Comparing diversification rates in lakes, rivers, and the sea

The diversity of species inhabiting freshwater relative to marine habitats is striking, given that freshwater habitats encompass <1% of Earth’s water. The most commonly proposed explanation for this pattern is that freshwater habitats are more fragmented than marine habitats, allowing more opportunities for allopatric speciation and thus increased diversification rates in freshwater. However, speciation may be generally faster in sympatry than in allopatry, as illustrated by lacustrine radiations such as African cichlids. Differences between rivers and lakes may be important to consider when comparing diversification broadly among freshwater and marine lineages. Here I compared diversification rates of teleost fishes in marine, riverine and lacustrine habitats. I found that lakes had faster speciation and net diversification rates than other aquatic habitats. However, most freshwater diversity arose in rivers. Surprisingly, riverine and marine habitats had similar rates of net diversification. Biogeographic models suggest that lacustrine habitats are evolutionary unstable, explaining the dearth of lacustrine species in spite of their rapid diversification. Collectively, these results suggest that diversification rate differences are unlikely to explain the freshwater paradox. Instead, this pattern may be attributable to the comparable amount of time spent in riverine and marine habitats over the 200-million-year history of teleosts.相较于海洋生境，淡水生境所承载的物种多样性之高令人瞩目——须知淡水生境仅占地球总水量的不足1%。针对这一现象，最广为接受的解释是：淡水生境比海洋生境更为破碎化，这为异域物种形成（allopatric speciation）提供了更多契机，进而提升了淡水类群的多样化速率。不过，同域物种形成（sympatric speciation）的速率通常快于异域物种形成，非洲慈鲷的湖泊辐射演化即为典型例证。在宽泛比较淡水与海洋类群的多样化速率时，河流与湖泊之间的差异不容忽视。本研究对海洋、河流及湖泊生境中的硬骨鱼（teleost fishes）多样化速率进行了对比分析。结果显示，湖泊生境的物种形成速率和净多样化速率均高于其他水生生境。然而，绝大多数淡水物种多样性起源于河流。令人意外的是，河流生境与海洋生境的净多样化速率相近。生物地理模型（biogeographic models）表明，湖泊生境在演化上并不稳定，这解释了为何尽管湖泊类群的多样化速率较快，但现存湖泊物种却较为稀少。综合来看，上述结果表明，多样化速率差异并不能解释淡水悖论（freshwater paradox）。取而代之的是，这一现象或许可归因于：在硬骨鱼长达2亿年的演化历史中，类群在河流生境与海洋生境中存续的时长相当。