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Interactions between individuals of different species are a major type of kith selection, where individuals are selected to affect their partners in ways that ultimately benefit themselves (or their kin). Such interactions need not be cooperative, but when they are, they typically involve exchange of different services that one partner needs and the other can easily provide, so partners can be very different. Accordingly, in Chapter 2, Joel Sachs and colleagues explore associations or symbiosis among partners that are very different indeed, one being eukaryotic and the other prokaryotic. Such symbioses, by leading to mitochondria and chloroplasts, were responsible for the evolution of the eukaryotic cell itself. But additional symbioses are widespread and sometimes ancient. These authors use a combination of broad-scale phylogenetic analyses and case histories of particular systems to explore several transitions. They find, for example, that there is little phylogenetic signal to indicate that some bacterial groups are preadapted for eukaryotic symbiosis. Instead, the genes required appear to be quite widely available through horizontal transmission. Mutualistic interactions appear to arise from both parasitic and free-living ancestors. Once acquired, these mutualistic interactions seem to be quite stable, with few reversions to nonmutualistic forms. Given the tendency of vertically transmitted symbionts to degrade and the propensity of horizontally transmitted ones to cheat, this stability is somewhat surprising.

The social insects have long been viewed as the pinnacle of cooperation. This view is most tenable if one ignores the cooperation that goes on in transitions that are already complete, such as to multicellular animals or the eukaryotic cell. But some social insect colonies are so cooperative and integrated that they are viewed as superorganisms (organisms made up of other organisms). The motive force behind the evolution of these societies, which consist of close relatives, is kin selection (Hamilton, 1964a). In Chapter 3, Peter Nonacs points out that predictions from kin selection theory have been both successful and also disappointing. The difference, he suggests, is not due to chance. The successful predictions from sex-ratio theory and worker-policing theory work because the predicted behaviors can be achieved using simple environmental cues that correlate with kinship. It is easy to treat males differently from females, or workers from queens. The less successful kin selection predictions, such as parts of skew theory, may fail because they require genetic kin recognition mechanisms sufficient to detect closer from more distant relatives within colonies. This may not explain everything, because genetic kin recognition systems do exist, at least for distinguishing colony members from noncolony members. The interaction between environmental and genetic recognition systems has scarcely been explored, and Nonacs runs computer simulations

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