Levels II: Bridging Levels |
What is the relation between functional integrity and its component processes, between part and whole, where selection acts on multiple levels or scales simultaneously? The problems are particularly acute in cases of the origin of new levels of organization or the creation of ontologically distinct entities ó whether slime mold colonies, multicellular organisms or populations of sexually reproducing organisms (populational lineages). This session offers new approaches to the problem of bridging levels by considering: (1) how emergent functionality affects the complexity of its parts; (2) why adaptations accrue primarily at the organismal level; and (3) how conflicts between selection at different levels of organization are resolved in the case of slime mold behavioral polymorphism.
- Dan McShea, Duke University
"Remodularizing the Organism"
- New hierarchical levels arise in evolution when lower-level entities combine to form higher-level individuals. Examples include the origin of multicellular organisms from solitary cells and the emergence of individuated colonies from solitary multicellular entities. Here I propose a hypothesis regarding the emergence of new levels, namely that as a higher-level individual arises and becomes able to performs functions, the number of functional demands on its lower-level entities declines. Assuming that number of part types reflects number of functions, the number of part types within the lower-level entities should decline also. The hypothesis is testable if the notion of "parts" can be operationalized. I suggest a way to do this using "object parts" as a proxy for all parts, and show how object parts can be identified in practice. Then I present data from a study at the cell level which show that cells in animals and land plants have fewer object parts than free-living protists, on average. Also, ---
- Eduardo Wilner, University of Alaska Fairbanks
"Why Adaptations Seem to Accumulate at the Organismal Level?"
- Most evolutionary biologists have come to agree that natural selection can work at many levels of organization. There can be, for instance, individual, kin, group, and even species selection. However, this pluralistic state of affairs generates a new question: why is it that adaptations seem to have "accumulated" mostly at one level, namely, the organismal level? This question can be put in a more general formulation. Darwinian evolution explains why different species are different. One must realize, however, that this explanation seems to leave open the question of "vertical" diversity: why is it that different levels of organization, at least in regards to adaptation, seem to be so different? Or, putting it in a more fundamental way, why are there different levels of life's organization? Here, I argue that the phenomenon of differential adaptive accumulation can be explained by the Darwinian framework. I also explore some of the most surprising implications regarding the nature of vertical diversity that follow from this explanation.
- Ian Nyberg, University of Texas at Austin
"Levels and Kin Selection"
- The cellular slime mold, Dictyostelium discoideum, the social bacterium, Myxococcus xanthus, and the colonial tunicate, Botryllus schlosseri, exhibit very similar conflicts between selection at different levels of organization. A central question is whether kin selection can resolve these
conflicts. When starved, individual cells of D. discoideum and M. xanthus respond by aggregating into fruiting bodies. A strain that makes the same proportion of non-spore to spore cells regardless of the presence of genetically different strains in a group could loose fitness. It has recently been shown that this happens in D. discoideum in nature. Previous mathematical results have largely failed to find a good rationale for this behavioral polymorphism. New mathematical results presented here show that intensity of competition at the cellular level should decrease because of fitness loss of the aggregate.
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