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This chapter examines how the logic of inclusive fitness theory can be mathematically formalized using the Price equation, and how that formalization can be used to derive Hamilton's rule in its simplest form, as applied to unconditional behaviors having additive effects on fitness. Various biological phenomena, such as sex allocation and working policing within eusocial insect colonies, have been analyzed by considering what strategies maximize individuals' inclusive fitness, and how observed social behaviors should correlate with quantities such as relatedness. The chapter derives Hamilton's rule by introducing some notation for the effects of behaviors on fitnesses of individuals that interact socially, to make explicit precisely how genes (and later phenotypes) affect fitness, and to give a general form of Hamilton's rule that will apply to any (unconditional, additive) behavior regardless of its details. It shows that inclusive fitness is a genuinely novel extension of the classical fitness studied by Charles Darwin, R. A. Fisher, and others.

This chapter examines which of the equivalent alternative partitions of fitness, including inclusive fitness and group fitness, can be interpreted as being subject to natural selection in a meaningful way. Inclusive fitness theory can deal with subtleties such as nonadditive fitness effects and conditionally expressed phenotypes. However, selection based on inclusive fitness gives equivalent predictions to other models of apparently different evolutionary processes, such as multilevel selection. The chapter considers how we can determine whether inclusive fitness really captures the essence of social evolution and whether inclusive fitness is really maximized by the action of selection, as suggested by William D. Hamilton. It also explains what heritability measures, and whether this makes sense biologically. Finally, it discusses the problem of classifying observed social behaviors in terms of their underlying evolutionary explanations.

This book has examined the genesis, the logic, and the generality of social evolution theory. In particular, it has presented evolutionary explanations of the many social behaviors we observe in the natural world by showing that William D. Hamilton's inclusive fitness theory provides the necessary generalization of classical Darwin–Wallace–Fisher fitness. This concluding chapter discusses the limitations of the analyses presented in this book and assesses the empirical support for inclusive fitness theory, focusing on microbial altruism, help in cooperative breeders, reproductive restraint in eusocial species, and the evolution of eusociality and cooperative breeding. It also considers more advanced topics in social evolution theory, including sex allocation, genetic kin recognition, spite, and the evolution of organismality. Finally, it reviews theoretical approaches to studying social evolution other than replicator dynamics and the Price equation, such as population genetics, class-structured populations, and maximization approaches.

This chapter considers the problem of correctly defining fitness costs and benefits in inclusive fitness theory, when competition occurs between offspring who are relatives. It reviews the definition of evolutionary fitness and shows how its misinterpretation explains many previous misunderstandings as to whether inclusive fitness theory always makes accurate predictions. The chapter begins with a discussion of Haldane's dilemma, which can be formalized with fitness equations that show that the risk of death can make fitness effects all-or-nothing. It then examines how inclusive fitness models can be constructed to deal with reproductive value and class-structured populations. It also shows how costs and benefits can be expressed as payoffs that are proportional to reproductive success, as changes in production of offspring, or as changes in evolutionary fitness. Finally, it presents examples that illustrate when fitness, payoffs, and fecundity are different, and how inclusive fitness analyses can be performed properly in such situations.

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.

This chapter examines the sociobiology of human cooperation. Given the tendency of people to copy the successful and the fact that natural selection favors the more fit, the chapter asks how our altruistic preferences overcame the cultural and biological evolutionary handicaps entailed by the reduced payoffs that they elicited. To answer this question, two major biological explanations of cooperation are discussed: inclusive fitness in either a kin-based or a multi-level selection model, and reciprocal altruism and its indirect reciprocity and costly signaling variants. The chapter explores a model of inclusive fitness based on group differentiation and competition, clarifying what is meant by multi-level selection and how it works. It also discusses models that address equilibrium selection, the link between standing strategy and indirect reciprocity, and positive assortment. Finally, it assesses the mechanisms and motives underlying helping behavior.

This book demonstrates the generality of inclusive fitness theory, with particular emphasis on its fundamental evolutionary logic. It presents the basic mathematical theory of natural selection and shows how inclusive fitness theory deals with more complicated social scenarios. Topics include the Price equation, Hamilton's rule, nonadditive interactions, conditional behaviors, heritability, and maximization of inclusive fitness. This chapter provides a brief historical introduction to the problem of apparent design in biology, evolutionary explanations of this, and in particular, evolutionary explanations of individual behaviors that appear designed to benefit not the individual themselves, but other members of their species. It examines how social behaviors can be shaped by natural selection and discusses the problem of providing an evolutionary explanation of self-sacrifice by individuals, altruism in group selection, and multilevel selection theory.

This chapter examines social behaviors that are expressed conditional on the phenotype of others. David Queller argued that inclusive fitness analyses need to be done on a per-behavior basis, citing as an example the decision over whether to reproduce directly, and whether to aid a reproductive. Queller showed that inclusive fitness predictions are only sensible when one analyzes what an individual should do, given it finds itself in a particular behavioral role. The chapter first provides an overview of implicit and explicit conditionality and presents two classic examples: William D. Hamilton's greenbeard traits and Robert Trivers's theory of reciprocal cooperation. It also introduces an extension of Hamilton's rule to deal with explicitly conditional behaviors; this extension features a measure of phenotypic assortment that appears not to be the classic genetic relatedness of Hamilton's rule.

This chapter attempts to demonstrate the plausibility, from an evolutionary standpoint, of the action-theoretic model presented so far. It begins with an overview of contemporary debates over the problem of altruism and the puzzle of human cooperation. It shows how evolutionary theorists have wound up focusing upon two features of human social behavior as the key to understanding the occurrence of large-scale cooperation amongst genetically unrelated individuals. The first is imitativeness with a conformist bias, the second is moralistic punishment. These two features, it is argued, correspond closely to the action-theoretic primitives being posited in the formal model of deontic constraint outlined in the previous chapters.

The basis of inclusive fitness theory is Hamilton's rule, which specifies the conditions for the spread of a gene for any of four types of social action, namely cooperation, altruism, selfishness, and spite. Hamilton's rule predicts that cooperation and selfishness can evolve at all levels of relatedness, whereas altruism requires positive relatedness and spite requires negative relatedness. When groups are non-clonal, within-group conflict arises because not all group members have identical inclusive fitness optima. A general form of within-group conflict involves the tragedy of the commons, whereby group members are selected to overexploit a public good for selfish gain. In support of inclusive fitness theory, phenotypic differences between social partners in nature arise from facultative gene expression of genes for social actions. Past and recent criticisms of inclusive fitness theory are flawed, and are mainly based on a lack of appreciation of its scope and flexibility.

This chapter looks briefly at evolutionary theory applied to human families, in particular inclusive fitness and parental investment theories, as well as some assumptions derived from evolutionary developmental psychology, including the concepts of ontogenetic, deferred, and conditional adaptations. It then examines factors that influence the investment decisions that mothers, fathers, grandparents, and stepparents make with regard to children. Lastly, it looks at issues related to living with siblings, including factors that influence both sibling conflict and cooperation and mechanism responsible for incest avoidance.

Hamilton introduced two conceptions of social fitness, which he termed neighbour-modulated fitness and inclusive fitness, and he argued that the two concepts are equivalent. This argument relies on two assumptions—actor’s control and additivity—that can be justified as approximations under ‘δ‎-weak selection’, which is selection on tiny differences between the mutant and the wild type. The assumption of δ‎-weak selection stems from a methodological stance that takes cumulative adaptation to be the explanatory target of social evolution research, together with an empirical commitment to a gradualist picture of how cumulative adaptation arises. In a process of gradual, cumulative adaptation, short-term change can be calculated using either fitness concept, but inclusive fitness has a special role to play as the criterion for improvement and the standard for optimality.

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.

Sibling relationships are unique. They are the longest lasting human social relationship exceeding, on average, the length of relationships with parents, mates, and children. The powerful underlying evolved psychological mechanisms activated in the contexts of sibling relationships are revealed in the attempts often made by parents to foster closeness between their children. Parents often attempt to groom young children for the arrival of a younger sibling. Put simply, there would be no need for such attempts if it were not for the evolved psychological mechanisms triggered in the minds of children that attempt to counteract the diversion of parental resources to siblings. This chapter shows that the most powerful guidance available to unmask this psychology is offered by evolutionary theories including inclusive fitness theory, parental investment theory, and parent-offspring conflict theory.

Inclusive fitness theory, originally due to W. D. Hamilton, is a popular approach to the study of social evolution, but shrouded in controversy. The theory contains two distinct aspects: Hamilton’s rule (rB > C); and the idea that individuals will behave as if trying to maximize their inclusive fitness in social encounters. These two aspects of the theory are logically separable but often run together. A generalized version of Hamilton’s rule can be formulated that is always true, though whether it is causally meaningful is debatable. However, the individual maximization claim only holds true if the payoffs from the social encounter are additive. The notion that inclusive fitness is the ‘goal’ of individuals’ social behaviour is less robust than some of its advocates acknowledge.

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.

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.

Ground squirrels don't always dive for safety at the first sign of a predator, but instead sometimes they stand stiffly and give a shrill alarm call that alerts other squirrels to the predator. Evolutionary biologists refer to behaviors like alarm calls that increase the fitness of the recipients but lower the fitness of the actor as altruism. The evolution of altruistic behavior had a strong role in defining sociobiology, and it continues to be one of its core problems. This chapter shows why natural selection can favor altruism when relatives interact. It introduces several new tools for building and solving analytical models, including George Price's covariance genetics framework. It also considers the prisoner's dilemma, positive assortment, common descent, and inclusive fitness. It reconstructs Hamilton's rule, using more-detailed genetic models of the evolution of altruism, and demonstrates why it can be used to understand the evolution of social behavior. It then derives Hamilton's rule using Price's method.

In his book Animal Dispersion in Relation to Social Behavior, ornithologist V. C. Wynne-Edwards argued that many enigmatic bird behaviors functioned to prevent over-population. The book generated a storm of controversy, and luminaries like George Williams and John Maynard Smith penned critiques explaining why this mechanism, then called “group selection,” could not work. The result was the beginning of an ongoing and highly successful revolution in our understanding of the evolution of animal behavior, a revolution that is rooted in carefully thinking about the individual and nepotistic functions of behavior. This chapter takes a general look at multilevel selection and shows that the Price equation also leads to a description of natural selection as going on in a series of nested levels: among genes within an individual, among individuals within groups, and among groups. It first discusses three views of selection according to personal fitness, inclusive fitness, and multilevel selection. It concludes by discussing the evolution of dispersal as an example of how multilevel selection can be used to clarify an important biological problem.