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10How Many Levels Are There? How Insights from Evolutionary Transitions in Individuality Help Measure the Hierarchical Complexity of Life Carl Simpson How the vast range of spatial and temporal scales on which biological processes interact and relate to each other is a fundamental problem in evolutionary biology. The standard solution was given by Darwin and formed the core of the modern synthesis. Observed processes of organisms interacting with their environment (which includes other organisms ) produce all the patterns at all spatial and temporal scales. From this perspective, biology is uniformitarian, patterns at all scales are thought to be caused by the observable local processes. Unfortunately, observations at large scales do not seem to be reducible to local patterns in all cases. The most critical of these observations are those transitions in evolution known as evolutionary transitions in individuality (Buss 1987; Maynard Smith and Szathmáry 1995; Michod 1999). What occurs during these transitions is the passing of the dominant fitness components from one level of organization to a higher, more inclusive level, and with it new levels of organization, evolution, and ecological interaction emerge. That is to say, as the new level of individuality emerges, we find new agents that not only have fitness values, the fitness of these new agents dominates the evolutionary trajectory of them and their components. From the view of some standard theory, these transitions are impossible. The lowest levels of selection are thought to always dominate because of higher heritability, shorter generation times, and other factors that seem to stack the cards in favor of the lowest levels. Yet, the transition from individuated cellular to well-individuated multicellular life occurred approximately sixteen times in largely unrelated groups (Bonner 2001; Maynard Smith and Szathmáry 1995), and that from individuated multicellular life to partially individuated colonial or social life occurred multiple times in ten animal phyla (Wilson 1975). That may not seem like a lot considering those occur over 4.6 billion years and among countless other origination events, including the origin of life, but though these events are rare, they also seem to be patterned: Often these transitions occur in clusters of independent events like during the Cambrian Explosion. Clearly, these are not just statistical possibilities. Since the modern synthesis, only conceptual work on the hierarchical nature of life has tried to explicitly incorporate the diversity of processes at the varying spatial and temporal scales in which organisms live and evolve. Curiously, all the hierarchies are rigid, having 200 Carl Simpson only a few discrete levels of fixed rank. And there are often two hierarchies, one for organisms and one for their interactions. There are the replicator/interactor hierarchy (Hull 1980), the replicator/vehicle hierarchy (Dawkins 1982), the genealogical/ecological hierarchy (Eldredge 1985; Eldredge and Greege 1992), the codical/material hierarchy (Williams 1992), and the fraternal/corporate hierarchy (Queller 2000). It is curious, at least when viewed with a desire to unify phenomena, that when hierarchy theory encounters biology, it splits its effort. After all, the level both hierarchies have in common is the organism. Because the majority of work on hierarchies did not realize the importance of the major transitions, most existing hierarchies are themselves static, unevolving, and synchronic. After Buss (1987), we know that any hierarchy must be diachronic and evolvable. The emergence of new hierarchical levels during a major transition explicitly allows for the hierarchy as a whole to evolve over time (Buss 1987; Maynard Smith and Szathmáry 1995; Michod 1999). But while theories of the major transitions are dynamic, they have not tried to incorporate ecology and evolution, which leaves the dual hierarchies, flawed as they may be, as the only hierarchy that takes ecology seriously. Buss’s notion that individuality is derived can teach us an additional lesson about the emergence of new levels. Individuality is derived, but poorly individuated organisms are still common today, yet they can themselves undergo additional major transitions. The sponges and cnidarians provide the best examples. Though they originated over a period of more than 550 million years, only a few lineages have increased their internal integration, but never as much as a simple bilaterian. Yet, both form aggregations and have many colonial species. Is a sleaze of sponges or a coral colony at the same level as colonial or social groups that evolved from highly individuated organisms such as social insects or mammals ? There is no clear answer, but individuality itself cannot be the sole criterion for a new level. The hierarchical levels...

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