Department of Biological Sciences, University of Arkansas Fayetteville, 1 University of Arkansas, SCEN 601, Fayetteville, AR, 72701.
Evolution. 2019 Dec;73(12):2497-2511. doi: 10.1111/evo.13832. Epub 2019 Sep 5.
Many clades that span the marine-freshwater boundary are disproportionately more diverse in the younger, shorter lived, and scarcer freshwater environments than they are in the marine realm. This disparity is thought to be related to differences in diversification rates between marine and freshwater lineages. However, marine and freshwaters are not ecologically homogeneous, so the study of diversification across the salinity divide should also account for other potentially interacting variables. In diatoms, freshwater and substrate-associated (benthic) lineages are several-fold more diverse than their marine and suspended (planktonic) counterparts. These imbalances provide an excellent system to understand whether these variables interact with diversification. Using multistate hidden-state speciation and extinction models, we found that freshwater lineages diversify faster than marine lineages regardless of whether they inhabit the plankton or the benthos. Freshwater lineages also had higher turnover rates (speciation + extinction), suggesting that habitat transitions impact speciation and extinction rates jointly. The plankton-benthos contrast was also consistent with state-dependent diversification, but with modest differences in diversification and turnover rates. Asymmetric and bidirectional transitions rejected hypotheses about the plankton and freshwaters as absorbing, inescapable habitats. Our results further suggest that the high turnover rate of freshwater diatoms is related to high turnover of freshwater systems themselves.
许多跨越海洋-淡水边界的分支在年轻、寿命短且稀缺的淡水环境中比在海洋环境中更加多样化。这种差异被认为与海洋和淡水谱系之间的多样化率差异有关。然而，海洋和淡水在生态上并不相同，因此跨越盐度分界线的多样化研究还应该考虑其他可能相互作用的变量。在硅藻中，淡水和与基质相关（底栖）的谱系比其海洋和悬浮（浮游）谱系多样化了好几倍。这些不平衡提供了一个极好的系统来了解这些变量是否与多样化相互作用。使用多态隐藏状态物种形成和灭绝模型，我们发现无论它们是否栖息在浮游生物或底栖生物中，淡水谱系的多样化速度都比海洋谱系快。淡水谱系的周转率（物种形成+灭绝）也更高，这表明生境转换共同影响物种形成和灭绝率。浮游生物-底栖生物的对比也与状态相关的多样化一致，但在多样化和周转率方面存在适度差异。不对称和双向转换否定了浮游生物和淡水是吸收性、不可避免栖息地的假设。我们的结果还表明，淡水硅藻的高周转率与淡水系统本身的高周转率有关。
