![]() The hierarchical organization of biological systems offers additional proxies for complexity, such as the length and interconnectedness of biochemical pathways and gene regulatory networks or the degree of integration and modularity in the form and function of organismal parts 23. This definition is the most common in empirical studies of complexity, partly because of its immediacy but also because it translates into simple indices 19, 20, 21, 22. In its simplest formulation, complexity is defined as the number of constituent parts, or types of parts, in an organism (for example, genes, cells, tissues and organs). It has long been recognized that biological complexity can be indexed at various levels and that changes across levels are often decoupled, such that one is a poor predictor of the others 18. ![]() Measuring complexity is a more challenging prospect than quantifying either diversity or disparity 11 but has vast potential for illuminating the origin of body plans 12, 13, the imbalances in species richness across groups 14 and the temporal and group-specific patterns of morphological diversification 15, 16, 17. In contrast, biological complexity remains remarkably understudied. Increasingly, diversity and disparity have been examined alongside each other, especially in analyses of extinct organisms 6, 7, 8, 9, 10. Further work should therefore focus on the ecological relevance of differences in complexity and a more detailed understanding of historical patterns.īiological complexity 1, taxonomic diversity 2, 3 and morphological disparity 4, 5 are three fundamental components of macroevolutionary dynamics. Different subclades evolve more complex vertebral columns in different configurations and probably under different selective pressures and constraints, with widespread convergence on the same formulae. We find support for multiple-rate models of evolution for all complexity metrics, suggesting that increases in complexity occurred in stepwise shifts, with evidence for widespread episodes of recent rapid divergence. Several increases are inferred to have coincided with major ecological or environmental shifts. We find strong evidence of a trend towards increasing complexity, where higher values propagate further increases in descendant lineages. Vertebral counts, but not complexity indices, differ significantly between major groups and exhibit greater within-group variation than recognized hitherto. Third, we ask whether evolutionary shifts in complexity depart from a uniform Brownian motion model. Second, we ask whether changes in complexity throughout the phylogeny are biased towards increases and whether there is evidence of driven trends. First, we ask whether the distribution of complexity values in major mammal groups is similar or whether clades have specific signatures associated with their ecology. We focus on the serial differentiation of the vertebral column in 1,136 extant mammal species, using two indices that quantify complexity as the numerical richness and proportional distribution of vertebrae across presacral regions and a third expressing the ratio between thoracic and lumbar vertebrae. Highly differentiated and serially repeated structures, such as vertebrae, are useful systems with which to investigate these patterns. However, it is unclear whether this increase is a purely diffusive process or whether it is at least partly driven, occurring in parallel in most or many lineages and with increases in the minima as well as the means. The maximum anatomical complexity of organisms has undoubtedly increased through evolutionary time. ![]() ![]() Complexity, defined as the number of parts and their degree of differentiation, is a poorly explored aspect of macroevolutionary dynamics.
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