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 to 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.

竞争通常被认为可促进生态分化，进而在进化辐射过程中助力竞争者实现共存。在大空间尺度下，物种亦可通过异域分布（allopatric distributions）达成共存，这引出了一个核心问题：在辐射演化进程中，分布范围扩张在物种增殖过程中扮演着何种角色。本研究整合了马达加斯加地区Nanos与Apotolamprus属粪甲虫（dung beetles，隶属于凯诺蜣螂族（Canthonini））一套采样充分（74个物种中已采集50个）且经时间校准的系统发育（phylogeny）数据，结合了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）殖民马达加斯加北部以来，世系积累速率整体呈随时间下降的趋势。一个起源于6.0 Ma、包含24个现生Nanos物种的演化支（clade L）呈现出物种形成速率的二次上升，这与体型大小的显著增加以及该类群物种极为突出的异域分布特征紧密相关。在粪甲虫中，大体型通常会赋予竞争优势，这一点在本研究中体现为：演化支L的物种在当地群落中拥有显著的数量主导地位。我们提出，大体型这一‘关键创新（key innovation）’通过竞争释放实现了分布范围扩张，进而为异域物种形成以及沿环境梯度的分化创造了大量机遇。当前多数解释马达加斯加地区物种分化模式的理论均依赖于异域物种形成模式，但它们无法解释祖先物种最初是如何实现广泛分布的。本研究提出的这一机制——即通过‘关键创新’实现竞争释放后伴随分布范围扩张——或许也适用于其他拥有大量地理分布范围狭窄的物种的马达加斯加类群。