A palaeontological case study for species delimitation in diverging fossil lineages

A general lineage concept is widely adopted in studies of species delimitation for extant taxa with DNA sequence data. In the general lineage concept, species are separately evolving metapopulations that acquire sets of properties during the process of speciation. Murine rodents from the Miocene Siwalik Group of Pakistan have an excellent fossil record showing temporal trends of morphological change, and are therefore ideal to observe the process of lineage divergence as fossil evidence. Here, we review the evolution of Siwalik murine rodents in the sense of the general lineage concept and draw lines of evidence for lineage separation. For currently available datasets, we applied the initial split criterion as well as three operational criteria (phenetic, diagnosable and ecological properties). In early Siwalik murine rodents, the acquisition of distinct phenetic properties emerges subsequent to the event of the evolutionary split. The phenetic evidence occurs closer in time to ‘true’ divergence, while distinct diagnosable and ecological properties fully arise later. Within a sequence of time-controlled samples, these observations explicitly show that phenetic properties (here, shape data and a combination of linear measurements) more precisely delimit the divergence of fossil species than does the fixation of diagnostic properties.

在依托DNA序列数据开展现生类群物种界定的相关研究中，谱系种概念（general lineage concept）已得到广泛应用。根据该概念，物种是独立演化的集合种群，在物种形成过程中会逐步获得一系列专属性状。产自巴基斯坦中新世锡瓦利克群的鼠科啮齿类拥有保存完好的化石记录，清晰呈现了形态变化的时间演化趋势，是依托化石证据探究谱系分化过程的理想类群。本文基于谱系种概念梳理了锡瓦利克群鼠科啮齿类的演化历程，并为谱系分化提供多维度佐证依据。针对当前可用的数据集，我们采用了初始分化判据以及三类操作判据：表型判据、可鉴别性状判据与生态性状判据。在锡瓦利克群早期鼠科啮齿类中，独特表型性状的出现时间晚于演化分化事件。表型证据的出现时间更接近‘真实’分化节点，而可鉴别性状与生态性状的完全形成则相对滞后。在一系列时间受控的样品序列中，上述观察结果清晰表明：相较于鉴别性状的固定，表型性状（本研究中指形态数据与线性测量值的组合）能够更精准地界定化石物种的分化事件。