Data from: Competitive release leads to range expansion and rampant speciation in Malagasy dung beetles

Competition is often thought to promote ecological diversification and thereby facilitate the coexistence of competitors during evolutionary radiations. At large spatial scales, species may also coexist by having allopatric distributions, which raises the question about the role of range expansion in the proliferation of species during radiations. Here, we integrate a well-sampled (50 out of 74 species) and timed phylogeny of Nanos and Apotolamprus dung beetles (Canthonini) in Madagascar with data on species’ geographical ranges, abundances, and body sizes. There is an overall decline in lineage accumulation through time since the colonization of northern Madagascar in the mid Miocene (24 to 13 Ma). A clade of 24 extant Nanos species (clade L) originating 6.0 Ma exhibits a secondary increase in speciation rate, which is associated with a significant increase in body size and strikingly allopatric distributions of the species. Large body size typically confers a competitive advantage in dung beetles, which is here reflected by strong numerical dominance of clade L species in local communities. We suggest that the ‘key innovation’ of large body size has allowed range expansion due to competitive release, which has created extensive opportunities for allopatric speciation and differentiation along environmental gradients. Most theories to explain diversification patterns in Madagascar rely on allopatric modes of speciation, but they fail to explain how ancestral species became widespread in the first place. The mechanism proposed here, involving range expansion following competitive release via a "key innovation", may have operated in other Malagasy taxa with large numbers of species with small geographic ranges.

学界普遍认为，在演化辐射（evolutionary radiation）过程中，种间竞争可推动生态位分化，进而促进竞争类群的共存。在大空间尺度下，物种也可通过异域分布（allopatric distribution）实现共存，这引发了一个核心问题：演化辐射过程中，分布区扩张在物种增殖环节中扮演了何种角色？本研究整合了马达加斯加地区50个（共74个已知物种）纳米蜣螂属（Nanos）与Apotolamprus蜣螂（隶属于蜣螂族Canthonini）的高采样覆盖度、时间校准系统发育树，以及各物种的地理分布范围、种群丰度与体型大小数据。自中新世中期（24至13 Ma）定殖马达加斯加北部以来，谱系积累速率整体随时间呈下降趋势。起源于6.0 Ma的包含24个现生纳米蜣螂物种的演化支（clade，支系L），其物种形成速率（speciation rate）呈现二次上升，这与体型的显著增大以及该支系物种显著的异域分布特征紧密相关。在蜣螂类群中，大体型通常能赋予竞争优势（competitive advantage），这一点在本研究中体现为：支系L的物种在当地群落中拥有显著的数量优势。我们提出，“关键创新”（key innovation）为大体型带来了竞争释放（competitive release）效应，进而推动了分布区扩张，这为异域物种形成（allopatric speciation）以及沿环境梯度（environmental gradients）的生态位分化创造了大量机遇。现有多数解释马达加斯加地区物种分化模式的理论均以异域物种形成模式为核心，但这些理论无法解释祖先物种最初是如何实现大范围分布的。本研究提出的这一机制——即通过“关键创新”达成竞争释放后发生分布区扩张——或许也存在于其他拥有大量地理分布范围狭小物种的马达加斯加类群（Malagasy taxa）中。