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This chapter examines the notorious issue of group selection in behavioural ecology, one of the mainstays of the traditional levels of selection debate. The history of the group selection controversy is briefly traced. The relationship between group selection, kin selection, and evolutionary game theory is discussed. An important debate between Sober and Wilson and Maynard Smith concerning the correct way to conceptualize group selection is explored. Lastly, some arguments of L. Nunney concerning the distinction between weak and strong altruism, and how individual and group selection should be defined, are examined.

Selection is often generated by interaction with other organisms: neighbours, partners, or antagonists. The force and direction of selection in these social contexts is very generally influenced by the density and composition of the population. It may result in some degree of cooperation or helpfulness, rather than unrestricted competition among individuals. The first section here is called Selection within a single uniform population: density-dependent selection and details density regulation; density-dependent fitness; the principle of frugality; resource competition in continuous culture; r-K selection; r-K selection experiments; and selection in seasonal environments. The second section is called Selection within a single diverse population: frequency-dependent selection and describes GxG; frequency-dependent fitness; and also frequency-dependence in complex environments. The third section is about social behaviour and describes the phenotypic theory of aggression and exploitation; cross-feeding; selfish cooperation; the prisoners' dilemmas; intransitive social interactions; and time-lagged social interactions. The final section is called Kin selection and group selection and describes kin selection; kin proximity and kin choice; spite; group selection in structured populations; productivity and diversity; artificial group selection; and cultural evolution.

The term quorum sensing (QS) is used to describe communication between bacterial cells, whereby a coordinated population response is controlled by diffusible signal molecules. QS has not only been described between cells of the same species (intraspecies), but also between bacterial species (interspecies) and between bacteria and higher organisms (interkingdom). This chapter compares the evolutionary literature on animal signalling and cooperation with the microbiological literature on QS, and discusses whether bacterial QS can be considered true signalling. From an evolutionary perspective, intraspecies signalling can be explained using models such as kin selection, but explanations become more difficult when communication is described between species. It is likely that this often involves QS molecules being used as ‘cues’ by other species as a guide to future action or as coercing molecules whereby one species will ‘coerce’ another into a response.

This chapter explores the modeling of multilevel selection (MLS) – and the related concepts of group selection and kin selection – using variance partitioning methods, using the Price equation to elucidate basic issues within MLS theory. An expansion of this theory, based on contextual analysis and direct fitness, is used to show that kin selection and MLS selection have the same mathematical roots, although they are not identical. Kin selection theory is oriented towards identifying the optimal group and individual level traits that maximize the fitness of an organism, while MLS theory is oriented towards identifying the rate of evolution of the group and individual level traits in a specified situation. Because of these differences, kin selection and group selection can be considered as complementary approaches. The chapter also addresses why heritable variation at one level often bears little relation to heritable variation at other levels. It is shown that interactions among units (e.g., individuals) cannot contribute to a response to selection at that level, but can contribute to response to selection at a higher level (e.g., the population). Thus, the response to selection at one level can be qualitatively different than the response to selection at other levels.

This chapter discusses the evolution of altruism through kin-selection. There is a great deal of evidence that humans and other animals are disposed to behave in accordance with Hamilton’s rule, sacrificing their interests for the sake of their kin and favoring those to whom they are most closely related, especially relatives with the highest reproductive potential. Kin-selection is a complex process that is widely misunderstood. The evidence suggests that humans (and many other animals) rely on three main cues to distinguish their kin from others—similarity (especially how much others look and smell like them), familiarity, and proximity (how close to them they reside). Because kin recognition mechanisms are designed in imperfect ways, people may end up helping others who look and act like their kin, thus contributing little or nothing to the propagation of their genes and rendering the behaviors genetically altruistic.

This chapter examines inclusive fitness, which is the conceptual paradigm that has dominated the field for the past three decades. This paradigm is shown to be wanting as a framework to understand how social wasps evolved.

This chapter examines cooperation between two species and how cooperation among related individuals of one species can also help maintain cooperation between species. It presents some examples of between-species cooperation, the evolutionary tradeoffs that can undermine such cooperation, and opportunities for improvement. The chapter begins by showing that cooperation and so-called cheating commonly occur between two species. It then considers how conflict evolves and how two-species partnerships may be improved such that they will be useful in agriculture. It also explores symbiotic nitrogen fixation and the dilemma termed “tragedy of the commons,” the link between kin selection and within-species cooperation, and microbial analogs of kin selection and rhizobial mutualism. Finally, the chapter discusses the sanctions hypothesis that explains the nature of microbial cooperation with plants, along with other opportunities for improved two-species cooperation.

This chapter considers the mechanisms by which aposematic signals might evolve and be maintained. Of particular importance are the roles of spatial aggregation and kin selection in the evolution of such signals, and the co-evolution of defence and signals of that defence. The initial evolution of aposematism is particularly interesting and challenging, since aposematic signals are expected to be more effective when they are commonplace, thus an initial rare mutant might be expected to be at a disadvantage.

Patients are phenotypes; thus all medical conditions are a product of genes and the environment. One genotype can produce many phenotypes depending on the environments encountered. Such phenotypic plasticity promotes reproductive success by creating a better fit between the genotype and the environment. Evolutionary insights into kin selection, life history, parental investment, and sexual selection help us to understand: the origins of child abuse and homicide in step-families; deadbeat dads; attachment disorders; failure to thrive; female infanticide; excess male mortality from accidents, suicide, and disease; risky behaviour; immunosuppression; reproductive cancer; marital violence; and genital cutting. Many of these problems reflect reproductive conflicts of interest between individuals. Other conflicts occur within individuals and involve life history trade-offs. Conflicts of interest within and between individuals constrain natural selection, and prevent an optimal world wherein adaptation is maximized at all levels simultaneously.

This chapter contains a discussion of Maynard Smith’s conceptual distinction between kin and group selection. The author discusses the design and experimental study of kin selection for and against cannibalism as a test of the definitional distinction. The fallacy that societies are vulnerable to cheaters is discussed along with the concept of a “kin-selection mutation balance.” The results of models with three levels of selection as well as models of the effect of inbreeding on kin selection are presented. The synergistic coevolution of mating system and sociality are illustrated and the underlying processes governing the run-away evolution of sociality are discussed. The advantage of the multilevel selection perspective over the inclusive fitness perspective is illustrated by drawing from counter-factual predictions of inclusive fitness theory and its inability to account for underlying processes of genetic change.

When do we care for others as we care for ourselves? William Hamilton showed that we should be expected to care for our family members in proportion to our degree of relationship to them. Such reasoning explains why eusociality evolved independently at least twelve times in the order Hymenoptera, which includes ants, bees, and wasps, but only three times elsewhere in the animal kingdom. It also verifies Thomas Hobbes' answer to the question: Why cannot mankind live sociably one with another as bees or ants?

The phenotypes of those individuals with which an focal individual interacts often influences the trait value in the focal individual. Maternal effects is a classic example of this phenomena, as is fitness. If these traits are heritable, then the selection response depends on both the change in the direct effects influencing a target trait and the associative effects contributed by interacting individuals. In such a setting, the breeder's equation no longer holds, as the problem is now a multiple trait one. This chapter examines the theory of response under models with both direct and associative effects, which can lead to a reversed response (a trait selected to increase instead decreases). The evolution of behavioral traits, including the evolution of altruism, is best handled using this approach. Further, kin and group selection follow as special cases of the gerenal model under multilevel selection. This chapter also examines how mixed models can be used estimate model parameters.

In this chapter I distinguish between two basic forms of altruism, which I call biological and psychological altruism, and explain how dispositions to emit each type evolved in the human species. Biological forms of altruism are defined in terms of the consequences of helping behaviors. They contribute to the survival and reproductive success of recipients at a cost to the survival and reproductive success of donors. Psychological forms of altruism are defined in terms of the motives and intentions of actors. They are aimed at improving the welfare of recipients as an end in itself. A form of conduct that has been labeled reciprocal altruism does not really qualify as biologically altruistic because it produces return benefits to donors. Mental mechanisms that evolved through kin selection are not very precisely designed. When people help others who resemble their kin, they may behave in biologically and genetically altruistic ways. Evolutionary theorists disagree about how mental mechanisms that dispose people to help strangers anonymously with no possibility of return benefits evolved. Researchers have concluded that in some contexts, people may be genuinely motivated to help others as an end in itself. Studies have produced the following findings. People are willing to help others in emergencies without any apparent concern for their own welfare. Empathy engenders the motive to help victims as an end in itself, rather than as a means of reducing vicariously-experienced distress, enhancing one’s public image, making one feel good about oneself, and so on. Identifying with a groups disposes people to sacrifice their interests for the sake of the group. People experience moral emotions that engender altruistic motives. Higher order cognitive abilities interact with more primitive emotional responses to structure moral emotions such as sympathy in ways that give rise to increasingly effective forms of altruism.

In group-structured populations in which some other assumptions are satisfied, kin and group selectionist methods provide formally equivalent conditions for change. However, this only shows an equivalence between two statistical methodologies, and this is compatible with there being a real, causal distinction between kin and group selection processes. This chapter pursues a Hamilton-inspired, population-centred approach to drawing that distinction, on which the differences between kin and group selection are differences of degree in the structural properties of populations. The relevant properties are K, the overall degree to which genealogical kin interact differentially, and G, the overall degree to which the population contains stable, internally integrated, and externally isolated social groups. A spatial metaphor (‘K-G space’) provides a useful framework for thinking about these differences.

The social and behavioral sciences have a long-standing interest in the factors that foster selfish (or individualistic) versus altruistic (or cooperative) behavior. Since the 1960s, evolutionary biologists have also devoted considerable attention to this issue. In the last 25 years, mathematical models (reviewed in Wilson and Sober 1994) have shown that, under particular demographic conditions, natural selection can favor traits that benefit group members as a whole, even when the bearers of those traits experience reduced reproductive success relative to other members of their group. This process, often referred to as "trait group selection" (D. S. Wilson 1975) can occur when the population consists of numerous, relatively small "trait groups," defined as collections of individuals who influence one another's fitness as a result of the trait in question. For example, consider a cooperative trait such as alarm calling, which benefits only individuals near the alarm caller. A trait group would include all individuals whose fitness depends on whether or not a given individual gives an alarm call. If the cooperative trait confers sufficiently large reproductive benefits on the average group member, it can spread. This is because trait groups that happen to include a large proportion of cooperators will send out many more offspring into the population as a whole than will groups containing few, or no cooperators. Thus, even though noncooperators out reproduce cooperators within trait groups (because they experience the benefits of the presence of cooperators without incurring the costs), this advantage can be offset by differences in rates of reproduction between trait groups. Numerous models of group selection (Wilson and Sober 1994) show that whether cooperative traits can spread depends on the relative magnitude of fitness effects at these two levels of selection (within and between trait groups). In addition, there is a growing body of empirical evidence for the operation of group selection in nature (e.g., Colwell 1981; Breden and Wade 1989; Bourke and Pranks 1995; Stevens et al. 1995; Seeley 1996; Miralles et al. 1997; Brookfield 1998) and under experimental conditions (reviewed in Goodnight and Stevens 1997).

At the heart of the debate concerning PCD as an adaptation lies the levels of selection question. PCD can only be selected for if something other than the dying cell is the target of selection. This chapter examines the possibilities. The importance of how terms like ‘gene’ or ‘group’ are defined and interpreted is discussed. In the gene-centric view, PCD is an adaptation only when the PCD gene is also a ‘selfish gene’. In these instances, selfish is used in the narrow sense such as in mobile genetic elements or replicons like plasmids. PCD can also be driven by kin selection or group selection, depending on relatedness and the cost / benefits to relatives or structured groups. The Price equation is used to formulate the question of group selection and PCD. The data from fitness and selection experiments are analyzed using a multilevel selection type 1 (MLS1) framework. A contextual analysis approach is included to separate individual cell selection from group level selection.

This chapter investigates recent proposals that a kin-selected communication system was adaptive in human evolution, and that in turn, interactions between closely-related individuals, particularly mothers and infants, drove the evolution of (proto)language. It argues that a proposed link between kin communication, teaching, and linguistic complexity is very weak; and that altruism is not characteristic of language use. The chapter is organized as follows. Section 5.2 examines the idea that protolanguage evolved for pedagogical purposes vital to a child's survival. Section 5.3 considers the proposed evolution of linguistic complexity out of either infant or maternal vocalizations. Section 5.4 questions the idea that language equates to altruistic information exchange. Section 5.5 concludes that a kin selection scenario has no explanatory power in investigations of the evolution of language.

Adaptations are the products of natural selection—traits that evolved because they proved useful at some level. Often adaptations evolved because they enhanced the survival or reproductive output of individuals, but selection can also operate at other levels—genes, individuals, populations, and species. Sometimes a genetic change has positive effects on all levels. A mutation that increases the survival of its carrier would increase in frequency; populations that become fixed for that allele may be less prone to extinction, which in turn may increase the longevity of that species. Other times there can be conflicting fitness effects at the different levels. This chapter explores the power of natural selection in shaping the living world by investigating the complexities of multilevel selection, biological solutions to heterogeneity and unpredictability in the environment, and how interactions between species can shape evolution. However, the chapter starts by investigating factors that may constrain adaptive evolution.

The theory that organisms become adapted to their environment through the process of natural selection has become so ingrained in modern biological thought, and more generally in Western culture of the late 20th century, that it is surely one of the great scientific paradigms of the present era. Evolution and adaptation were both well-accepted concepts by the mid-19th century, at least among French and British natural philosophers. The theory of natural selection, developed by Wallace (1858) and Darwin (1859), provided a functional connection between the two processes. However, despite its logical consistency, natural selection was not accepted as a necessary or sufficient explanation for adaptation until the “evolutionary synthesis” of the mid-20th century, when knowledge from population and quantitative genetics, natural history (e.g., biogeography, ecology, behavior), systematics, and paleontology merged to form the unified theory of adaptive evolution known as neo-Darwinism (see Futuyma 1998 for a concise review of this history). Since that time, natural selection has been accepted as the universal mechanism leading to adaptation, and the two terms have become so closely associated as to be almost tautological. Adaptationist hypotheses are now fundamental to much of modern biology and are becoming increasingly apparent in more disparate fields, such as anthropology, medicine, biochemistry, and psychology (Futuyma 1999). Nevertheless, there is much that natural selection cannot explain. For example, chance events may strongly influence macroevolutionary trends (i.e., the origin and extinction of species and higher taxa), some aspects of molecular evolution, and evolution within small or subdivided populations (Mazer and Damuth, this volume, chapter 2; Nunny, this volume). For this reason, adaptationist hypotheses should be viewed with skepticism until adequately tested (Reznick and Travis, this volume). In this chapter, we carefully define natural selection and discuss methods of measuring selection in natural populations as a means of testing adaptationist hypotheses. These methods are most appropriate for testing hypotheses concerning the adaptive significance of contemporary trait distributions within and among populations (“microevolutionary” hypotheses) and thus have particular relevance for evolutionary ecologists. Readers will find many additional examples of these and other methods of testing microevolutionary adaptationist hypotheses throughout this volume, such tests being an essential component of most research programs in evolutionary ecology.

Group selection theory was, and to some extent still is, one of the most controversial topics in biology. It is interesting that the origins of life and death syntheses invoke group selection in some form or another. At the origin of life when relatedness between replicating units, whatever they may have been, was not a feature group selection of compartmentalized replicators seems to be one of the few feasible explanations. At the origin of PCD, cooperation can be explained by a few driving forces like reciprocity, kin selection and group selection. At some point, however, particularly when relatedness between individuals was low, selection for groups of cooperating cells seems most likely. Selection may have acted upon traits that result from aggregation or emergence. Examples of the first instance are the population densities and frequencies of PCD that are critical for groups with PCD to outcompete those without. In the second instance, group selection acted on the properties that emerged out of the sociobiological interactions between replicating units at the origin of life. Despite the controversies, group selection appears essential for a complete account of the origins of both life and death.