Data from: Shaping the latitudinal diversity gradient: New perspectives from a synthesis of paleobiology and biogeography

An impediment to understanding the origin and dynamics of the latitudinal diversity gradient (LDG) -- the most pervasive large-scale biotic pattern on Earth -- has been the tendency to focus narrowly on a single causal factor, when a more synthetic, integrative approach is needed. Using marine bivalves as a model system, and drawing on other systems where possible, we review paleobiologic and biogeographic support for two supposedly opposing views, that the LDG is shaped primarily by (a) local environmental factors that determine the number of species and higher taxa at a given latitude (in-situ hypotheses), or (b) the entry of lineages arising elsewhere into a focal region (spatial-dynamics hypotheses). Support for (a) includes the fit of present-day diversity trends in many clades to environmental factors such as temperature, and the correlation of extinction intensities in Pliocene bivalve faunas with net regional temperature changes. Support for (b) includes the age-frequency distribution of bivalve genera across latitudes, which is consistent with an out-of-the-tropics dynamic, as are the higher species diversities in temperate southeast Australia and southern Japan than in the tropical Caribbean. Thus, both in-situ and spatial-dynamics processes must shape the bivalve LDG, and are likely to operate in other groups as well. The relative strengths of the two processes may differ among groups showing similar LDGs, but dissecting their effects will require improved methods of integrating fossil data with molecular phylogenies. We highlight several potential research directions, and argue that many of the most dramatic biotic patterns, past and present, are likely to have been generated by diverse, mutually reinforcing drivers.

长期以来阻碍我们理解纬度多样性格局（latitudinal diversity gradient, LDG）——地球上最普遍的大型生物分布模式——的起源与动态的一大瓶颈，在于学界往往狭隘地仅聚焦单一驱动因素，而该领域亟需更具综合性与整合性的研究路径。本研究以海洋双壳类（marine bivalves）为模式系统，并尽可能吸纳其他类群的研究证据，系统梳理了支持两种看似对立观点的古生物学与生物地理学依据：纬度多样性格局主要由以下两类因素塑造，（a）决定特定纬度带物种及高阶类群数量的局域环境因素（原位假说，in-situ hypotheses），或（b）源自其他区域的演化支迁入目标研究区域的过程（空间动态假说，spatial-dynamics hypotheses）。对于原位假说的支持证据包括：诸多演化支的现代多样性格局与温度等环境因素高度契合，以及上新世（Pliocene）双壳类动物群的灭绝强度与区域净温度变化存在显著相关性。空间动态假说的支持依据则包括：双壳类属的年龄-频率分布随纬度的变化格局，与热带起源向外扩散动态（out-of-the-tropics dynamic）过程相符；澳大利亚东南部温带海域及日本南部海域的物种多样性高于热带加勒比海（Caribbean）海域，同样印证了该假说。由此可见，原位过程与空间动态过程共同塑造了双壳类的纬度多样性格局，且这两种过程极可能同样作用于其他生物类群。尽管对于呈现相似纬度多样性格局的不同类群而言，这两种过程的相对强度可能存在差异，但要厘清二者的具体效应，亟需开发将化石数据与分子系统发育（molecular phylogenies）相结合的改良研究方法。本研究梳理了若干极具潜力的研究方向，并提出：诸多历史与现代的典型生物分布模式，极有可能由多种相互协同的驱动因素共同塑造而成。