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.

在利用DNA序列数据开展现生类群的物种界定研究中，通用谱系概念（general lineage concept）已被广泛采用。该概念将物种定义为在物种形成过程中逐步获得一系列特征的独立演化集合种群（metapopulations）。产自巴基斯坦中新世锡瓦利克群（Siwalik Group）的鼠科啮齿类（murine rodents）拥有保存极佳的化石记录，能够展现形态演化的时间序列趋势，因此是利用化石证据探究谱系分化过程的理想研究对象。本文基于通用谱系概念的视角，对锡瓦利克群鼠科啮齿类的演化历程进行综述，并梳理了谱系分化的相关证据线索。针对当前可用的数据集，本研究采用了初始分化判定标准，以及三类操作性判定标准：表型（phenetic）特征标准、可鉴别（diagnosable）特征标准与生态（ecological）特征标准。在锡瓦利克群早期鼠科啮齿类中，独特表型特征的出现时间晚于演化分化事件本身。表型证据的出现时间更接近‘真实’的分化事件，而可鉴别特征与生态特征则要在更晚的阶段才会完全形成。在一系列经时间管控的化石样本序列中，本研究的观测结果明确显示：相较于诊断特征的固定过程，表型特征（本文中指形态数据与线性测量数据的组合）能够更精准地界定化石物种的分化事件。