Data from: The ecology of a continental evolutionary radiation: Is the radiation of sigmodontine rodents adaptive?

Evolutionary radiations on continents are less well-understood and appreciated than those occurring on islands. The extent of ecological influence on species divergence can be evaluated to determine whether a radiation was ultimately the outcome of divergent natural selection or else arose mainly by nonecological divergence. Here, we used phylogenetic comparative methods to test distinct hypotheses corresponding to adaptive and nonadaptive evolutionary scenarios for the morphological evolution of sigmodontine rodents. Results showed that ecological variables (diet and life-mode) explain little of the shape and size variation of sigmodontine skulls and mandibles. A Brownian model with varying rates for insectivory versus all other diets was the most likely evolutionary model. The insectivorous sigmodontines have a faster rate of morphological evolution than mice feeding on other diets, possibly due to stronger selection for features that aid insectivory. We also demonstrate that rapid early-lineage diversification is not accompanied by high morphological divergence among subclades, contrasting with island results. The geographic size of continents permits spatial segregation to a greater extent than on islands, allowing for allopatric distributions and escape from interspecific competition. We suggest that continental radiations of rodents are likely to produce a pattern of high species diversification coupled with a low degree of phenotypic specialization.

相较于岛屿上发生的演化辐射（evolutionary radiation），大陆上的演化辐射迄今尚未得到充分的认知与重视。我们可通过评估生态作用对物种分化的影响程度，判断某一辐射演化事件究竟源于趋异自然选择的最终结果，还是主要由非生态性分化所促成。本研究采用系统发育比较方法（phylogenetic comparative methods），针对棉鼠亚科啮齿类（sigmodontine rodents）的形态演化，检验对应适应性与非适应性演化情景的不同假说。研究结果显示，生态变量（食性与生活型）仅能极小程度解释棉鼠亚科啮齿类的头骨与下颌骨形状及大小变异。以食虫性与其他所有食性类群的演化速率差异为参数的布朗运动模型（Brownian model），为最符合数据的演化模型。食虫性棉鼠亚科啮齿类的形态演化速率快于其他食性的鼠类，这可能源于针对辅助食性相关性状的更强选择压。本研究同时证实，早期谱系的快速分化并未伴随亚支系间显著的形态分化，这与岛屿类群的研究结果形成鲜明对比。大陆的地理尺度远大于岛屿，使得类群更易实现空间隔离，进而形成异域分布并规避种间竞争。我们据此提出，啮齿类的大陆辐射演化大概率会呈现出“高物种分化度与低表型特化程度”并存的模式。