Animal Ecology and Systematics, Justus Liebig University, Heinrich-Buff-Ring 26-32 (IFZ), D-35392, Giessen, Germany.
Department of Invertebrate Zoology, Smithsonian Institution, 10th and Constitution Ave, NW, Washington DC, 20013-7012, USA.
BMC Evol Biol. 2018 Apr 19;18(1):55. doi: 10.1186/s12862-018-1169-2.
Differences in species richness among phylogenetic clades are attributed to clade age and/or variation in diversification rates. Access to ecological opportunity may trigger a temporary increase in diversification rates and ecomorphological variation. In addition, lower body temperatures in poikilothermic animals may result in decreasing speciation rates as proposed by the metabolic theory of ecology. For strictly freshwater organisms, environmental gradients within a river continuum, linked to elevation and temperature, might promote access to ecological opportunity and alter metabolic rates, eventually influencing speciation and extinction processes. To test these hypotheses, we investigated the influence of environmental temperature and elevation, as proxies for body temperature and ecological opportunity, respectively, on speciation rates and ecomorphological divergence. As model systems served two closely related gastropod genera with unequal species richness and habitat preferences - Pseudamnicola and Corrosella.
Lineage-through-time plots and Bayesian macroevolutionary modeling evidenced that Pseudamnicola species, which typically live in lower reaches of rivers, displayed significantly elevated speciation rates in comparison to the 'headwater genus' Corrosella. Moreover, state-dependent speciation models suggested that the speciation rate increased with decreasing elevation, supporting the ecological opportunity hypothesis. In contrast, a significant effect of environmental temperature, as proposed by the metabolic theory of ecology, could not be observed. Disparity-through-time plots, models of ecomorphological evolution, and ancestral habitat estimation showed for Pseudamnicola species rapid morphological divergence shortly after periods of elevational and habitat divergence. In contrast, Corrosella species did not deviate from null models of drift-like evolution.
Our finding that speciation rates are correlated with elevation and ecomorphological disparity but not with environmental temperatures suggests that differences in ecological opportunity may have played a key role in Corrosella and Pseudamnicola diversifications. We propose that Pseudamnicola lineages experienced higher ecological opportunity through dispersal to new locations or habitats in lowlands, which may explain the increase in speciation rates and morphological change. In contrast, the evolution of Corrosella in headwaters is likely less facilitated by the environment and more by non-ecological processes.
系统发育进化枝之间的物种丰富度差异归因于进化枝的年龄和/或多样化速率的变化。获得生态机会可能会暂时增加多样化速率和生态形态变异。此外，由于生态生理学提出的代谢理论，变温动物的体温较低可能会导致物种形成率下降。对于严格的淡水生物，与海拔和温度相关的河流连续体内部的环境梯度可能会促进获得生态机会并改变代谢率，最终影响物种形成和灭绝过程。为了检验这些假设，我们调查了环境温度和海拔（分别作为体温和生态机会的代表）对物种形成率和生态形态分歧的影响。作为模型系统，我们使用了两个具有不等物种丰富度和栖息地偏好的密切相关的腹足纲贝类属——Pseudamnicola 和 Corrosella。
谱系时间图和贝叶斯宏观进化模型表明，通常生活在河流下游的 Pseudamnicola 物种的物种形成率明显高于“源头属”Corrosella。此外，状态依赖的物种形成模型表明，随着海拔的降低，物种形成率增加，支持了生态机会假说。相比之下，代谢理论所提出的环境温度的显著影响无法观察到。时间差异图、生态形态进化模型和祖先栖息地估计表明，Pseudamnicola 物种在海拔和栖息地分化后不久就发生了快速的形态分化。相比之下，Corrosella 物种没有偏离随机漂变似的进化模型。
我们的发现表明，物种形成率与海拔和生态形态差异相关，而与环境温度无关，这表明生态机会的差异可能在 Corrosella 和 Pseudamnicola 多样化中发挥了关键作用。我们提出，Pseudamnicola 谱系通过向低地的新地点或栖息地扩散获得了更高的生态机会，这可能解释了物种形成率的增加和形态变化。相比之下，Corrosella 在源头的进化可能较少受到环境的促进，而更多地受到非生态过程的影响。
