Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093, USA.
Proc Natl Acad Sci U S A. 2009 Dec 15;106(50):21224-9. doi: 10.1073/pnas.0902887106. Epub 2009 Dec 8.
In contrast with speciation in terrestrial organisms, marine plankton frequently display gradual morphological change without lineage division (e.g., phyletic gradualism or gradual evolution), which has raised the possibility that a different mode of evolution dominates within pelagic environments. Here, we reexamine a classic case of putative gradual evolution within the Globorotalia plesiotumida-G. tumida lineage of planktonic foraminifera, and find both compelling evidence for the existence of a third cryptic species during the speciation event and the abrupt evolution of the descendant G. tumida. The third morphotype, not recognized in previous analyses, differs in shape and coiling direction from its ancestor, G. plesiotumida. This species dominates the globorotaliid population for 414,000 years just before the appearance of G. tumida. The first population of the descendant, G. tumida, evolves abruptly within a 44,000-year interval. A combination of morphological data and biostratigraphic evidence suggests that G. tumida evolved by cladogenesis. Our findings provide an unexpected twist on one of the best-documented cases of within-lineage phyletic gradualism and, in doing so, revisit the limitations and promise of the study of speciation in the fossil record.
与陆生生物的物种形成相反，海洋浮游生物经常表现出逐渐的形态变化，而没有谱系分裂（例如，分支渐进主义或渐进进化），这使得一种不同的进化模式在浮游环境中占主导地位的可能性增加。在这里，我们重新审视了浮游有孔虫Globorotalia plesiotumida-G. tumida 谱系内假定渐进进化的一个经典案例，并且在物种形成事件中发现了存在第三个隐生种的令人信服的证据，以及后代 G. tumida 的突然进化。第三种形态类型，在前一次分析中未被识别，在形状和旋卷方向上与其祖先 G. plesiotumida 不同。这种形态类型在 G. tumida 出现之前的 414,000 年中主导了 globorotaliid 种群。其后代 G. tumida 的第一个种群在 44,000 年的时间内突然进化。形态数据和生物地层学证据的结合表明，G. tumida 通过分支进化演变而来。我们的发现为谱系内分支渐进主义的最佳记录案例之一带来了意想不到的转折，同时也重新审视了化石记录中物种形成研究的局限性和前景。
