Data from: Locating evolutionary precursors on a phylogenetic tree

Conspicuous innovations in the history of life are often preceded by more cryptic genetic and developmental precursors. In many cases, these appear to be associated with recurring origins of very similar traits in close relatives (parallelisms) or striking convergences separated by deep time (“deep homologies”). Although the phylogenetic distribution of gain and loss of traits hints strongly at the existence of such precursors, no models of trait evolution currently permit inference about their location on a tree. Here we develop a new stochastic model which explicitly captures the dependency implied by a precursor and permits estimation of precursor locations. We apply it to the evolution of extrafloral nectaries (EFNs), an ecologically significant trait mediating a widespread mutualism between plants and ants. In legumes, a species-rich clade with morphologically diverse EFNs, the precursor model fits the data on EFN occurrences significantly better than conventional models. The model generates explicit hypotheses about the phylogenetic location of hypothetical precursors, which may help guide future studies of molecular genetic pathways underlying nectary position, development and function.

生命演化史上的显著创新，往往先于更多隐秘的遗传与发育前驱事件出现。在诸多案例中，这些前驱事件似乎与近缘类群中重复起源的极相似性状（平行演化，parallelisms），或是历经漫长演化时间后出现的惊人趋同（深层同源性，deep homologies）密切相关。尽管性状得失的系统发育分布强烈暗示了这类前驱事件的存在，但当前的性状演化模型均无法推断其在系统发育树上的位置。本研究构建了一种全新的随机模型，可显性捕获前驱事件所隐含的依存关系，并能估算前驱事件的发生位置。我们将该模型应用于花外蜜腺（extrafloral nectaries, EFNs）的演化分析——该性状是介导植物与蚂蚁间广泛互利共生的重要生态特征。在豆科植物这一物种丰富、花外蜜腺形态多样的演化支中，前驱模型对花外蜜腺出现数据的拟合效果显著优于传统模型。该模型可针对假说中的前驱事件的系统发育位置生成明确的研究假说，有望为后续探索蜜腺位置、发育与功能背后的分子遗传通路提供指引。