Biased extinction and evolutionary trends
Biased extinction and evolutionary trends
复制标题
偏向灭绝和进化趋势
作者：
R. Norris
长期趋势的方向性可能会受到进化枝内在力量的强烈影响。身体大小和骨骼形状的趋势可能更多地取决于反映生态差异的生存率变化，而不是环境的长期方向变化。因此，通过选择性地消除某些形态并允许幸存者发现下一次辐射，大规模的辐射可以帮助推动进化趋势。例子包括重复的趋势，以更大的最大身体大小和龙骨物种的进化，从那些球状测试在南极有孔虫。这两种趋势的大小和形状的发展，因为小的物种与球状测试是显着更大的物种，比灭绝或周边龙骨。因此，在白垩纪-第三纪和始新世-渐新世灭绝事件中幸存下来的物种都是小型的、无龙骨的类群。大型物种和有龙骨的物种在两次大规模灭绝后都趋同进化，因为创始人重新辐射。三个辐射的南极有孔虫的比较表明，相似的测试形状和大小的趋同进化是由于在海洋学的同步变化，跟踪进化趋势。相反，需要重建栖息地异质性，以允许随后的辐射展开，而不是密切引导其进展。类似的进化趋势将在每个辐射中发展，只要创始人具有相似的形态，并且在前一个辐射中存在的变体的进化不被环境排除。
The directionality of long-term trends can be strongly biased by forces intrinsic to a clade. Trends in body size and skeletal shape may be dictated more by variations in survivorship that reflect differences in ecology than by long-term directional changes in the environment. Hence, mass extinctions can help drive evolutionary trends by selectively eliminating some morphologies and permitting the survivors to found the next radiation. Examples include repeated trends toward larger maximum body size and the evolution of keeled species from those with globose tests in planktonic foraminifera. Both the trends in size and shape develop because small species with globose tests are significantly more resistant to extinction than species that are large or have peripheral keels. Hence, the survivors of both the Cretaceous-Tertiary and Eocene-Oligocene extinction episodes are small, unkeeled taxa. Large species and species with keels evolved convergently after both mass extinctions as the founders radiated anew. Comparison of three radiations of planktonic foraminifera suggest that the convergent evolution of similar test shapes and sizes is not due to synchronous changes in oceanography that track evolutionary trends. Instead, the reestablishment of habitat heterogeneity is needed to permit the ensuing radiation to unfold rather than to closely guide its progress. Similar evolutionary trends will develop in each radiation as long as the founders have similar morphology and the evolution of variants present in the previous radiation is not precluded by the environment.