The Evolution of Hierarchical Structure in Organisms: A Test for an Increasing Tendency

生物体等级结构的进化：一种增长趋势的检验丹尼尔·麦克谢，杜克大学-0208156摘要生命史上最明显的趋势之一是生物体等级结构的增长，也就是说，它们嵌套的程度，整体中的部分。众所周知的等级增加的例子包括进化中所谓的“重大转变”，例如多细胞生物的各种起源（例如，植物和动物）从自由生活细胞的聚集体，以及在更高水平上，集落的起源（例如，社会昆虫群体）从多细胞个体的克隆。 趋势是明确的，但我们对机制知之甚少，对推动趋势的因素知之甚少。一种可能性是，存在一种偏向，一种等级增加的积极趋势，也许是因为自然选择倾向于更大的嵌套。但也有可能不存在这样的趋势，等级的减少和增加一样频繁，这种趋势是进化谱系在等级光谱中被动传播的结果。为了了解这是如何工作的，假设第一批生物（可能是细菌）的等级尽可能浅，所以最初的等级只会增加。然后，随着生活的多样化，在不同的群体中发生增加和减少，等级结构的外壳将被推向更高的层次。史蒂芬·杰伊·古尔德在他的书《满屋》中有力地论证了这一机制。 关于这个问题的证据-是否存在任何增加的趋势-是模棱两可的。我们知道，在向独居生活的过渡中，等级结构的丧失已经发生，特别是在殖民地一级。但其频率，相对于增加，是未知的。这里提出的研究是有史以来第一次尝试以严格的方式回答这个问题。调查的一个障碍是，已知的主要转变太少，无法以统计上可靠的方式估计增加和减少的相对频率。这项研究解决了这个问题，采用最近开发的高分辨率层次规模。该等级标识主要等级之间的中间等级（例如，在多细胞个体和群体之间），实际上在主要转变内插入一系列“次要转变”，从而增加了可用于研究的转变的数量。为了测试增加的趋势，该研究将检查从细菌到超级菌落的等级组织光谱中的微小转变。现代系统发育的方法将被用来重建历史的等级与每个组的研究，计算数量的增加和减少的层次，并评估其相对概率。

The Evolution of Hierarchical Structure in Organisms: A Test for an Increasing TendencyDaniel McShea, Duke UniversityEAR-0208156ABSTRACT One of the clearest trends in the history of life is the increase in the hierarchical structure of organisms, that is, in the degree to which they are nested, parts within wholes. Well-known cases of hierarchy increase include the so-called "major transitions" in evolution, such as the various origins of multicellular organisms (e.g., plants and animals) from aggregates of free-living cells, and also at a higher level, the origins of colonies (e.g., social-insect colonies) from clones of multicellular individuals. The trend is clear, but we know little about the mechanism, about what drives the trend. One possibility is that there has been a bias, a positive tendency for hierarchy to increase, perhaps because greater nestedness is favored by natural selection. But it could also be that there is no such tendency, that hierarchy decreases as often as it increases, and that the trend is the result of the passive spread of evolutionary lineages across the hierarchy spectrum. To see how this might work, suppose that the first organisms (presumably bacteria) were hierarchically as shallow as possible, so that initially hierarchy could only increase. Then, as life diversified, with both increases and decreases occurring in various groups, the outer envelope of hierarchical structure would be pushed toward ever-higher levels. Stephen Jay Gould argued forcefully for this mechanism in his book, Full House. The evidence on this question - whether any increasing tendency exists - is equivocal. We know that losses of hierarchical structure have occurred, especially at the colony level, in the transitions back to solitary life. But their frequency, relative to increases, is unknown. The study proposed here is a first-ever attempt to answer the question in a rigorous way. One barrier to investigation has been the fact that too few major transitions are known to estimate relative frequencies of increase and decrease in a statistically robust way. This study solves the problem by employing a recently developed high-resolution hierarchy scale. The scale identifies intermediate levels between the major levels (e.g., between multicellular individual and colony), in effect interpolating a series of "minor transitions" within the major transitions and thereby increasing the number of transitions available to be studied. To test for an increasing tendency, the study will examine minor transitions across the spectrum of hierarchical organization, from bacteria to supercolony. Modern phylogenetic methods will be used to reconstruct the history of hierarchy with each group studied, to count numbers of increases and decreases in hierarchy, and to assess their relative probabilities.