Department of Paleobiology and Department of Invertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20013-7012, USA.
Proc Natl Acad Sci U S A. 2013 Aug 20;110(34):13892-7. doi: 10.1073/pnas.1308588110. Epub 2013 Aug 7.
Evolutionary processes leading to adaptive radiation regularly occur too fast to be accurately recorded in the fossil record but too slowly to be readily observed in living biota. The study of evolutionary radiations is thereby confronted with an epistemological gap between the timescales and approaches used by neontologists and paleontologists. Here we report on an ongoing radiation of extant Bellamya species (n = 4) from the African Rift Lake Malawi that provides an unusual opportunity to bridge this gap. The substantial molecular differentiation in this monophyletic Bellamya clade has arisen since Late Pleistocene megadroughts in the Malawi Basin caused by climate change. Morphological time-series analysis of a high-resolution, radiocarbon-dated sequence of 22 faunas spanning the Holocene documents stasis up to the middle Holocene in all traits studied (shell height, number of whorls, and two variables obtained from geometric morphometrics). Between deposition of the last fossil fauna (~5 ka) and the present day, a drastic increase in morphological disparity was observed (3.7-5.8 times) associated with an increase in species diversity. Comparison of the rates of morphological evolution obtained from the paleontological time-series with phylogenetic rates indicates that the divergence in two traits could be reconstructed with the slow rates documented in the fossils, that one trait required a rate reduction (stabilizing selection), and the other faster rates (divergent selection). The combined paleontological and comparative approach taken here allows recognition that morphological stasis can be the dominant evolutionary pattern within species lineages, even in very young and radiating clades.
导致适应性辐射的进化过程通常发生得太快，无法在化石记录中准确记录，但又太慢，无法在现存生物群中轻易观察到。因此，进化辐射的研究面临着古生物学家和现代生物学家使用的时间尺度和方法之间的认识论差距。在这里，我们报告了来自马拉维裂谷湖的现存 Bellamya 物种（n = 4）正在进行的辐射，这为弥合这一差距提供了一个不同寻常的机会。在这个单系的 Bellamya 进化枝中，大量的分子分化是由于马拉维盆地晚更新世的大规模干旱而发生的，这些干旱是由气候变化引起的。对跨越全新世的 22 个动物群的高分辨率、放射性碳定年序列进行的形态时间序列分析记录了从全新世中期到全新世中期的所有研究特征（壳高、螺层数和从几何形态测量学获得的两个变量）的稳定状态。在最后一个化石动物群（约 5 千年前）沉积和今天之间，观察到形态差异急剧增加（增加了 3.7-5.8 倍），同时物种多样性也增加了。从古生物学时间序列中获得的形态进化率与系统发育率的比较表明，两个特征的分歧可以用化石中记录的缓慢率来重建，一个特征需要降低率（稳定选择），而另一个特征需要更快的率（分歧选择）。这里采用的综合古生物学和比较方法可以认识到，即使在非常年轻和辐射的进化枝中，形态稳定也可以是物种谱系中的主导进化模式。
