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Inclusive fitness: 50 years on



The cardinal problem of evolutionary biology is to explain adaptation, or the appearance of design in the living world [[1][1],[2][2]]. Darwin [[3][3]] convincingly argued that the process of adaptation is driven by natural selection: those heritable variations—i.e. genes—that are associated
Cite this article: Gardner A, West SA. 2014
Inclusive fitness: 50 years on. Phil.
Trans. R. Soc. B 369: 20130356.
One contribution of 14 to a Theme Issue
‘Inclusive fitness: 50 years on’.
Subject Areas:
behaviour, evolution, genetics, ecology,
theoretical biology, health and disease
and epidemiology
social evolution, adaptation, altruism,
kin selection, natural selection, theory
Author for correspondence:
Andy Gardner
Inclusive fitness: 50 years on
Andy Gardner
and Stuart A. West
School of Biology, University of St Andrews, Dyers Brae, St Andrews KY16 9TH, UK
Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK
1. Introduction
The cardinal problem of evolutionary biology is to explain adaptation, or the
appearance of design in the living world [1,2]. Darwin [3] convincingly
argued that the process of adaptation is driven by natural selection: those heri-
table variations—i.e. genes—that are associated with greater individual
reproductive success are those that will tend to accumulate in natural popu-
lations. To the extent that the individual’s genes are causally responsible for
her improved fitness, natural selection leads to the individual appearing
designed as if to maximize her fitness. Thus, Darwinism is a theory of both
the process and the purpose of adaptation.
However, correlations between an individual’s genes and her fitness need
not reflect a direct, causal relationship. For example, genes for altruism can
be associated with greater fitness, despite the direct cost that they inflict on
their bearer, if relatives interact as social partners. This is because an individual
who carries genes for altruism will tend to have more altruistic social partners.
That altruism can be favoured by natural selection suggests that the purpose of
adaptation is not, in general, to maximize the individual’s personal fitness [4].
Although Darwin [3] recognized the potential for such indirect effects to
drive the evolution of social behaviours, discussing the logic of kin selection
theory in connection with the adaptations of sterile insect workers, it was
William D. Hamilton (figure 1), more than a century later, who developed
these insights into a full mathematical theory. By quantifying the relative
strengths of direct selection, acting via the individual’s own reproduction,
and indirect selection, acting via the reproduction of the individual’s relatives,
Hamilton [4] revealed the ultimate criterion that natural selection uses to judge
the fate of genes.
Hamilton’s rule states that any trait—altruistic or otherwise—will be
favoured by natural selection if and only if the sum of its direct and indirect fit-
ness effects exceeds zero [47]. That is c þ
. 0, where c is the impact
that the trait has on the individual’s own reproductive success, b
is its impact
on the reproductive success of the individual’s ith social partner and r
is the
genetic relatedness of the two individuals. This mathematical partition of fit-
ness effects underpins the kin selection approach to evolutionary biology [8].
The general principle is that with regards to social behaviours, natural selection
is mediated by any positive or negative consequences for recipients, according
to their genetic relatedness to the actor. Consequently, individuals should show
greater selfish restraint, and can even behave altruistically, when interacting
with closer relatives [4].
Ha ving clarified the process of social adapta t ion, Hamilton [4] revealed its true
purpose: to maximize inclusive fitness (figure 2). That is, Darwinian individuals
should strive to maximize the sum of the fitness effects that the y hav e on all
their r ela tiv es (including themselves), each increment or decrement being weighte d
by their genetic relatedness. This is the most fundamental revision that has been
made to the logic of Darwinism and—aside fr om a possibly apocryphal quip
attributed to J. B. S. Haldane, to the effect that he would giv e his life to sav e the
lives of two brothers or eight cousin s—it was wholly original to Hamilton.
Since its inception 50 years ago, inclusive fitness theory has grown to
become one of the most successful approaches in evolutionary biology. In
addition to igniting an explosive interest in altruistic behaviour, it also ener-
gized the investigation of many other social traits (table 1). In all its
2014 The Author(s) Published by the Royal Society. All rights reserved.
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applications, the usefulness of inclusive fitness theory, and
its encapsulation in Hamilton’s rule, lies in how it provides
a simple conceptual framework that can be applied with rela-
tive ease to a wide range of scenarios, successfully translating
between the dynamical process of natural selection and the
design objective of Darwinian adaptation, on paper, in the
laboratory and in the field [911].
In addition to its traditional focus upon individual
organisms, inclusive fitness theory has been applied equally
successfully to explain social interactions between genes,
illuminating the evolution of selfish genetic elements
and genomic imprinting [12,13]. Indeed, by translating
between—and characterizing conflicts of interest within—
different levels of biological organization, inclusive fitness
theory provides a framework for understanding major
transitions in individuality (table 2; [14]).
Clearly, inclusive fitness is not a single hypothesis, but
rather represents an entire programme of research. Scientific
hypotheses are judged according to how amenable they are
for empirical testing and how well they resist attempts at
empirical falsification. By contrast, scientific research pro-
grammes are judged according to how well they facilitate
the formulation and testing of hypotheses—that is, stimulat-
ing the interplay between theory and empiricism that drives
progress in scientific understanding. For example, inclusive
fitness theory has yielded a number of hypotheses concerning
the factors driving the evolution of insect eusociality, includ-
ing the ‘haplodiploidy hypothesis’ [4,18] and the ‘monogamy
hypothesis’ [ 1921]. The former hypothesis has not with-
stood detailed theoretical and empirical scrutiny, whereas
the latter goes from strength to strength [1925]. This is
exactly what we expect of a productive research programme.
In order to better assess the health of inclusive fitness
theory on its 50th anniversary, here we showcase research
showing the research programme in action, from the ext-
remely pure, mathematical realm, through basic empirical
science, to bold applications in a variety of disciplines.
The first three papers of this theme issue explore the
connections between inclusive fitness and the classical foun-
dations of evolutionary theory, with Laurent Lehmann and
Franc¸ois Rousset focusing upon population genetics, Allen
Moore and co-workers focusing upon quantitative genetics,
and David Queller revisiting the central mathematical result
of Darwinian theory—Fisher’s fundamental theorem of natu-
ral selection—from a social evolutionary perspective. These
contributions are followed by an exploration of alternative
mathematical approaches to inclusive fitness, with Peter
Taylor and Wes Maciejewski considering social evolution in
structured populations from a graph-theoretic angle and
Hisashi Ohtsuki developing connections with game theory.
Moving on to specific biological questions, Geoff Wild
and Cody Koykka explore the evolution of cooperative breed-
ing from a theoretical perspective, whereas Andrew Bourke
and Ben Hatchwell and co-workers take the stock of the
empirical successes of inclusive fitness theory on this front,
illustrating comparative and focused field approaches,
respectively. Descending to the level of the gene, Ben Nor-
mark and Laura Ross investigate the role for inclusive
fitness conflicts to drive the evolution of genetic systems.
This basic research is then followed by more applied uses
of inclusive fitness theory. Helen Leggett and co-workers
explore the insights that inclusive fitness theory yields for
infectious disease, and Bernard Crespi and co-workers
broaden out this exploration to consider non-infectious dis-
ease and ‘Hamiltonian medicine’, in general. We close the
theme issue with Toby Kiers and Ford Denison’s exploration
of applications of inclusive fitness theory to agriculture,
and Thom Scott-Phillips and co-workers on its application
to understanding human culture.
These contributions confirm that inclusive fitness theory
is in excellent shape. It still dominates the study of social
interactions in behavioural ecology, and continues to break
new ground in other disciplines. This is a testament not
only to the generality and flexibility of the theory, but also
to the efforts of its practitioners, including both theoreti-
cians who maintain a firm grasp on the natural world and
empiricists who keep a close eye on the latest theoretical
developments. Hamilton was one of those rare individuals
Figure 1. William D. Hamilton (19362000). Copyright: Tokyo Zoological
Park Society.
Figure 2. Inclusive fitness comprises the effects that the actor has on her
own reproductive success and the reproductive success of her relatives
(solid arrows), but not the effects that her relatives have on her reproductive
success or on their own reproductive success (dashed arrows). If an action
incurs a direct fitness cost of c to the actor’s own fitness, and provides an
indirect fitness benefit of b to her social partner, then natural selection
favours that action if rb 2 c . 0, where r is the genetic relatedness of
the two individuals [4]. Phil. Trans. R. Soc. B 369: 20130356
on July 10, 2014rstb.royalsocietypublishing.orgDownloaded from
who effortlessly combined theory with forays into the field,
but most of the rest of us who specialize one way or the
other need to communicate and collaborate to achieve
the requisite interplay of theory and empiricism. Science is
a social enterprise, so it may be unsurprising that inclusive
fitness theory epitomizes the successful scientific research
programme. The next 50 years promise to be very exciting.
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Table 2. The major transitions in individuality, according to Bourke [14].
major transition in
individuality details
prokaryotic cell separate replicators (genes) ! cell
enclosing genome
eukaryotic cell separate unicells ! symbiotic unicell
sexual reproduction asexual unicells ! sexual unicell
multicellularity unicells ! multicellular organism
eusociality multicellular organisms ! eusocial
interspecific mutualism separate species ! interspecific
Table 1. Some example areas where inclusive fitness theory has facilitated
insights and understanding. Inclusive fitness theory is not the only way to
model evolution, but it has proved to be an immensely productive and
useful approach for studying social behaviours [917].
research areas
adoption, alarm calls, altruism, cannibalism, conflict resolution,
cooperation, dispersal, division of labour, eusociality, kin
discrimination, genomic imprinting, multicellularity, mutualism,
parasite virulence, parentoffspring conflict, policing, selfish genetic
elements, sex allocation, sibling conflict, spite, suicide and
symbiosis. Phil. Trans. R. Soc. B 369: 20130356
on July 10, 2014rstb.royalsocietypublishing.orgDownloaded from
... Although the theory of inclusive fitness offers a possible interpretation of how the germ/soma separation can evolve, we think that this interpretation is more complex than this theory suggests, as many more mechanisms, not necessarily altruistic, may underlie its evolution (Okasha 2006;Nowak et al. 2010;Durand 2020). This is because the concept of biological altruism has a very specific meaning (Gardner and West 2014), and not every biological mechanism underlying the evolution of the germ/soma separation is of this type. To say that a given behavior is altruistic, it must be performed by the organism as a means of reducing its direct fitness and substantially enhancing its indirect fitness. ...
... Rather, we wish to show that the observation of some costly behaviors that diminish the reproductive capabilities of their bearers, while simultaneously leading to an increase in the number of these very genes in the population, does not necessarily mean that this behavior is caused by altruism driven by kin selection. For this behavior to be caused by altruism, the actor must have been in control of it (Gardner and West 2014). Our article shows that, in many situations, somatic cells are not in control; on the contrary, they are coerced to behave in this costly way, and thus even if an increase in their number of genes in the population follows, this is only accidental. ...
Full-text available
An understanding of the factors behind the evolution of multicellularity is one of today's frontiers in evolutionary biology. This is because multicellular organisms are made of one subset of cells with the capacity to transmit genes to the next generation (germline cells) and another subset responsible for maintaining the functionality of the organism, but incapable of transmitting genes to the next generation (somatic cells). The question arises: why do somatic cells sacrifice their lives for the sake of germline cells? How is germ/soma separation maintained? One conventional answer refers to inclusive fitness theory, according to which somatic cells sacrifice themselves altruistically, because in so doing they enhance the transmission of their genes by virtue of their genetic relatedness to germline cells. In the present article we will argue that this explanation ignores the key role of policing mechanisms in maintaining the germ/soma divide. Based on the pervasiveness of the latter, we argue that the role of altruistic mechanisms in the evolution of multicellularity is limited and that our understanding of this evolution must be enriched through the consideration of coercion mechanisms.
... Reduced relatedness can be problematic for groups because it can shift the balance of selection away from favoring individuals acting for the good of the group toward maximizing their selfish interests. However, under all relatedness conditions, selection should favor individuals who are able to contribute to public goods at the level that maximizes their inclusive fitness (given their relatedness), which can be accomplished by strategically adjusting contributions depending on their relatedness to the group (2,(14)(15)(16)(17)(18). The logic of such strategic cooperation is captured in the "Collective Investment" game (17,18) in which the "players" are different genotypes interacting in groups and each player decides what fraction of their resources to invest into production of public goods. ...
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Organisms often cooperate through the production of freely available public goods. This can greatly benefit the group but is vulnerable to the “tragedy of the commons” if individuals lack the motivation to make the necessary investment into public goods production. Relatedness to groupmates can motivate individual investment because group success ultimately benefits their genes’ own self-interests. However, systems often lack mechanisms that can reliably ensure that relatedness is high enough to promote cooperation. Consequently, groups face a persistent threat from the tragedy unless they have a mechanism to enforce investment when relatedness fails to provide adequate motivation. To understand the real threat posed by the tragedy and whether groups can avert its impact, we determine how the social amoeba Dictyostelium discoideum responds as relatedness decreases to levels that should induce the tragedy. We find that, while investment in public goods declines as overall within-group relatedness declines, groups avert the expected catastrophic collapse of the commons by continuing to invest, even when relatedness should be too low to incentivize any contribution. We show that this is due to a developmental buffering system that generates enforcement because insufficient cooperation perturbs the balance of a negative feedback system controlling multicellular development. This developmental constraint enforces investment under the conditions expected to be most tragic, allowing groups to avert a collapse in cooperation. These results help explain how mechanisms that suppress selfishness and enforce cooperation can arise inadvertently as a by-product of constraints imposed by selection on different traits.
... Curry's (2016) interdisciplinary research uses psychological theories-kin altruism, mutualism, reciprocal altruism, and competitive altruism-to explain different types of kindness. Kin altruism means that people will be kind to their families (Gardner & West, 2014). This type of kindness can take the form of love, care, sympathy, and compassion. ...
... Multi-level selection proposes that trade-offs between benefits and costs to the lower-level units can be scaled up to determine fitness at the collective level (Michod, 2007). Similarly to the sociobiological approach, that is based on translating individual-level costs and benefits into inclusive fitness as a property of a whole population (Gardner and West, 2014;B Kerr and Godfrey-Smith, 2009), the statistical description of the outcome of interactions does not inform on the processes underlying population-level success. Though these approaches have the great advantage of permitting elegant generalizations and exploitation of tools developed for population genetics, the existence and magnitude of genetically-determined, individual fitness costs and benefits are not easy to assess without elucidating the mechanisms underlying population-level statistics. ...
Full-text available
The social amoeba Dictyostelium discoideum, where aggregation of genetically heterogeneous cells produces functional collective structures, epitomizes social conflicts associated with multicellular organization. 'Cheater' populations that have a higher chance - quantified by a positive spore bias - of surviving to the next generation when mixed with cooperators bear a selective advantage. Their spread is thus expected to undermine collective functions over evolutionary times. In this review, we discuss the two main approaches adopted to conceptualize social conflicts in Dictyostelium discoideum: describing social interactions as a property of cell populations (strains), or as a result of individual cell choices during the developmental process. These two points of view are often held equivalent and used interchangeably. While the population-level view grants more direct evolutionary inference, however, the cell-level interpretation reveals that such evolutionary predictions may be modified if mechanisms such as dependence on the environment, development and intrinsic unpredictability of cell fate choices are taken into account. We conclude by proposing a set of open questions that in our opinion lie at the core of a multi-scale description of aggregative life cycles, where the formulation of predictive evolutionary models would include cell-level mechanisms responsible for spore bias alongside population-level descriptors of multicellular organization.
... Natural selection will favour genes that incur a cost to provide a greater benefit to copies of themselves in other individuals (Hamilton 1963;Dawkins 1979). This theory of 'kin selection' predicts that, under some conditions, organisms will possess adaptations for detecting and delivering benefits (or avoiding doing harm) to kin, and thus explains many instances of altruism in many species (Gardner and West 2014), including humans (Kurland and Gaulin 2005;Lieberman et al. 2007). Humans detect kin (Lieberman et al. 2003(Lieberman et al. , 2007, invest in offspring (Smith et al. 1987;Geary 2000), and live in family structures in all societies (Chapais 2014). ...
Full-text available
What is morality? How many moral values are there? And what are they? According to the theory of morality-as-cooperation, morality is a collection of biological and cultural solutions to the problems of cooperation recurrent in human social life. This theory predicts that there will be as many different types of morality as there are different types of cooperation. Previous research, drawing on evolutionary game theory, has identified at least seven different types of cooperation, and used them to explain seven different types of morality: family values, group loyalty, reciprocity, heroism, deference, fairness and property rights. Here we explore the conjecture that these simple moral ‘elements’ combine to form a much larger number of more complex moral ‘molecules’, and that as such morality is a combinatorial system. For each combination of two elements, we hypothesise a candidate moral molecule, and successfully locate an example of it in the professional and popular literature. These molecules include: fraternity, blood revenge, family pride, filial piety, gavelkind, primogeniture, friendship, patriotism, tribute, diplomacy, common ownership, honour, confession, turn taking, restitution, modesty, mercy, munificence, arbitration, mendicancy, and queuing. These findings indicate that morality – like many other physical, biological, psychological and cultural systems – is indeed a combinatorial system. Thus morality-as-cooperation provides a principled and powerful theory, that explains why there are many moral values, and successfully predicts what they will be; and it generates a systematic framework that has the potential to explain all moral ideas, possible and actual. Pursuing the many implications of this theory will help to place the study of morality on a more secure scientific footing.
... In fact, when asked to identify principles that have led to the formulation of empirical consequences in SET, theoretical biologists tend to mention one of its explanatory principles-most often kin selection. For instance, according to Gardner & West (2014): ...
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Social evolution theory provides a wide array of successful evolutionary explanations for cooperative traits. However and surprisingly, a number of cases of unexplained cooperative behaviour remain. Shouldn't they cast doubt on the relevance of the theory, or even disconfirm it? This depends on whether the theory is akin to a research programme such as adaptationism, or closer to a theory - a set of compatible, (dis)confirmable hypotheses. In order to find out, we focus on the two main tenets of social evolution theory, namely reciprocity explanations and kin selection. Reciprocity-based explanations are extremely hard to (dis)confirm. This is due, first to the multiple realisability of explanatory processes, factors and strategies, despite apparent reasons to the contrary; second, to the high quantity, and limited availability of data needed to eliminate or back up such explanations. One of our target cases vividly illustrates these limitations. Moreover, kin selection, while relatively easy to disconfirm in particular cases, seems to enjoy a more limited explanatory scope than previously thought. Overall, social evolution theory turns out to be neither a research programme nor a theory, but a heterogeneous scientific entity, composed of parts that are amenable to (dis)confirmation and others barely so.
... Inclusive fitness accounts for both an individual's own reproductive success and that of relatives who share identical-by-descent alleles. It provides key evolutionary insights (Gardner and West, 2014), perhaps most iconically explaining self-sacrificial behaviour by appealing to the increased reproductive success of related beneficiaries (Hamilton, 1964a), and identifying causes of conflict between parents and offspring over parental investment (hereafter 'PI'; Trivers, 1972Trivers, , 1974. ...
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Inbreeding increases parent-offspring relatedness and commonly reduces offspring viability, shaping selection on reproductive interactions involving relatives and associated parental investment (PI). Nevertheless, theories predicting selection for inbreeding versus inbreeding avoidance and selection for optimal PI have only been considered separately, precluding prediction of optimal PI and associated reproductive strategy given inbreeding. We unify inbreeding and PI theory, demonstrating that optimal PI increases when a female's inbreeding decreases the viability of her offspring. Inbreeding females should therefore produce fewer offspring due to the fundamental trade-off between offspring number and PI. Accordingly, selection for inbreeding versus inbreeding avoidance changes when females can adjust PI with the degree that they inbreed. In contrast, optimal PI does not depend on whether a focal female is herself inbred. However, inbreeding causes optimal PI to increase given strict monogamy and associated biparental investment compared to female-only investment. Our model implies that understanding evolutionary dynamics of inbreeding strategy, inbreeding depression, and PI requires joint consideration of the expression of each in relation to the other. Overall, we demonstrate that existing PI and inbreeding theories represent special cases of a more general theory, implying that intrinsic links between inbreeding and PI affect evolution of behaviour and intra-familial conflict.
... The multi-level selection approach proposes that trade-offs between benefits and costs to the lower-level units can be scaled up to determine fitness at the collective level (Michod, 2007). Similarly to the sociobiological approach, that is also based on translating individual-level costs and benefits into inclusive fitness as a property of a whole population (Gardner & West, 2014;B. Kerr & Godfrey-Smith, 2009), the statistical description of the outcome of interactions does not inform on the underlying processes. ...
Full-text available
The ’social amoeba’ Dictyostelium discoideum, where aggregation of genet- ically heterogeneous cells produces functional collective structures, epitomizes social conflicts associated with multicellular organization. ’Cheater’ populations that have a higher chance – quantified by a positive spore bias – of surviving to the next generation are selectively advantaged. Their spread is thus expected to undermine collective functions over evolutionary times. In this review, we discuss the two main approaches adopted to conceptualize social conflicts in Dictyostelium discoideum: describing spore bias as a property of cell popula- tions (strains), or as a result of individual cell choices during the developmental process. These two points of view are often held equivalent and used inter- changeably. While the population-level view allows for more direct evolutionary inference, however, the cell-level interpretation reveals that such evolutionary predictions may be modified if developmental mechanisms, such as dependence on the environment and intrinsic unpredictability of cell fate choices, are taken into account. We conclude by proposing a set of open questions that in our opinion lie at the core of a multi-scale description of aggregative life cycles, where the formulation of predictive evolutionary models would include cell-level mechanisms responsible for spore bias alongside population-level descriptors of multicellular organization.
The explanatory power of Hamilton's rule, the main explanatory principle of social evolution theory, is an ongoing subject of controversy. In this paper, we reinforce the case for the considerable value of the regression-based version of the rule in explaining the evolution of social traits. Although we agree that the rule can have an organizing role in social evolution research, we maintain that it does not explain in virtue of citing causes or providing an organizing framework. Instead, we argue it either provides an explanation by constraint or a non-causal counterfactual explanation.
The evolution of cooperation in Prisoner's Dilemmas with additive random cost and benefit for cooperation cannot be accounted for by Hamilton's rule based on mean effects transferred from recipients to donors weighted by coefficients of relatedness, which defines inclusive fitness in a constant environment. Extensions that involve higher moments of stochastic effects are possible, however, and these are connected to a concept of random inclusive fitness that is frequency-dependent. This is shown in the setting of pairwise interactions in a haploid population with the same coefficient of relatedness between interacting players. In an infinite population, fixation of cooperation is stochastically stable if a mean geometric inclusive fitness of defection when rare is negative, while fixation of defection is stochastically unstable if a mean geometric inclusive fitness of cooperation when rare is positive, and these conditions are generally not equivalent. In a finite population, the probability for cooperation to ultimately fix when represented once exceeds the probability under neutrality or the corresponding probability for defection if the mean inclusive fitness of cooperation when its frequency is 1/3 or 1/2 exceeds 1. All these results rely on the simplifying assumption of a linear fitness function. It is argued that meaningful applications of random inclusive fitness in complex settings (multi-player game, diploidy, population structure) would generally require conditions of weak selection and additive gene action.
Full-text available
Abstract Hamilton's haplodiploidy hypothesis suggests that the relatively higher relatedness of full sisters in haplodiploid populations promotes altruistic sib rearing and, consequently, the evolution of eusociality. This haplodiploidy effect works when some broods have a relatively female-biased sex ratio and other broods have a relatively male-biased sex ratio, termed split sex ratios. There is empirical evidence for two scenarios having potentially led to split sex ratios en route to eusociality: unmated queens and queen replacement. A recent analysis of these two scenarios has suggested that haplodiploidy can either promote or inhibit the evolution of eusociality and that the effect is usually small. However, this work made the simplifying assumptions that there is only negligible reproduction by workers and that their offspring have the same sex ratio as those produced by the queen. Here, we relax these assumptions and find that worker reproduction has a negative influence on the evolution of helping, either reducing the extent to which it is promoted or leading to it being inhibited. This is particularly so when workers are unmated and hence constrained to produce only sons, by arrhenotoky. Overall, when parameterized with empirical data, our results suggest that split sex ratios in haplodiploid species have not played an important role in facilitating the evolution of eusociality.
Recent decades have witnessed an explosion of theoretical and empirical studies of sex allocation, transforming how we understand the allocation of resources to male and female reproduction in vertebrates, invertebrates, protozoa, and plants. In this landmark book, Stuart West synthesizes the vast literature on sex allocation, providing the conceptual framework the field has been lacking and demonstrating how sex-allocation studies can shed light on broader questions in evolutionary and behavioral biology. West clarifies fundamental misconceptions in the application of theory to empirical data. He examines the field's successes and failures, and describes the research areas where much important work is yet to be done. West reveals how a shared underlying theoretical framework unites findings of sex-ratio variation across a huge range of life forms, from malarial parasites and hermaphroditic worms to sex-changing fish and mammals. He shows how research on sex allocation has been central to many critical questions and controversies in evolutionary and behavioral biology, and he argues that sex-allocation research serves as a key testing ground for different theoretical approaches and can help resolve debates about social evolution, parent-offspring conflict, genomic conflict, and levels of selection. Certain to become the defining book on the subject for the next generation of researchers, Sex Allocation explains why the study of sex allocation provides an ideal model system for advancing our understanding of the constraints on adaptation among all living things in the natural world.