Although most of this book is about the new topics that are being treated as part of social evolution, such as genes, microbes, and medicine, the old fundamental subjects still matter and remain the object of vigorous research. The first four chapters revisit some of these standard arenas, including social insects, cooperatively breeding birds, mutualisms, and how to model social evolution.
There are many ways to think about and model social evolution. Inclusive fitness is one of the most venerable and most useful, and is the framework used by many authors in this book. In Chapter 1, David Queller revisits why inclusive fitness has been so useful and suggests ways to expand it to make it speak more directly to interactions besides kin selection. He delimits two other kinds of social selection that can be treated more explicitly in Hamilton’s rule. “Kind selection,” which involves synergisms between individuals expressing the same traits, groups together greenbeards (genes that in effect can identify the presence of copies of themselves in other individuals) and many cases of frequency-dependent games because these share the feature that individuals expressing the trait have different effects on other expressers compared to nonexpressers and they also share many differences from pure kin selection. “Kith selection” requires neither kin nor kind, but instead involves actors affecting partners in ways that feed back to the actor’s fitness. Mutualism and manipulation are included in this category. The expanded version of Hamilton’s rule with kin, kith, and kind could bring the advantages of Hamilton’s methods to a broader range of social interactions.
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
showing how greenbeard loci can perturb the outcomes expected under pedigree relatedness alone.
After the social insects, cooperative birds and mammals have attracted the most attention. Many bird species have helpers at the nest, usually offspring from previous broods who have remained at their natal site (Cockburn, 2006). Kinship is important here too. Helping systems usually evolve from monogamous ones, and discrimination evolves in systems that show variation in relatedness (Cornwallis et al., 2010). But the story is more complicated, for two reasons. First, although, some helpers gain kin-selected benefits through helping close kin, others may gain direct benefits. Compared with the social insects, more research on birds has addressed the particular benefits of remaining at home and on the ecological constraints that may limit independent breeding. Variance in reproductive success has played a role in these discussions, but in Chapter 4, Dustin Rubenstein moves it to a more central position. He suggests that cooperative breeders may be bet hedgers, gaining advantage from a more uniform reproductive output in cooperative groups. Rubenstein draws on many years of his field data on starlings in Africa, where there is much variation in both time and space, and he finds support for several predictions of this hypothesis.