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

生物等级结构的进化：对日益增长趋势的测试生命历史上最明显的趋势之一是生物等级结构的增加，也就是说，它们嵌套的程度，整体中的部分。等级增加的著名案例包括进化中所谓的“重大转变”，例如多细胞生物（如植物和动物）的各种起源来自自由活细胞的聚集，以及在更高层次上，多细胞个体克隆的菌落（如社会性昆虫菌落）的起源。趋势很明显，但我们对其机制知之甚少，不知道是什么驱动了这种趋势。一种可能是存在一种偏见，一种等级制度增加的积极趋势，也许是因为自然选择更倾向于更大的巢性。但也有可能没有这样的趋势，等级制度的减少和增加一样频繁，这种趋势是进化谱系在等级制度范围内被动传播的结果。为了了解这是如何工作的，假设第一批生物（可能是细菌）在等级上越浅越好，所以最初的等级只会增加。然后，随着生命的多样化，在不同的群体中既有增加又有减少，等级结构的外层将被推向更高的层次。史蒂芬·杰伊·古尔德在他的书《欢乐满屋》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，等级结构的丧失已经发生，特别是在群体层面，在向孤独生活的过渡中。但它们的频率，相对于增长，是未知的。这里提出的研究是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，无法以统计可靠的方式估计增加和减少的相对频率。本研究采用最新开发的高分辨率层次量表解决了这一问题。该量表确定了主要水平之间的中间水平（例如，在多细胞个体和群体之间），实际上在主要转变中插入了一系列“次要转变”，从而增加了可供研究的转变数量。为了测试这种增加的趋势，这项研究将检查从细菌到超级菌落的整个等级组织光谱的微小转变。现代系统发育方法将用于重建每个研究群体的等级历史，计算等级增加和减少的数量，并评估它们的相对概率。

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.