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Concept: Inclusive fitness


Why females of some species cease ovulation prior to the end of their natural lifespan is a long-standing evolutionary puzzle [1-4]. The fitness benefits of post-reproductive helping could in principle select for menopause [1, 2, 5], but the magnitude of these benefits appears insufficient to explain the timing of menopause [6-8]. Recent theory suggests that the cost of inter-generational reproductive conflict between younger and older females of the same social unit is a critical missing term in classical inclusive fitness calculations (the “reproductive conflict hypothesis” [6, 9]). Using a unique long-term dataset on wild resident killer whales, where females can live decades after their final parturition, we provide the first test of this hypothesis in a non-human animal. First, we confirm previous theoretical predictions that local relatedness increases with female age up to the end of reproduction. Second, we construct a new evolutionary model and show that given these kinship dynamics, selection will favor younger females that invest more in competition, and thus have greater reproductive success, than older females (their mothers) when breeding at the same time. Third, we test this prediction using 43 years of individual-based demographic data in resident killer whales and show that when mothers and daughters co-breed, the mortality hazard of calves from older-generation females is 1.7 times that of calves from younger-generation females. Intergenerational conflict combined with the known benefits conveyed to kin by post-reproductive females can¬†explain why killer whales have evolved the longest post-reproductive lifespan of all non-human animals.

Concepts: Scientific method, Human, Natural selection, Male, Reproduction, Evolution, Kin selection, Inclusive fitness


Despite its short-term costs, behaviour that appears altruistic can increase an individual’s inclusive fitness by earning direct (selfish) and/or indirect (kin-selected) benefits. An evolved preference for other-regarding or helping behaviour in potential mates has been proposed as an additional mechanism by which these behaviours can yield direct fitness benefits in humans.

Concepts: Evolutionary biology, Motivation, Kin selection, Evolutionary psychology, The Selfish Gene, Altruism, Inclusive fitness, Richard Dawkins


The evolution of sterile worker castes in eusocial insects was a major problem in evolutionary theory until Hamilton developed a method called inclusive fitness. He used it to show that sterile castes could evolve via kin selection, in which a gene for altruistic sterility is favored when the altruism sufficiently benefits relatives carrying the gene. Inclusive fitness theory is well supported empirically and has been applied to many other areas, but a recent paper argued that the general method of inclusive fitness was wrong and advocated an alternative population genetic method. The claim of these authors was bolstered by a new model of the evolution of eusociality with novel conclusions that appeared to overturn some major results from inclusive fitness. Here we report an expanded examination of this kind of model for the evolution of eusociality and show that all three of its apparently novel conclusions are essentially false. Contrary to their claims, genetic relatedness is important and causal, workers are agents that can evolve to be in conflict with the queen, and eusociality is not so difficult to evolve. The misleading conclusions all resulted not from incorrect math but from overgeneralizing from narrow assumptions or parameter values. For example, all of their models implicitly assumed high relatedness, but modifying the model to allow lower relatedness shows that relatedness is essential and causal in the evolution of eusociality. Their modeling strategy, properly applied, actually confirms major insights of inclusive fitness studies of kin selection. This broad agreement of different models shows that social evolution theory, rather than being in turmoil, is supported by multiple theoretical approaches. It also suggests that extensive prior work using inclusive fitness, from microbial interactions to human evolution, should be considered robust unless shown otherwise.

Concepts: Natural selection, Evolution, Evolutionary biology, Population genetics, Kin selection, The Selfish Gene, Altruism, Inclusive fitness


The length of stay in inpatient and outpatient rehabilitation after an injury or illness has declined in recent years, exposing those with newly acquired neurologic disability to a risk of significant postrehabilitation health decline. Following a short stay in outpatient rehabilitation, individuals with neurologic disability have few, if any, options to continue their physical recovery after discharge, thus further increasing their risk for functional decline and secondary conditions. Professionals who work in community-based fitness facilities have the potential to assist therapists in extending the recovery process and preventing this decline. The focus of this article was to address a conceptual framework for better understanding how rehabilitation and health/fitness professionals can work together to help with this growing need. To that end, the antecedents to and effects of postrehabilitation health decline are discussed, followed by the introduction of a theoretical model illustrating a therapist-to-trainer system that facilitates the use of community-based fitness facilities by individuals with neurologic disabilities to continue their recovery postrehabilitation. Finally, a thorough description of an exemplary existing community-based inclusive fitness program is presented, followed by examples of select disability groups using these programs for continued recovery.Video Abstract available (see Video, Supplemental Digital Content 1, for more insights from the authors.

Concepts: Mental disorder, Disability, Concept, Decline, Recovery model, Kin selection, Inclusive fitness, Reproductive success


Kin selection, which can lead organisms to behave altruistically to their genetic relatives, works differently when-as is often the case in human societies-altruism can be boosted by social pressure. Here I present a model of social norms enforced by indirect reciprocity. In the model there are many alternative stable allocations of rewards (“distributional norms”); a stable norm is stable in the sense that each player is best off following the norm if other players do the same. Stable norms vary widely in how equally they reward players with unequal abilities. In a population of mixed groups (some group members follow one norm, some follow another, and some compromise) with modest within-group coefficients of relatedness, selection within groups favors those who compromise, and selection between groups favors generous generalized reciprocity rather than balanced reciprocity. Thus evolved social norms can amplify kin altruism, giving rise to a uniquely human mode of kin-based sociality distinct from spontaneous altruism among close kin, or cooperation among non-kin.

Concepts: Natural selection, Evolution, Sociology, Society, Kin selection, Altruism, Inclusive fitness, Social evolution


In this Formal Comment, the authors challenge the claims of a recent theoretical study that genetic relatedness is important in the evolution of eusociality.

Concepts: Evolution, Evolutionary biology, Population genetics, Kin selection, Altruism, Inclusive fitness, Coefficient of relationship, Reproductive success


The authors of “Relatedness, Conflict, and the Evolution of Eusociality” respond to objections raised by Martin Nowak and Benjamin Allen.

Concepts: Natural selection, Evolution, Evolutionary biology, Population genetics, Kin selection, Inclusive fitness, Eusociality, W. D. Hamilton


The evolution of altruism-costly self-sacrifice in the service of others-has puzzled biologists since The Origin of Species. For half a century, attempts to understand altruism have developed around the concept that altruists may help relatives to have extra offspring in order to spread shared genes. This theory-known as inclusive fitness-is founded on a simple inequality termed Hamilton’s rule. However, explanations of altruism have typically not considered the stochasticity of natural environments, which will not necessarily favour genotypes that produce the greatest average reproductive success. Moreover, empirical data across many taxa reveal associations between altruism and environmental stochasticity, a pattern not predicted by standard interpretations of Hamilton’s rule. Here we derive Hamilton’s rule with explicit stochasticity, leading to new predictions about the evolution of altruism. We show that altruists can increase the long-term success of their genotype by reducing the temporal variability in the number of offspring produced by their relatives. Consequently, costly altruism can evolve even if it has a net negative effect on the average reproductive success of related recipients. The selective pressure on volatility-suppressing altruism is proportional to the coefficient of variation in population fitness, and is therefore diminished by its own success. Our results formalize the hitherto elusive link between bet-hedging and altruism, and reveal missing fitness effects in the evolution of animal societies.

Concepts: Scientific method, Gene, Natural selection, Evolution, Biology, Evolutionary biology, Kin selection, Inclusive fitness


For honey bee and other social insect colonies the ‘queen substance’ regulates colony reproduction rendering workers functionally sterile. The evolution of worker reproductive altruism is explained by inclusive fitness theory, but little is known of the genes involved or how they regulate the phenotypic expression of altruism. We previously showed that application of honeybee queen pheromone to virgin fruit flies suppresses fecundity. Here we exploit this finding to identify genes associated with the perception of an ovary-inhibiting social pheromone. Mutational and RNAi approaches in Drosophila reveal that the olfactory co-factor Orco together with receptors Or49b, Or56a and Or98a are potentially involved in the perception of queen pheromone and the suppression of fecundity. One of these, Or98a, is known to mediate female fly mating behaviour, and its predicted ligand is structurally similar to a methyl component of the queen pheromone. Our novel approach to finding genes associated with pheromone-induced sterility implies conserved reproductive regulation between social and pre-social orders, and further helps to identify candidate orthologues from the pheromone-responsive pathway that may regulate honeybee worker sterility.

Concepts: Evolution, Insect, Honey bee, Beekeeping, Kin selection, Altruism, Inclusive fitness, Queen bee


In four experiments, we found that the presence of self-interest in the charitable domain was seen as tainting: People evaluated efforts that realized both charitable and personal benefits as worse than analogous behaviors that produced no charitable benefit. This tainted-altruism effect was observed in a variety of contexts and extended to both moral evaluations of other agents and participants' own behavioral intentions (e.g., reported willingness to hire someone or purchase a company’s products). This effect did not seem to be driven by expectations that profits would be realized at the direct cost of charitable benefits, or the explicit use of charity as a means to an end. Rather, we found that it was related to the accessibility of different counterfactuals: When someone was charitable for self-interested reasons, people considered his or her behavior in the absence of self-interest, ultimately concluding that the person did not behave as altruistically as he or she could have. However, when someone was only selfish, people did not spontaneously consider whether the person could have been more altruistic.

Concepts: Psychology, Behavior, Human behavior, Morality, Kin selection, Altruism, Inclusive fitness, Selfishness