The latitudinal gradient in species richness is perhaps the most fundamental pattern of biodiversity, yet a satisfactory explanation for its existence remains elusive. A geometric “mid-domain effect” is often cited as having potential to help explain the latitudinal gradient in species richness, but the logic underpinning this hypothesis is apparently built on two incorrect assumptions: (1) that a given great circle—usually the Equator—can constitute the geometric “mid-domain” of the Earth’s surface, and (2) that geophysical or bioclimatic boundaries are of geometric relevance in the context of a global-scale mid-domain effect. This article gives a brief overview of the relevant literature and history of thought on the subject, and describes in clear and simple terms why a global-scale mid-domain effect cannot arise, and thus cannot possibly represent a mechanistic basis for the latitudinal gradient in species richness. Explicit acknowledgement of this fact is of great importance, as it allows us to dispense with a commonly cited hypothesis for the latitudinal gradient in species richness.

中文翻译：

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全球尺度的中域效应无法解释物种丰富度的纬度梯度

物种丰富度的纬度梯度可能是生物多样性最基本的模式，但对其存在的令人满意的解释仍然难以捉摸。几何“中域效应”通常被认为有可能有助于解释物种丰富度的纬度梯度，但支持这一假设的逻辑显然建立在两个错误的假设上：(1) 给定的大圆——通常是赤道——可以构成地球表面的几何“中域”，以及 (2) 地球物理或生物气候边界在全球尺度中域效应的背景下具有几何相关性。本文简要概述了有关该主题的相关文献和思想史，并清晰而简单地描述了为什么不会出现全球范围的中域效应，因此不可能代表物种丰富度的纬度梯度的机械基础。明确承认这一事实非常重要，因为它使我们能够免除普遍引用的物种丰富度纬度梯度假设。