际系统发育中通常无法得到满足。许多作者将同塑性的普遍存在解释为进化替代数量有限的证据。然而，系统发育对祖先和后代之间可跨越的形态距离的限制也能预测同塑性。系统发育稀疏化（子抽样）表明，有限状态空间和惯性状态空间在同塑性分布上确实会产生不同的趋势。两个分支显示出系统发育惯性的特征趋势，随着我们对亲缘关系更远的类群进行子抽样，同塑性降低（一致性指数增加）。一个分支显示同塑性增加，表明有限状态已耗尽。因此，不同的分支可能表现出不同的特征进化模式。然而，当排除简约性无信息特征（在分支系统学研究中可能未记录就出现）时，可能就无法再区分惯性状态空间和有限状态空间了。有趣的是，惯性模型预测同塑性应聚集在亲缘关系相对较近的类群之间（平行进化），而有限状态模型则不然。如果形态进化本质上通常是惯性的，那么同塑性（假同源性）可能主要发生在亲缘关系较近的类群之间，也许在更高的分类尺度上会被功能类比所取代。
