Iterative Evolution of Depth Stratification in Neogene Planktonic Foraminifera

人们普遍认为，尽管很少进行测试，但远洋生物通过入侵水柱中空置的部分来实现物种和多样性。辐射被认为是当人群跨越海洋锋面或垂直分层水团之间的边界变得孤立时发生的。浮游有孔虫和其他浮游类群的广泛趋同进化表明，同一栖息地在地质时间上反复受到不同谱系的入侵。例如，球状、多刺的浮游有孔虫反复产生扁平的、壁光滑的物种，上面装饰着外围的隆突或龙骨。我们建议测试一个流行的模型，该模型解释龙骨浮游有孔虫的迭代进化是根据逐渐深入的水柱部分的入侵来解释的。我们的研究将为两组独立进化龙骨的球状藻类物种提供del 18O同位素排名：Fohsella和Menardella/Globoconella分支的早期成员。表层浮游有孔虫、底栖有孔虫和这两个谱系的成员之间的同位素差异将被用来估计深层生境的变化。由于这些谱系是从球状祖先独立进化而来的，我们可以观察到两个谱系在栖息地深度上是否发生了类似的变化，同时也发生了类似的形态变化。这些结果将使我们能够检验这样的假设，即表层栖息的物种通过入侵水柱更深的部分而辐射。我们的发现将表明，有孔虫的反复收敛进化是否可以用谱系内深度栖息地的变化来解释。我们还可以确定这些海洋浮游生物的形态进化是否与生态进化相关。此外，这项研究将记录一次重大辐射发生的生态背景。

It has been widely assumed, despite few tests, that pelagic organisms speciate and diversify by invading unoccupied parts of the water column. Radiation is thought to occur when populations become isolated across oceanic fronts or across boundaries between vertically stratified water masses. Extensive convergent evolution in planktonic foraminifera and other planktonic groups suggests that the same habitats are repeatedly invaded by different lineages over geologic time. For example, globular, spiny-walled planktonic foraminifera repeatedly give rise to compressed, smooth-walled species ornamented with a peripheral carina or keel. We propose to test the popular model that explains the iterative evolution of keeled planktonic foraminifera in terms of invasion of progressively deeper parts of the water column. Our study will produce del 18 O isotopic rankings for species in two groups of globorotaliids that independently evolved the keel: Fohsella and early members of the Menardella/Globoconella clade. Isotopic differences between surface-dwelling planktonic foraminifera, benthic foraminifera, and members of these two lineages will be used to estimate changes in depth habitats. Since these lineages independently evolved from globular ancestors, we can observe whether similar changes in depth habitat occurred in concert with similar morphological changes in both lineages. These results will permit us to test the hypothesis that surface-dwelling species radiated by invading deeper parts of the water column. Our findings will show if the repeated convergent evolution of foraminifera can be explained in terms of changes of depth habitat within lineages. We can also determine whether or not morphological evolution is correlated with ecological evolution in these marine plankton. In addition, this study will document the ecological context in which a major radiation unfolded.