Wagner Peter J
5734 South Ellis Avenue, Department of Geophysical Sciences, The University of Chicago, Chicago, Illinois, 60637.
Evolution. 1996 Jun;50(3):990-1007. doi: 10.1111/j.1558-5646.1996.tb02341.x.
Gastropod evolution during the early Paleozoic featured active trends (i.e., differential replacement of morphologies) for at least three shell characters. Selective sorting, either of individual organisms or of whole species, is an obvious mechanism for such active trends. Sorting of individuals should result in a disproportionate number of ancestor to descendant transitions being in the same direction as the trend, whereas sorting of species should result in species with particular morphologies producing more daughter species. Sorting of species can occur over long periods of time or it can be concentrated over a particular interval, such as an extinction event. Constraints on morphologic evolution also can drive trends, especially in cases where it is easier to produce a particular morphology than it is to change it. Finally, active trends can be artifacts of unrelated differential diversification within a clade (i.e., species hitchhiking), which might result from sorting of species based on phylogenetically associated characters or simply by chance. Unlike other active trends, trends attributable to species hitchhiking do not support hypotheses about selection or evolutionary constraints.
在早古生代，腹足类动物的进化至少在三个壳特征方面呈现出活跃的趋势（即形态的差异替代）。对个体生物或整个物种进行选择性筛选，是产生这种活跃趋势的一种明显机制。对个体进行筛选会导致在祖先到后代的过渡中，与趋势方向相同的过渡数量不成比例，而对物种进行筛选会导致具有特定形态的物种产生更多的子代物种。对物种的筛选可以在很长一段时间内发生，也可以集中在特定的时间段内，比如一次灭绝事件期间。形态进化的限制也可能推动趋势的发展，特别是在某些情况下，产生特定形态比改变它更容易。最后，活跃趋势可能是一个分支内不相关的差异多样化的产物（即物种搭便车），这可能是由于基于系统发育相关特征对物种进行筛选，或者仅仅是偶然造成。与其他活跃趋势不同，归因于物种搭便车的趋势并不支持关于选择或进化限制的假说。
