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'Unit ofselection' is a polysemic expression, meaning interactor, replicator/reproducer or manifestor of adaptation/type-1 agent in today's biological research. This Element presents each of these concepts and demonstrates the necessity of their isolation.

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The controversy regarding theunit ofselection is fundamentally a dispute about what is the correct causal structure of the process of evolution by naturalselection and its ontological commitments. By characterizing the process as consisting of two essential steps--interaction and transmission--a singular answer to theunit question becomes ambiguous. With such an account on hand, two recent defenses of competing units ofselection are considered. Richard Dawkins maintains that the gene is the appropriate (...) class='Hi'>unit ofselection and Robert Brandon, in response, argues that the individual organism is better suited to the role. This paper argues that by making explicit the underlying questions that each of these views addresses, the apparent conflict can be resolved. Furthermore, such a resolution allows for a more complete and realistic understanding of the process of evolution by naturalselection. (shrink)

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The reductionistic vision of evolutionary theory, "the gene's eye view of evolution" is the dominant view among evolutionary biologists today. On this view, the gene is the onlyunit with sufficient stability to act as aunit ofselection, with individuals and groups being more ephemeral units of function, but not ofselection. This view is argued to be incorrect, on several grounds. The empirical and theoretical bases for the existence of higher-level units ofselection (...) are explored, and alternative analyses discussed critically. The success of a multi-levelselection theory demands the recognition and development of a multi-level genetics. The way to accomplish this is suggested. The genotype/phenotype distinction also requires further analysis to see how it applies at higher levels of organization. This analysis provides a way of defining genotype and phenotype for cultural evolution, and a treatment of the innate-acquired distinction which are both generalizeable to analyze problems of the nature and focus of scientific change. (shrink)

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A correct analysis of hierarchicalselection processes must specify 1) the objects that succeed differentially as units, and 2) the properties that provide the causal bases for differential success. Here I illustrate how failing to recognize the units/bases distinction creates a contradiction in Elliott Sober's recent account ofselection. A revised criterion for units ofselection is developed and applied to examples at several biological levels. Criteria for bases ofselection are discussed in terms of the (...) degree of context-dependence and directness of a property's effect on the success of units. The significance of previous work by Sober, Wimsatt and Brandon is thereby clarified. (shrink)

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The aim of this article is to develop an approach to theunit ofselection concept that fits the theory of evolution by naturalselection without reproduction. I review the history of the concept to show that predominant approaches, which have recently been categorized by Suárez and Lloyd using the labels “unitary project” and “disambiguating project,” cannot be used in the context of evolution by naturalselection without reproduction. I therefore propose a new and more inclusive (...) understanding of theunit ofselection that upholds the unitary project’s central claim that the concept has a unitary extension. I define theunit ofselection as an entity whose interactions with the environment influence the ratio of units with similar traits in the population. That is, aunit ofselection is a fitness bearer, where fitness is defined in terms that correspond to its operationalization in population genetics (without reference to heritability). This revamped unitary concept covers cases in which fitness can be reduced to reproductive output, but also cases where differential persistence or reconstitution are more important than reproduction. (shrink)

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Holobionts, consisting of a host and diverse microbial symbionts, function as distinct biological entities anatomically, metabolically, immunologically, and developmentally. Symbionts can be transmitted from parent to offspring by a variety of vertical and horizontal methods. Holobionts can be considered levels ofselection in evolution because they are well-defined interactors, replicators/reproducers, and manifestors of adaptation. An initial mathematical model is presented to help understand how holobionts evolve. The model offered combines the processes of horizontal symbiont transfer, within-host symbiont proliferation, vertical (...) symbiont transmission, and holobiontselection. The model offers equations for the population dynamics and evolution of holobionts whose hologenomes differ in gene copy number, not in allelic or loci identity. The model may readily be extended to include variation among holobionts in the gene identities of both symbionts and host. (shrink)

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Richard Lewontin's (1970) early work on the units ofselection initiated the conceptual and theoretical investigations that have led to the hierarchical perspective onselection that has reached near consensus status today. This paper explores other aspects of his work, work on what he termed continuity and quasi-independence, that connect to contemporary explorations of modularity in development and evolution. I characterize such modules and argue that they are the true units ofselection in that they are what (...) evolution by naturalselection individuates, selects among, and transforms. (shrink)

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Theunit ofselection is the concept of that ‘something’ to which biologists refer when they speak of an adaptation as being ‘for the good of’ something. Darwin identified the organism as theunit ofselection because for him the ‘struggle for existence’ was an issue among individuals. Later on it was suggested that, in order to understand the evolution of social behavior, it is necessary to argue that groups, and not individuals, are the units of (...)selection. The last addition to this debate was the formulation by Dawkins, in 1976, that the genes themselves are the units ofselection while organisms are merely the temporary receptacles and vehicles for such genes. Thus, the preposterous dissolution of the organism into genes and the proteins coded by such genes has been introduced in the evolutionary discourse by neglecting that the explanations for biological phenomena can be either synchronic or diachronic, depending on the phenomenon to be explained. Therefore explanations in molecular biology are synchronic while evolutionary biology needs diachronic explanations. Nevertheless, for ultra-Darwinians such as Dawkins, efficient replication is all that biology is about. Here I develop an argument in order to show that there is nothing in molecular and cell biology that might support such a contention and that the idea of the gene as theunit ofselection is incompatible with the evident evolution of biological complexity. (shrink)

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The Units ofSelection debate is a dispute about the causes of population change. I argue that it is generated by a particular `dynamical'' interpretation of naturalselection theory, according to which naturalselection causes differential survival and reproduction of individuals and naturalselection explanations cite these causes. I argue that the dynamical interpretation is mistaken and offer in outline an alternative, `statistical'' interpretation, according to which naturalselection theory is a fancy kind of `bookkeeping''. (...) It explains by citing the statistical structure of a population and not by citing the causes of survival and reproduction. From the perspective of the statistical interpretation there is no substantive Units ofSelection issue. (shrink)

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Sober and Lewontin's critique of genic selectionism is based upon the principle that aunit ofselection should make a context‐independent contribution to fitness. Critics have effectively shown that this principle is flawed. In this paper I show that the context independence principle is an instance of a more general principle for characterizing causes,called the contextual unanimity principle. I argue that this latter principle, while widely accepted, is erroneous. What is needed is to replace the approach to causality (...) characterized by the contextual unanimity criterion with an approach based on the concept of causal mechanism. After sketching such an approach, I show how it can be used to shed light on the units ofselection problem. (shrink)

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"Additive variance in fitness" is an important concept in the formal apparatus of population genetics. Wimsatt and Lloyd have argued that this concept can also be used to decide the "unit ofselection" in an evolutionary process. The paper argues that the proposed criteria of Wimsatt and Lloyd are ambiguous, and several interpretations of their views are presented. It is argued that none of these interpretations provide acceptable criteria for deciding units ofselection. The reason is that (...) additive variance in fitness can be both a cause of evolution, but also a byproduct ofselection at another level. (shrink)

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I consider two attempts to combine realism with pluralism about the units ofselection: Sober and Wilson’s combination of “model” and “unit” pluralism, and Sterelny and Griffiths’ “local pluralism”. I argue that both of these attempts fail to show that realism and pluralism are compatible. Sober and Wilson’s pluralism turns out, on closer inspection, to be a kind of monism in disguise, while Sterelny and Griffiths’ local pluralism involves a combination of realism and anti-realism about interactors, and the (...) units ofselection, that is fundamentally unstable. My conclusion is that one must choose whether to be a realist or a pluralist in this area: one cannot be both. The question of which we should choose is a further question that I do not take a stand on. (shrink)

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Genic selectionism holds that allselection can be understood as operating on particular genes. Critics (and conventional biological wisdom) insist that this misrepresents the actual causal structure of selective phenomena at higher levels of biological organization, but cannot convincingly defend this intuition. I argue that the real failing of genic selectionism is pragmatic – it prevents us from adopting the most efficient corpus of causal laws for predicting and intervening in the course of affairs – and I offer a (...) Pragmatic account of causation itself which ultimately bears out the claim that genic selectionism misrepresents the causal structure of selective contexts. (shrink)

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Developing a definition of groupselection, and applying that definition to the dispute in the social sciences between methodological holists and methodological individualists, are the two goals of this paper. The definition proposed distinguishes between changes in groups that are due to groupselection and changes in groups that are artefacts ofselection processes occurring at lower levels of organization. It also explains why the existence of groupselection is not implied by the mere fact that (...) fitness values of organisms are sensitive to the composition of groups. And, lastly, the definition explains why groupselection need not involveselection for altruism. Groupselection is thereby seen as an evolutionary force which is objectively distinct from other evolutionary forces. Applying the distinction between group and individualselection to the holism/individualism dispute has the desirable result that the dispute is not decidable a priori. This way of looking at the dispute yields a conception of individualism which is untainted by atomism and a conception of holism which is unspoiled by hypostatis. (shrink)

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The conflation of two fundamentally distinct issues has generated serious confusion in the philosophical and biological literature concerning the units ofselection. The question of how aunit ofselection of defined, theoretically, is rarely distinguished from the question of how to determine the empirical accuracy of claims--either specific or general--concerning whichunit(s) is undergoingselection processes. In this paper, I begin by refining a definition of theunit ofselection, first presented in (...) the philosophical literature by William Wimsatt, which is grounded in the structure of naturalselection models. I then explore the implications of this structural definition for empirical evaluation of claims about units ofselection. I consider criticisms of this view presented by Elliott Sober--criticisms taken by some (for example, Mayo and Gilinsky 1987) to provide definitive damage to the structuralist account. I shall show that Sober has misinterpreted the structuralist views; he knocks down a straw man in order to motivate his own causal account. Furthermore, I shall argue, Sober's causal account is dependent on the structuralist account that he rejects. I conclude by indicating how the refined structural definition can clarify which sorts of empirical evidence could be brought to bear on a controversial case involving units ofselection. (shrink)

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This anthology collects some of the most important papers on what is believed to be the major force in evolution, naturalselection. An issue of great consequence in the philosophy of biology concerns the levels at which, and the units upon whichselection acts. In recent years, biologists and philosophers have published a large number of papers bearing on this subject. The papers selected for inclusion in this book are divided into three main sections covering the history of (...) the subject, explaining its conceptual foundations, and focusing on kin and groupselection and higher levels ofselection.One of the book's interesting features is that it draws together material from the biological and philosophical literatures. The philosophical literature, having thoroughly absorbed the biological material, now offers conceptual tools suitable for the reworking of the biological arguments. Although a full symbiosis has yet to develop, this anthology offers a unique resource for students in both biology and philosophy.Robert N. Brandon is Professor in the Philosophy Department, Duke University. Richard M. Burian is Professor of Philosophy and Department Chairman, Virginia Polytechnic Institute and State University.A Bradford Book. (shrink)

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Brandon ([1982] 1984, 1990) has argued that Salmon's (1971) concept of screening-off can be used to characterize (i) the idea that naturalselection acts directly on an organism's phenotype, only indirectly on its genotype, and (ii) the biological problem of the levels ofselection. Brandon also suggests (iii) that screening-off events in a causal chain are better explanations than the events they screen off. This paper critically evaluates Brandon's proposals.

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My general aim is to clarify the foundational difference between Stephen Jay Gould and Richard Dawkins concerning what biological entities are the units ofselection in the process of evolution by naturalselection. First, I recapitulate Gould’s central objection to Dawkins’s view that genes are the exclusive units ofselection. According to Gould, it is absurd for Dawkins to think that genes are the exclusive units ofselection when, after all, genes are not the exclusive interactors: (...) those agents directly engaged with, directly impacted by, environmental pressures. Second, I argue that Gould’s objection still goes through even when we take into consideration Sterelny and Kitcher’s defense of gene selectionism in their admirable paper “The Return of the Gene.” Third, I propose a strategy for defending Dawkins that I believe obviates Gould’s objection. Drawing upon Elisabeth Lloyd’s careful taxonomy of the various understandings of theunit ofselection at play in the philosophy of biology literature, my proposal involves realizing that Dawkins endorses a different understanding of theunit ofselection than Gould holds him to, an understanding that does not require genes to be the exclusive interactors. (shrink)

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In an important article, Kim Sterelny and Philip Kitcher challenge the common assumption that for any biological phenomenon requiring a selectionist explanation, it is possible to identify a uniquely correct account of the relevantselection process. They argue thatselection events can be modeled in any of a number of different, equally correct ways. They call their view ' Pluralism,' and explicitly connect it with various antirealist positions in the philosophy of science. I critically evaluate Sterelny and Kitcher's (...) Pluralism along with its attendant antirealist theses. In particular, I argue that there are serious problems with their pluralistic antirealism regarding units ofselection. By correctly diagnosing these problems a more adequate position can be constructed. I defend such a position, which I designate Inclusive Hierarchical Monism, and show how it captures the important virtues of Sterelny and Kitcher's approach while avoiding its problems. (shrink)

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The evolutionary problem of the units ofselection has elicited a good deal of conceptual work from philosophers. We review this work to determine where the issues now stand.

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This paper inquires into the very possibility of the units ofselection debate’s origin in the problem of altruism, function in articulating the evolutionary synthesis, and philosophical status as a problem in clarifying what makes something a level orunit ofselection. What makes the debate possible? In terms of origins, there are a number of logically possible ways to deviate from the model of Darwinian individualselection to explain evolved traits. In terms of function, adherence (...) to the evolutionary synthesis yields norms which restrict these possibilities to a manageable few. In terms of philosophical status, the abstract structure ofselection mechanisms permits a causal construal, on which theunit ofselection is identified with the “unit of possession”, that which possesses the causally efficacious trait selected for. It also allows a teleological interpretation, on which theunit ofselection is identified with the “unit of benefit”, that for the sake of which the causally efficacious trait is selected. It is proposed that aunit ofselection is really a pair of units, consisting of both aunit of possession and aunit of benefit. (shrink)

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Sober argues that the units ofselection problem in evolutionary biology is to be understood and solved by applying the general analysis of what it means for C to cause E in a population. The account he utilizes is the unanimity account, according to which C causes E in a population when C raises the probability of E in each causal context. I argue that he does not succeed here, both because the unanimity account is not well grounded in (...) the general case, and because there are important differences between cases of population causation which do involveselection and those which do not. (shrink)

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Naturalselection is an important force that shapes the evolution of all living things by determining which individuals contribute the most descendents to future generations. The biologicalunit upon whichselection acts has been the subject of serious debate, with reasonable arguments made on behalf of populations, individuals, individual phenotypic characters and, finally, individual genes themselves. In this essay, I argue that the usualunit ofselection is the gene. There are powerful logical arguments in (...) favor of this conclusion, as well as many real-world examples. I also explore the possibility that epigenetic differences between individuals may be heritable between generations. Although few such examples exist, epigenetic differences provide an exciting source of potentially heritable variation that may allow rapid evolutionary change to occur, perhaps in response to environmental influences. (shrink)

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I discuss two subjects in Samir Okasha’s excellent book, Evolution and the Levels ofSelection. In consonance with Okasha’s critique of the conventionalist view of the units ofselection problem, I argue that conventionalists have not attended to what realists mean by group, individual, and genicselection. In connection with Okasha’s discussion of the Price equation and contextual analysis, I discuss whether the existence of these two quantitative frameworks is a challenge to realism.

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Naturalselection is an important force that shapes the evolution of all living things by determining which individuals contribute the most descendents to future generations. The biologicalunit upon whichselection acts has been the subject of serious debate, with reasonable arguments made on behalf of populations, individuals, individual phenotypic characters and, finally, individual genes themselves. In this essay, I argue that the usualunit ofselection is the gene. There are powerful logical arguments in (...) favor of this conclusion, as well as many real-world examples. I also explore the possibility that epigenetic differences between individuals may be heritable between generations. Although few such examples exist, epigenetic differences provide an exciting source of potentially heritable variation that may allow rapid evolutionary change to occur, perhaps in response to environmental influences. (shrink)

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Review of Javier Suárez and Elisabeth A. Lloyd, _Units of Selection_. Cambridge: Cambridge University Press, 2023, 75 pp, £49.99 HB.

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Much recent work in sociobiology can be understood as designed to demonstrate the sufficiency ofselection operating at lower levels of organization by the development of models at the level of the gene or the individual. Higher level units are accordingly viewed as artifacts ofselection operating at lower levels. The adequacy of this latter form of argument is dependent upon issues of the complexity of the systems under consideration. A taxonomy is proposed elaborating a series of types, (...) or grades, of hierarchically organized systems. These range from aggregative systems, in which there is no organization relevant to systemic properties, through several graded variations reflecting various degrees of functional interdependence of components, to integrated systems, which manifest component specialization and diversification as well as a subordination of component function to systemic function. It is suggested that the most complex form of organization is plausibly treated as indicative of higher level units ofselection. (shrink)

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This chapter contains section titled: Introduction Historical Remarks The Gene's Eye View of Evolution GroupSelection and KinSelection SpeciesSelection and Macroevolution MultilevelSelection Theory and the Major Transitions in Evolution Conclusion References Further Reading.

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This paper defends global realism about the units ofselection, the view that there is always (or nearly always) an objective fact of the matter concerning the level at which naturalselection acts. The argument proceeds in two stages. First, it is argued that global conventionalist-pluralism is false. This is established by identifying plausible sufficient conditions for irreducibleselection at a particular level, and showing that these conditions are sometimes satisfied in nature. Second, it is argued that (...) local pluralism – the view that while realism is true of someselection regimes, pluralist conventionalism holds for others – should also be rejected. I show that the main arguments for local pluralism are consistent with global realism. I also suggest that local pluralism offers an unacceptably disunified view of the metaphysics ofselection. It follows that we should accept global realism. But this leaves open the question of how to classify so called ‘multi-levelselection type 1’ (MLS1) processes, such as Wilson’s classic trait-group model for the evolution of altruism: should they be interpreted as particleselection or collectiveselection? On the assumption of global realism, at most one of these is correct. I argue, against global realists such as Sober, that MLS1 processes should be understood as particle, not collective,selection, due to three features of MLS1: the reducibility of collective fitness, the absence of collective reproduction, and the dispensable role of collectives. (shrink)

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This article proposes two conditions to assess whether an entity at a level of description is aunit ofselection qua interactor. These two conditions make it possible to (1) distinguish biologically relevant entities from arbitrary ones and (2) distinguish units that can _potentially_ enter aselection process from those that have already done so. I show that the classical approaches used in the literature on units and levels ofselection do not fare well with respect (...) to either or both of these desiderata. (shrink)

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In recent years philosophers have attempted to clarify the units ofselection controversy in evolutionary biology by offering conceptual analyses of the term 'unit ofselection'. A common feature of many of these analyses is an emphasis on the claim that units ofselection are entities exhibiting heritable variation in fitness. In this paper I argue that the demand that units ofselection be characterized in terms of heritability is unnecessary, as well as undesirable, on (...) historical, theoretical, and philosophical grounds. I propose a positive account of the proper referent of the term 'unit ofselection', distinguishing between the processes of evolution and phenotypicselection. The main result of this analysis is greater clarity about the conceptual structure of evolutionary theory. (shrink)

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The theory of evolution is supported by the theory of genetics, which provides a single causal mechanism to explain the activities of replicators and interactors. A common misrepresentation of the theory of evolution, however, is that interaction (involving interactors), and transmission (involving replicators), are distinct causal processes. Sandra Mitchell (1987) is misled by this. I discuss why only a single causal mechanism is working in evolution and why it is sufficient. Further, I argue that Mitchell's mistaken view of the causal (...) mechanism in evolution prevents her from resolving the conflict between Dawkins and Brandon. I conclude that theunit-of-selection question remains very much alive. (shrink)

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On the basis of distinctions between those properties of entities that can be defined without reference to other entities and those that (in different ways) cannot, this note argues that non-trivial forms of frequency-dependentselection of entities should be interpreted asselection occurring at a level higher than that of those entities. It points out that, except in degenerately simple cases, evolutionary game-theoretic models ofselection are not models of individualselection. Similarly, models of genotypic (...) class='Hi'>selection such as heterosis cannot be legitimately interpreted as models of genicselection. The analysis presented here supports the views that: (i)selection should be viewed as a multi-level process; (ii) upper-levelselection is ubiquitous; (iii) kinselection should be viewed as a type of groupselection rather than individualselection; and (iv) inclusive fitness is not an individual property. (shrink)

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A set of constraints forces trade-offs which prevent us from achieving the best possible definitions of the ‘level’ and ‘unit’ of naturalselection. This set consists in decisions concerning conflicting pre-analytic intuitions in problematic cases, the relative roles of various conceptual resources in the definitions, which facts need to be accounted for using the definitions, how the relation betweenselection and evolution orients the definitions, and the relation between the level andunit concepts. Systematic reconstruction and (...) evaluation of leading analyses along these dimensions favors a new functional analysis over Williams’ consequentialist analysis, Sober’s causal analysis, and Dawkins’ teleological analysis. (shrink)

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This article provides an overview of work on the levels ofselection in the philosophy of biology.

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The question "what is (are) theunit(s) ofselection" can be interpreted in three different ways. These interpretations are discussed and it is shown that they prompt different answers; such units are shown to be individuals in the context of the given interpretation. One of these interpretations is argued, by examples, not always to have an unambiguously correct answer. An alternative approach to this question is sketched.

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In recent years evolutionary theorists have been engaged in a protracted and bitter disagreement concerning how naturalselection affects units such as genes, individuals, kin groups, and groups. Central to this debate has been whether selective pressures affecting group success can trump the selective pressures that confer advantage at the individual level. In short, there has been a debate about the utility of groupselection, with noted theorist Steven Pinker calling the concept useless for the social sciences. We (...) surveyed 175 evolutionary anthropologists to ascertain where they stood in the debate. We found that most were receptive to groupselection, especially in the case of cultural groupselection. The survey also revealed that liberals and conservatives, and males and females, all displayed significant differences of opinion concerning which selective forces were important in humanity’s prehistory. We conclude by interpreting these findings in the context of recent research in political psychology. (shrink)

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A set of constraints forces trade-offs which prevent us from achieving the best possible definitions of the ‘level’ and ‘unit’ of naturalselection. This set consists in decisions concerning conflicting pre-analytic intuitions in problematic cases, the relative roles of various conceptual resources in the definitions, which facts need to be accounted for using the definitions, how the relation betweenselection and evolution orients the definitions, and the relation between the level andunit concepts. Systematic reconstruction and (...) evaluation of leading analyses along these dimensions favors a new functional analysis over Williams’ consequentialist analysis, Sober’s causal analysis, and Dawkins’ teleological analysis. (shrink)

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Two major clarifications have greatly abetted the understanding and fruitful expansion of the theory of naturalselection in recent years: the acknowledgment that interactors, not replicators, constitute the causalunit ofselection; and the recognition that interactors are Darwinian individuals, and that such individuals exist with potency at several levels of organization (genes, organisms, demes, and species in particular), thus engendering a rich hierarchical theory ofselection in contrast with Darwin’s own emphasis on the organismic level. (...) But a piece of the argument has been missing, and individuals at levels distinct from organisms have been denied potency (although granted existence within the undeniable logic of the theory), because they do not achieve individuality with the same devices used by organisms and therefore seem weak by comparison. We show here that different features define Darwinian individuality across scales of size and time. In particular, species-individuals may develop few emergent features as direct adaptations. The interactor approach works with emergent fitnesses, not with emergent features; and species, as a consequence of their different mechanism for achieving individuality (reproductive exclusivity among subparts, that is, among organisms), express many effects from other levels. Organisms, by contrast, suppress upwardly cascading effects, because the organismic style of individuality (by functional integration of subparts) does not permit much competition or differential reproduction of parts from within. Species do not suppress the operation of lower levels; such effects therefore become available as exaptations conferring emergent fitness—a primary source of the different strength that species achieve as effective Darwinian individuals in evolution. (shrink)

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