Bridging macroecology and macroevolution in the radiation of sigmodontine rodents

Investigations of phenotypic disparity across geography often ignore
macroevolutionary processes. As a corollary, the random null expectations
to which disparity is compared and interpreted may be unrealistic. We
tackle this issue by representing, in geographical space, distinct
processes of phenotypic evolution underlying ecological disparity. Under
divergent natural selection, assemblages in a given region should have
empirical disparity higher than expected under an evolutionarily-oriented
null model, while the opposite may indicate constraints on phenotypic
evolution. We gathered phylogenies, biogeographic distributions, and data
on the skull morphology of sigmodontine rodents to discover which regions
of the Neotropics were more influenced by divergent, neutral, or
constrained phenotypic evolution. We found that regions with higher
disparity than expected by the evolutionary-oriented null model, in
terms of both size and shape, were concentrated in the
Atlantic Forest, suggesting a larger role for divergent natural selection
there. Phenotypic disparity in the rest of South America, mainly the
Amazon basin, northeastern Brazil and Southern Andes, was constrained —
lower than predicted by the evolutionary model. We also demonstrated
equivalence between the disparity produced by randomization-based null
models and constrained-evolution null models. Therefore, including
evolutionary simulations into the null modeling framework used in
ecophylogenetics can strengthen inferences on the processes
underlying phenotypic evolution.

针对地理梯度上表型差异度（phenotypic disparity）的研究，往往忽略了宏进化（macroevolutionary）过程。由此衍生的问题是，用于比对和阐释该差异度的随机零假设期望往往并不符合实际。本研究通过在地理空间中刻画支撑生态差异的不同表型进化过程，以解决这一问题。在趋异自然选择（divergent natural selection）作用下，特定区域的物种集合的经验表型差异度应高于进化导向型零模型的预测结果；反之则可能意味着表型进化受到约束。我们收集了南美稻鼠类啮齿动物（sigmodontine rodents）的系统发育树（phylogenies）、生物地理分布数据以及颅骨形态（skull morphology）数据，以探明新热带界（Neotropics）的哪些区域更多受到趋异、中性或受约束表型进化的影响。研究发现，在体型与形态两方面均高于进化导向型零模型预测值的差异度区域，集中分布于大西洋森林（Atlantic Forest），这表明趋异自然选择在该区域发挥了更强的作用。南美洲其余区域——主要包括亚马孙盆地、巴西东北部与安第斯山脉南部——的表型差异度则受到约束，即低于该进化模型的预测值。我们还证实，基于随机化的零模型所产生的差异度，与受约束进化零模型所产生的结果并无显著差异。因此，将进化模拟纳入生态系统发育学（ecophylogenetics）所采用的零模型框架中，能够强化针对表型进化背后过程的推断结论。