McShea Daniel W
Museum of Paleontology, University of Michigan, Ann Arbor, Michigan, 48109.
Evolution. 1994 Dec;48(6):1747-1763. doi: 10.1111/j.1558-5646.1994.tb02211.x.
Large-scale evolutionary trends may result from driving forces or from passive diffusion in bounded spaces. Such trends are persistent directional changes in higher taxa spanning significant periods of geological time; examples include the frequently cited long-term trends in size, complexity, and fitness in life as a whole, as well as trends in lesser supraspecific taxa and trends in space. In a driven trend, the distribution mean increases on account of a force (which may manifest itself as a bias in the direction of change) that acts on lineages throughout the space in which diversification occurs. In a passive system, no pervasive force or bias exists, but the mean increases because change in one direction is blocked by a boundary, or other inhomogeneity, in some limited region of the space. Two tests have been used to distinguish these trend mechanisms: (1) the test based on the behavior of the minimum; and (2) the ancestor-descendant test, based on comparisons in a random sample of ancestor-descendant pairs that lie far from any possible lower bound. For skewed distributions, a third test is introduced here: (3) the subclade test, based on the mean skewness of a sample of subclades drawn from the tail of a terminal distribution. With certain restrictions, a system is driven if the minimum increases, if increases significantly outnumber decreases among ancestor-descendant pairs, and if the mean skew of subclades is significantly positive. A passive mechanism is more difficult to demonstrate but is the more likely mechanism if decreases outnumber increases and if the mean skew of subclades is negative. Unlike the other tests, the subclade test requires no detailed phylogeny or paleontological time series, but only terminal (e.g., modern) distributions. Monte Carlo simulations of the diversification of a clade are used to show how the subclade test works. In the empirical cases examined, the three tests gave concordant results, suggesting first, that they work, and second, that the passive and driven mechanisms may correspond to natural categories of causes of large-scale trends.
大规模的进化趋势可能源于驱动力，也可能源于有限空间内的被动扩散。这类趋势是跨越重要地质时期的较高分类单元中持续的定向变化；例如包括常被提及的整个生命在大小、复杂性和适应性方面的长期趋势，以及较低的超特定分类单元中的趋势和空间趋势。在驱动趋势中，分布均值由于一种作用于发生多样化的整个空间内谱系的力（可能表现为变化方向上的偏差）而增加。在被动系统中，不存在普遍的力或偏差，但均值增加是因为在空间的某些有限区域，一个方向的变化被边界或其他不均匀性所阻挡。已经使用两种检验来区分这些趋势机制：（1）基于最小值行为的检验；（2）祖先 - 后代检验，基于对远离任何可能下限的祖先 - 后代对随机样本的比较。对于偏态分布，这里引入第三种检验：（3）子分支检验，基于从终端分布尾部抽取的子分支样本的平均偏度。在某些限制条件下，如果最小值增加、祖先 - 后代对中增加显著多于减少，并且子分支的平均偏度显著为正，则系统是由驱动的。被动机制更难证明，但如果减少多于增加且子分支的平均偏度为负，则更可能是被动机制。与其他检验不同，子分支检验不需要详细的系统发育或古生物学时间序列，只需要终端（例如现代）分布。一个分支多样化的蒙特卡罗模拟用于展示子分支检验的工作原理。在所研究的实证案例中，这三种检验给出了一致的结果，首先表明它们是有效的，其次表明被动和驱动机制可能对应于大规模趋势原因的自然类别。
