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Group selection

Group selection is a concept in evolutionary biology that refers to the process by which natural selection acts on groups of organisms, favoring traits that increase the survival and reproductive success of the group relative to other groups, even if those traits reduce the fitness of individuals within the group.^[1] This mechanism contrasts with individual selection, where traits evolve primarily based on their direct benefits to the organism's personal reproductive success.^[2] Group selection has been invoked to explain the evolution of altruism and cooperation, where individuals perform behaviors that benefit others at a cost to themselves, provided such actions contribute to group-level advantages like resource conservation or collective defense.^[3] The idea traces back to Charles Darwin, who in The Descent of Man (1871) suggested that natural selection could operate on communities to promote traits beneficial to the group, such as moral instincts in early human societies.^[1] It gained prominence in the mid-20th century through Vero Wynne-Edwards' work, particularly Animal Dispersion in Relation to Social Behaviour (1962), which argued that population-regulating behaviors, like reduced breeding rates to avoid overexploitation of resources, evolve via group-level selection to ensure long-term group viability.^[2] However, this view faced sharp criticism in the 1960s and 1970s from evolutionary biologists like George C. Williams and John Maynard Smith, who contended that within-group individual selection overwhelmingly dominates, rendering true group selection rare and theoretically implausible without mechanisms like limited migration between groups.^[2] The controversy subsided with the rise of kin selection theory, proposed by W.D. Hamilton in 1964, which explained altruism through inclusive fitness benefits to genetic relatives, often obviating the need for group-level explanations.^[2] Nonetheless, group selection was revitalized in the 1970s and 1980s through David Sloan Wilson's trait-group model (1975), which demonstrated mathematically how partitioned populations could sustain altruistic traits under certain conditions of group formation and dispersal.^[3] George Price's equation (1970, 1972) further formalized multilevel selection, partitioning evolutionary change into within- and between-group components, showing that group selection can occur alongside individual selection when between-group variance in fitness exceeds within-group variance.^[1] In contemporary evolutionary theory, group selection is often framed within the broader paradigm of multilevel selection (MLS), which recognizes selection acting simultaneously at multiple hierarchical levels, from genes to societies.^[1] Proponents like David Sloan Wilson and Elliott Sober argue that MLS resolves historical debates by integrating group benefits with individual and kin selection, and empirical evidence supports its role in phenomena such as the evolution of eusociality in insects and human cultural cooperation.^[1] Critics persist in viewing it as redundant or prone to misinterpretation, but among experts in social evolution, it is increasingly accepted as a valid process, particularly in structured populations with low gene flow.^[4] Applications extend to cultural group selection, where group-beneficial norms and practices spread through intergroup competition in human societies.^[5]

Definitions and Concepts

Definition and Basic Principles

Group selection refers to a mode of natural selection in evolutionary biology that operates at the level of groups of organisms, rather than solely on individuals, whereby traits that increase the fitness of the group as a whole can evolve and spread, even if those traits decrease the fitness of the individual bearer within the group.^[6] This process typically involves altruism or cooperation, where individuals perform behaviors that benefit others at a personal cost, such as reduced survival or reproductive success.^[7] In essence, groups containing higher proportions of such altruistic individuals gain a competitive edge over less cooperative groups, allowing the altruistic traits to proliferate in the broader population through the success of those groups.^[6] The core principles of group selection hinge on intergroup competition as the driving force for group-level adaptation. Groups vary in their composition of traits, and those with more individuals exhibiting group-beneficial behaviors—such as resource sharing or collective defense—tend to survive and reproduce more effectively than rival groups, thereby propagating those traits.^[7] However, this is often counteracted by within-group selection, where selfish individuals exploit altruists, gaining higher personal fitness and potentially eroding cooperative traits inside the group.^[8] For group selection to prevail, the between-group variance in fitness must outweigh the within-group variance, ensuring that group-level advantages dominate.^[7] A classic conceptual example of group-beneficial traits, often framed within multilevel selection theory, arises in social insects, such as ants and bees, where non-reproductive workers altruistically forage, defend the colony, and care for the queen's offspring, forgoing their own reproduction to enhance the colony's overall productivity and survival.^[8] This extreme form of altruism boosts group fitness by enabling larger, more efficient colonies that outcompete others, despite the individual workers' reproductive sacrifice, though kin selection also plays a key role.^[8] Charles Darwin initially proposed group selection in his 1871 work The Descent of Man to account for the evolution of moral instincts in humans, arguing that tribes with members prone to mutual aid and self-sacrifice for the common good would triumph over less cohesive groups in competition. Group selection forms the basis for broader frameworks like multilevel selection theory, which examines selection across hierarchical levels from genes to societies.^[8]

Types of Groups in Selection

In group selection models, trait groups refer to temporary assemblages of individuals that share similar phenotypic traits, such as cooperative behaviors, which influence group-level fitness through interactions like resource sharing or defense.^[9] These groups form due to assortment mechanisms, including spatial clustering or behavioral preferences, creating variance in trait composition across groups without requiring permanent isolation.^[9] This structure enables multilevel selection by allowing differential survival or reproduction of groups based on their overall composition, such as those with higher proportions of cooperators outperforming groups dominated by defectors.^[10] Demes, in contrast, are spatially structured subpopulations within a larger metapopulation, characterized by limited migration and gene flow that maintain distinct genetic compositions over time. In these units, selection operates at the group level when differential extinction or proliferation of demes occurs, driven by collective traits like reduced virulence in pathogen populations. The restricted dispersal in demes amplifies between-group variance, facilitating adaptation to local environments while individual-level selection persists within them.^[11] The primary distinction between trait groups and demes lies in their formation and isolation: trait groups rely on transient, trait-based assortment that promotes multilevel processes without geographic barriers, whereas demes depend on spatial separation and limited migration to sustain group-level dynamics.^[12] For instance, in microbial populations, cells forming biofilms exemplify trait groups, where cooperative producers of public goods enhance matrix stability and group persistence against cheaters, independent of strict boundaries.^[10] In demic structures, such as host-associated bacterial communities, limited gene flow between spatially separated clusters allows selection on group productivity, as seen in gut microbiomes where deme extinction favors efficient consortia.^[11] Trait groups can overlap with kin selection when assortment occurs among related individuals, but they broadly encompass non-kin interactions as well.^[12]

Historical Development

Early Ideas from Darwin to Mid-20th Century

Charles Darwin laid the foundational ideas for group selection in his 1871 book The Descent of Man, and Selection in Relation to Sex, where he applied natural selection to explain the origins of human altruism and morality at the tribal level. Darwin posited that natural selection could favor entire groups over individuals, particularly in scenarios of intertribal competition, such as warfare over resources or territory. He argued that traits promoting cooperation and self-sacrifice, which might disadvantage individuals within a tribe, could nonetheless spread if they enhanced the tribe's overall survival and reproductive success against rival groups. This group-level process, Darwin suggested, accounted for the evolution of social instincts that underpin moral behavior, allowing sympathetic and obedient tribes to outcompete more selfish ones.^[13] A key example Darwin provided involved the selective advantage of cooperative tribes in conflict: "A tribe including many members who, from possessing in a high degree the spirit of patriotism, fidelity, obedience, courage, and sympathy, were always ready to aid one another, and to sacrifice themselves for the common good, would be victorious over most other tribes; and this would be natural selection." He emphasized that such victories would propagate the associated moral qualities across generations, raising the standard of ethics in advancing civilizations, even as within-group selection might favor less altruistic individuals. Darwin also noted that communities with the most sympathetic members would flourish best and produce more offspring, linking group selection directly to the development of human sociality.^[14] In the early 20th century, Herbert Spencer built on these notions in his Principles of Sociology (1876–1896), conceptualizing society as a "superorganic" entity subject to evolutionary forces beyond individual biology. Spencer described superorganic evolution as the adaptation of social aggregates—such as institutions, customs, and communities—through processes analogous to organic natural selection, where groups compete and adapt as integrated wholes. This framework implied selection at the societal level, with cooperative structures enabling groups to survive environmental and competitive pressures more effectively than disorganized ones. Spencer's ideas extended Darwin's tribal dynamics to larger social scales, influencing early sociological thought on collective evolution.^[15] Before the mid-20th century, group selection concepts enjoyed broad acceptance in ecology and sociology, often invoked to explain adaptive traits in populations and communities without emphasizing conflicts over altruism. This perspective prevailed in discussions of species interactions and social organization, treating groups as primary units of adaptation. However, the emergence of modern population genetics in the 1930s and 1940s, spearheaded by Ronald Fisher, J.B.S. Haldane, and Sewall Wright, redirected focus to individual-level mechanisms, highlighting how genic variation and selection within populations could account for most evolutionary change and marginalizing group-oriented views.

Wynne-Edwards and the Group Selection Controversy

In 1962, Vero Copner Wynne-Edwards published Animal Dispersion in Relation to Social Behaviour, a seminal work proposing that group selection plays a central role in the evolution of animal social behaviors to maintain population stability.^[16] He argued that mechanisms such as territorial displays and epideictic behaviors—social signals that convey population density—evolve to prevent overpopulation and resource depletion, prioritizing the welfare of the group over individual reproductive gains.^[16] Wynne-Edwards posited that these traits arise through selection among demes (local populations), where groups practicing restraint outlast those that do not, even if it imposes costs on individuals.^[16] A key aspect of his theory involved interpreting behaviors like territoriality in birds as adaptations for the collective good, where dominant individuals exclude subordinates from breeding to keep group numbers sustainable and avoid famine.^[16] Similarly, he viewed lekking systems in species such as grouse, where males gather to display without providing parental care or resources, as forms of group altruism that signal density and curb excessive reproduction, thereby benefiting the population's long-term survival.^[16] Wynne-Edwards claimed these examples demonstrated how group selection could override selfish individual advantages, challenging the dominance of genic or individual-level explanations in evolutionary theory.^[16] The book ignited a fierce controversy, with critics contending that Wynne-Edwards' formulation of "naive" group selection lacked empirical support and theoretical rigor.^[17] David Lack, in Population Studies of Birds (1966), rebutted the idea by showing that population regulation in birds results from density-dependent mortality and individual competitive advantages, not group-level heritability or altruism; he argued that Wynne-Edwards overestimated the role of social displays in controlling numbers, as cheater individuals would undermine such systems.^[18] George C. Williams, in Adaptation and Natural Selection (1966), delivered a more comprehensive critique, asserting that group selection requires implausibly strong between-group variation and heritability while ignoring how within-group individual selection favors exploiters over altruists.^[19] Williams dismissed Wynne-Edwards' examples, including lekking and territoriality, as misinterpretations better explained by individual mating success or resource competition, concluding that "group-related adaptations do not, in fact, exist."^[19] This backlash marginalized group selection in mainstream evolutionary biology for decades, shifting focus to individual and genic perspectives until later theoretical revivals.^[17]

Theoretical Foundations

Kin Selection and Inclusive Fitness

Kin selection, proposed by W. D. Hamilton in his seminal 1964 papers, provides a gene-centered explanation for the evolution of altruistic behaviors by emphasizing the role of genetic relatedness among individuals within groups.^[20]^[21] This theory posits that individuals can increase their genetic representation in future generations not only through direct reproduction but also by aiding relatives who share their genes, thereby serving as an alternative or complement to traditional group selection models that focus on benefits to the collective.^[20] Central to kin selection is the concept of inclusive fitness, which Hamilton defined as the total effect of an individual's actions on the propagation of genes identical by descent from common ancestors, encompassing both personal reproductive success and the reproductive success of relatives weighted by their genetic relatedness.^[20] Unlike classical fitness, which measures only an organism's own offspring production, inclusive fitness accounts for indirect contributions to gene transmission via kin, allowing selfish genes to favor altruism when it enhances overall genetic propagation.^[20] This framework resolves apparent paradoxes in social evolution, such as sterile castes in eusocial insects, by demonstrating that such sacrifices can yield net genetic benefits through elevated relatedness within the group.^[21] Hamilton formalized this idea in his rule for the evolution of altruism: rB > C, where r is the genetic relatedness between actor and recipient, B is the reproductive benefit to the recipient, and C is the reproductive cost to the actor.^[21] The rule derives from a genetical model tracking changes in allele frequency under natural selection, using Sewall Wright's coefficient of relationship to quantify r as the probability that a gene in the actor is identical by descent to a gene in the recipient.^[20] Specifically, the change in gene frequency \Delta p due to a social behavior is approximated as \Delta p \approx \frac{p(1-p)}{ \bar{w} } (rB - C), where p is the initial gene frequency, \bar{w} is mean fitness, and positive \Delta p occurs when rB > C, indicating selection favors the behavior.^[21] This condition ensures that the inclusive fitness gain from aiding kin outweighs the direct fitness loss, promoting the spread of altruism even if it reduces the actor's personal reproduction.^[21] In haplodiploid species like social insects (Hymenoptera order, including bees), Hamilton's rule gains particular explanatory power through the haplodiploid sex-determination system, where females develop from fertilized diploid eggs and males from unfertilized haploid eggs.^[21] Under this system, full sisters share, on average, 75% of their genes identical by descent (r = 0.75), higher than the 50% relatedness to their own offspring, because sisters inherit the father's entire genome and half from the mother.^[21] Workers (sterile females) thus gain greater inclusive fitness by raising sisters (B to sisters at r = 0.75) than by producing sons (r = 0.5), satisfying rB > C for altruistic foraging and colony defense even at high personal costs.^[21] This asymmetry resolves the puzzle of eusociality in bees—such as honeybees (Apis mellifera), where workers forgo reproduction to support the queen—without relying solely on group-level benefits, as the high sister relatedness amplifies indirect fitness returns.^[21] Hamilton's 1964 publications marked a pivotal shift in evolutionary biology, redirecting attention from group-centric to gene-level explanations of sociality and diminishing the prominence of earlier group selection ideas.^[22] This gene-focused perspective profoundly influenced Richard Dawkins' 1976 book The Selfish Gene, which popularized inclusive fitness and kin selection by framing evolution as competition among replicators, with altruism emerging as a strategy to propagate shared genes.^[23]

Multilevel Selection Theory

Multilevel selection theory (MLS) posits that natural selection operates simultaneously across multiple hierarchical levels of biological organization, including genes, individuals, and groups, rather than solely at the individual level. This framework reconciles group-level adaptations with individual-level selection by emphasizing that evolutionary change can be partitioned into components attributable to different levels, allowing for the emergence of traits that benefit groups even if they impose costs on individuals within those groups. The theory gained prominence through the work of biologist David Sloan Wilson, who introduced foundational ideas in his 1975 paper outlining conditions under which group selection could predominate. Philosopher Elliott Sober joined Wilson in the 1990s to formalize MLS as a compatible extension of standard evolutionary theory, arguing that group selection does not contradict individual selection but rather integrates it within a broader multilevel perspective. A central element of MLS is the partition of variance in fitness across levels, which quantifies how much of the total change in a trait's frequency is due to selection within groups versus between groups. This partitioning reveals when group-level selection can drive the evolution of altruism or cooperation by favoring groups composed of cooperative individuals over less cooperative ones. The theory distinguishes between two primary approaches: MLS1, which focuses on contextual analysis by statistically decomposing fitness differences to assess the relative contributions of individual and group effects without invoking group-level causation, and MLS2, which emphasizes causal processes where groups themselves function as adaptive units with their own heritable properties and differential replication.^[24] MLS1 treats groups as contexts influencing individual fitness, while MLS2 posits that groups can evolve as entities akin to organisms, enabling explanations for phenomena like the major transitions in evolution.^[25] This dual framework, often underpinned by the Price equation for variance partitioning, provides a unified lens for analyzing social behaviors.^[26] In their seminal 1998 book Unto Others: The Evolution and Psychology of Unselfish Behavior, Sober and Wilson demonstrated that MLS is fully compatible with gene-level and individual-level selection, countering earlier dismissals of group selection by showing how multilevel processes can explain unselfish behavior in both biological and psychological contexts. The book argues that traits like altruism evolve when between-group selection outweighs within-group selection, using conceptual models to illustrate how group benefits can propagate despite individual costs. Following this revival in the 1990s, MLS has achieved broader acceptance in the study of social evolution by the 2020s, with applications extending to cultural and ecological systems. For instance, a 2025 study on wild animal populations provided evidence of multilevel selection shaping social structures, highlighting its relevance to ongoing research in behavioral ecology.^[27]^[28]

Mathematical Models

Price Equation Applications

The Price equation, developed by George Price in the early 1970s, provides a general mathematical framework for describing evolutionary change in any trait under natural selection, including scenarios involving group-level processes.^[29] Price's initial formulation appeared in 1970, with an extension in 1972 that explicitly allowed for recursive partitioning across hierarchical levels, such as individuals within groups.^[30] This work mathematically resolved longstanding controversies from the 1960s over group selection by demonstrating its compatibility with individual-level mechanisms, and it directly facilitated W.D. Hamilton's derivation of his inclusive fitness rule in 1970 as well as David Sloan Wilson's subsequent trait-group models in the mid-1970s.^[31] The core Price equation states that the change in the average value of a trait G across a population, denoted \Delta \bar{G}, equals the covariance between relative fitness w and the trait G, divided by the mean fitness \bar{w}, plus the expected value of the fitness-weighted change in the trait within each reproductive unit:

\Delta \bar{G} = \frac{\text{Cov}(w, G)}{\bar{w}} + E\left( \frac{w \Delta G_i}{w_i} \right),

where \Delta G_i is the change in G within the i-th unit (e.g., individual or group), and the expectation is taken over all units.^[30] To derive this for multilevel selection, consider a population structured into groups, where G_{ij} is the trait value of individual j in group i, w_{ij} is its fitness, \bar{G}_i = E[G_{ij} \mid i] is the group mean trait, and W_i = E[w_{ij} \mid i] is the group mean fitness. The total change \Delta \bar{G} can be partitioned by applying the Price equation first at the individual level and then aggregating across groups. Substituting the within-group Price equation into the overall form yields the multilevel version:

\Delta \bar{G} = \frac{\text{Cov}(W_i, \bar{G}_i)}{\bar{W}} + E\left( \frac{W_i}{\bar{W}} \Delta \bar{G}_i \right),

where the first term, \frac{\text{Cov}(W_i, \bar{G}_i)}{\bar{W}}, captures between-group selection (the differential success of groups based on their trait composition), and the second term accounts for within-group processes (selection and transmission biases inside groups).^[32] This partition holds exactly under Price's assumptions of no assumptions about the form of fitness or trait transmission, making it a covariance-based identity rather than an approximation. In applications to group selection, the equation formalizes conditions under which group-level processes drive trait evolution: the between-group term must be positive and sufficiently large to outweigh any negative within-group term. For altruism—a trait where an individual's fitness decreases (c > 0) but provides benefits (b > 0) to others—the within-group covariance is typically negative, as altruists are outcompeted by selfish individuals inside groups. However, if population structure generates positive between-group variance in altruist frequency (e.g., via limited dispersal or assortment), the between-group term can favor altruism overall when \frac{\text{Cov}(W_i, \bar{G}_i)}{\bar{W}} > -E\left( \frac{W_i}{\bar{W}} \Delta \bar{G}_i \right). A representative example involves a structured population of haploid organisms with two types: altruists (trait G = 1, paying cost c to benefit groupmates by b/m, where m is group size) and selfish individuals (G = 0). Assume groups form randomly with mean altruist proportion p, and group fitness W_i = 1 + p_i b - p_i c (additive effects). The within-group term is E\left( \frac{W_i}{\bar{W}} \Delta \bar{G}_i \right) \approx -p(1-p)c / \bar{W} (negative due to individual selection against altruists). The between-group term is \frac{\text{Cov}(W_i, p_i)}{\bar{W}} = \frac{\text{Var}(p_i) b}{\bar{W}}, where \text{Var}(p_i) depends on assortment (e.g., relatedness r \approx \text{Var}(p_i)/[p(1-p)]). Altruism increases if r b > c, mirroring Hamilton's rule; here, group structure amplifies between-group variance to enable evolution despite within-group costs. This framework has been pivotal in modeling altruism in viscous populations, such as microbial biofilms or animal societies, where spatial structure enhances group-level covariances.^[32]

Trait Group and Haystack Models

The haystack model, introduced by John Maynard Smith in 1964 and revised in 1976, illustrates group selection dynamics in a structured population with discrete generations and periodic mixing. In this model, the population is conceptualized as mice colonizing isolated "haystacks" (groups), each founded by a single mated female carrying alleles for altruism (A, which benefits the group at individual cost) or selfishness (S, which exploits others). During a growth phase lasting G non-overlapping generations within each haystack, the relative fitness of altruists is reduced by a cost factor c (e.g., lower individual reproduction), while selfish individuals have fitness 1; however, groups with higher proportions of altruists exhibit greater overall productivity, measured by the benefit parameter b representing increased disperser output from cooperative groups. At the end of the growth phase, all survivors disperse and randomly recolonize new haystacks, with migration effectively resetting group composition unless mating occurs primarily within groups (probability m close to 1).^[33]^[34] The model's key insight emerges from analyzing allele frequency changes across cycles: within haystacks, the selfish allele S increases rapidly due to individual selection, but between haystacks, groups founded with more altruists produce more dispersers, favoring A at the group level if the inter-group differential outweighs intra-group losses. Maynard Smith derived the condition for the altruist allele to spread as bR > c, where R is the average relatedness within groups, which rises with larger G (more generations per group) and higher m (limited dispersal and within-group mating). In numerical examples, if G = 10 and the within-group fitness advantage of S is modest (e.g., 1.1 relative to A), group selection can maintain A only if m approaches 1 and b is sufficiently large (e.g., group productivity 2-3 times higher for pure A groups than pure S); otherwise, complete mixing (m = 0) leads to rapid fixation of S. The 1976 revision emphasized that realistic migration rates (m < 0.5) render group selection negligible, as random reassortment dilutes group-level benefits.^[33]^[34]^[35] David S. Wilson's trait group model, developed in 1975 and expanded in 1980, shifts focus to stochastically formed assemblages in a single randomly mating deme, where "trait groups" arise temporarily during interactions affecting fitness, such as mating, competition, or predation. Unlike fixed demes, trait groups form via binomial sampling of individuals into subgroups of size N, with composition varying due to assortment (random or patchy distribution of altruist A and non-altruist B genotypes); altruists incur a donor cost f_d < 0 but provide recipient benefit f_r > 0, enhancing average group fitness. The model quantifies multilevel selection through the change in A frequency in the deme. For altruism to evolve, between-group genetic variance (V_g) must exceed random levels, such as through kin clustering or patchiness; under random assortment, individual selection requires f_d > 0, but with positive assortment, altruism can spread if the group selection term outweighs the negative individual term (e.g., f_d > -(N-1)f_r at the group level, adjusted for variation). Weak altruism evolves with modest assortment, while strong altruism requires high structure. The 1980 extension formalized trait groups as any fitness-relevant interaction unit, showing their equivalence to structured demes under limited dispersal.^[9]^[36] Both models demonstrate group selection's viability under conditions of limited dispersal and positive assortment, contrasting with panmictic populations where individual selection dominates. In the haystack model, low migration (high m) allows group-level advantages to persist across G generations, enabling altruist dominance if b > c / R (e.g., migration below 10% sustains A at 20% frequency in simulations with G = 5). Wilson's approach generalizes this by showing stochastic trait group formation suffices for weak altruism even in large demes, with group selection strength proportional to V_g / V_t. These simulations highlight that group selection prevails when within-group exploitation is offset by between-group competition, such as in metapopulations with 5-20% migration rates, but collapses with free mixing. The Price equation underpins both as a foundational decomposition of variance, though here applied to specific scenarios of patchiness and stochasticity.^[33]^[34]^[9]

Applications

In Non-Human Populations

Group selection has been invoked to explain cooperative behaviors in non-human animal populations, particularly where individual actions benefit the collective survival of flocks or groups. In bird species such as the Florida scrub jay (Aphelocoma coerulescens), 1980s studies on cooperative breeding and sentinel behaviors, including alarm calls, demonstrated how helpers at the nest contribute to group defense against predators, potentially evolving through group-level advantages alongside kin selection.^[37] These alarm calls in family groups alert members to threats, reducing overall predation risk and enhancing group persistence, as observed in long-term field observations where group cohesion correlated with higher reproductive success.^[38] In microbial populations, quorum sensing in bacteria exemplifies a group adaptation where cells coordinate gene expression based on population density to produce public goods like virulence factors or biofilms. This mechanism allows bacterial groups to collectively respond to environmental challenges, such as antibiotic exposure, favoring the evolution of cooperative traits at the group level over individual cheaters.^[39] Among plants, kin recognition in Arabidopsis thaliana leads to competitive restraint, where individuals grown with siblings exhibit reduced root proliferation compared to those with non-kin, allocating fewer resources to aggressive foraging and more to overall growth.^[40] This behavior, mediated by root exudates, enhances inclusive fitness by minimizing resource depletion within family groups, supporting multilevel selection dynamics in dense populations.

In Human Evolution and Culture

Gene-culture coevolution refers to the reciprocal influence between genetic and cultural evolution, where cultural practices shape selective pressures on genes, and genetically influenced cognitive biases affect cultural transmission. In humans, this process has been pivotal in the evolution of cooperative behaviors through cultural group selection, as modeled by Boyd and Richerson starting in their 1985 work. Their dual inheritance theory posits that cultural variants, such as norms favoring group-beneficial actions, can spread rapidly via imitation and conformist bias, leading to between-group differences that natural selection acts upon at multiple levels. For instance, cultural norms promoting fairness and punishment have selected for genetic predispositions toward prosociality, as evidenced by correlations between cultural practices and genetic markers like those for oxytocin receptors influencing social bonding.^[41] A key application is parochial altruism, where individuals exhibit costly cooperation within their group while showing hostility toward out-groups, facilitating success in intergroup conflicts such as warfare. Boyd and Richerson's models demonstrate that cultural transmission allows parochial altruism to evolve even when individual-level selection opposes it, as groups with strong norms of in-group favoritism outcompete others through coordinated aggression and defense. Empirical support comes from simulations and historical cases, like the expansion of pastoralist societies via raiding, where cultural enforcement of cooperation sustains large-scale efforts beyond kin ties. This coevolutionary dynamic explains how human warfare has driven the spread of cooperative cultural traits, with genetic adaptations reinforcing them over millennia.^[41] In human evolution, traits like language and morality have emerged as adaptations enhancing group cohesion and competitiveness. Language facilitates precise cultural transmission and coordination in collective tasks, evolving under group selection pressures to enable larger, more effective social units, as seen in models where linguistic complexity correlates with societal scale. Morality, encompassing norms of reciprocity and justice, likely coevolved similarly, with cultural group selection favoring groups whose moral systems promote internal harmony and external vigilance, as Darwin first suggested and later reinforced by multilevel models integrating kin and group benefits. Recent 2020s research on small-scale societies, such as pastoralist groups in northern Kenya (Borana, Rendille, Samburu, and Turkana), provides empirical validation, showing that intergroup competition selects for cooperative cultural variants, leading to higher within-group prosociality even among non-kin. Field experiments in these societies reveal that readiness to cooperate scales with cultural similarity between groups, supporting cultural group selection as a mechanism for human ultrasociality.^[42]^[43] The implications extend to large-scale human cooperation, explaining phenomena like religious and national institutions that bind millions beyond genetic relatedness. Cultural group selection via "big gods" and moralizing religions has amplified cooperation by enforcing norms across vast groups, as evidenced by historical correlations between religious adherence and empire stability. In modern nations, similar dynamics persist through shared ideologies and institutions, where group competition—economic, military, or ideological—selects for cultures fostering collective action, underscoring how gene-culture coevolution has enabled humanity's unique societal complexity.^[41]

Criticisms and Debates

Challenges from Individual Selection

One of the most influential critiques of group selection emerged from George C. Williams' 1966 book Adaptation and Natural Selection, where he argued that apparent group-level traits and adaptations are illusory, representing mere statistical byproducts of individual-level adaptations rather than genuine evolutionary designs for group benefit.^[44] Williams contended that natural selection primarily operates to maximize individual reproductive success, rendering group-related adaptations nonexistent, as they lack the functional organization required for true evolutionary purpose.^[19] He emphasized that population survival is incidental to individual fitness, with no empirical evidence supporting adaptive mechanisms at the group level, such as in behaviors like schooling in fish, which serve individual predator avoidance rather than collective welfare.^[19] Building on this foundation, Richard Dawkins reinforced the individualist perspective in his 1976 book The Selfish Gene, promoting a gene-centered view of evolution that portrays genes as "selfish" replicators whose propagation undermines group-level altruism. Dawkins criticized naive or parochial forms of group selection—those assuming unstructured mixing within groups without mechanisms for assortment—as inherently unstable, because selfish "free-riders" (individuals who benefit from group efforts without contributing) rapidly proliferate, outcompeting altruists and causing altruistic groups to collapse. Without strong mechanisms like genetic relatedness to prevent exploitation, individual selection invariably dominates, making group selection ineffective or redundant in explaining adaptive traits.^[45] Philosophically, these challenges highlight a tension between reductionism, which reduces evolutionary explanations to the lowest level (genes or individuals), and emergence, where higher-level (group) properties might arise independently but are dismissed as non-causal illusions by critics like Williams and Dawkins.^[44] Multilevel selection theory is often viewed by individualists as merely kin selection in disguise, repackaging group benefits through relatedness without introducing novel emergent dynamics beyond gene-level selfishness.^[1] Proponents of multilevel approaches have responded by formalizing assortment mechanisms to counter free-rider dominance, though debates persist on whether this truly elevates group selection beyond reductionist critiques.^[1]

Empirical Evidence and Recent Advances

Experimental evolution in microorganisms has provided strong empirical support for group selection, particularly through studies demonstrating the emergence of multicellular traits under group-level pressures. In a landmark experiment, Ratcliff et al. selected for rapid sedimentation in yeast populations (Saccharomyces cerevisiae), leading to the evolution of multicellular "snowflake" clusters within approximately 60 generations. This process favored groups that stayed intact longer, enhancing survival against predation and demonstrating multilevel selection type 2 (MLS2), where group-level fitness directly influences propagation independent of individual-level effects.^[46] Subsequent work by the same group showed that these multicellular forms evolved coordinated cell division and increased size by over 20,000 times compared to ancestors, with group selection stabilizing the trait against individual cheaters.^[47] Field studies in natural populations have also uncovered evidence of group selection, particularly in social behaviors. In rock hyraxes (Procavia capensis), a wild mammal, multilevel selection acts on both individual traits and group social structures, with groups exhibiting higher connectivity and centrality showing greater reproductive success and survival.^[48] Analysis of long-term data revealed that selection at the group level, quantified via social network metrics, explained variance in fitness beyond individual behaviors, providing causal MLS2 evidence through differential group persistence in varying environments.^[28] Similarly, in cooperatively breeding birds like white-browed sparrow-weavers, group augmentation—where group size directly boosts individual fitness—supports multilevel selection, as larger groups with altruistic members outcompete smaller ones in resource defense.^[49] Recent advances in the 2020s have integrated genomic tools to detect group-level signatures, countering earlier skepticism by identifying heritable variation at higher levels. A 2023 study on mitochondrial genomes demonstrated multilevel selection through varying levels of selection operating on the mitochondrial genome and the consequences they have on biological hierarchies, revealing forces shaping mitochondrial genetic variation.^[50] In cancer evolution, multiomics analyses have uncovered selection signatures at tissue and organismal levels, with altruistic cell behaviors persisting due to group-level benefits, as evidenced by allele frequency shifts in tumors.^[51] A bibliometric review of 280 studies up to 2025 confirmed abundant empirical support for MLS across taxa, including artificial and natural systems, with growing consensus on its role in complex traits.^[52] Shifts in scientific opinion underscore these advances, with a 2014 survey of evolutionary anthropologists showing 55% endorsement of multilevel selection over strict individual selection, and 80.7% rejecting its outright dismissal.^[53]^[54] This paradigm change, highlighted by the Multilevel Selection Initiative, emphasizes causal MLS2 evidence from experiments and field data, fostering applications in prosocial evolution.^[55]

References

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Eight Criticisms Not to Make About Group Selection - PMC - NIH
It is impossible to evaluate whether a given trait evolves by group selection unless local and global selection differentials are compared to each other. Based ...Missing: papers | Show results with:papers
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https://doi.org/10.1111/j.1420-9101.2009.01876.x
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A theory of group selection - PubMed
A simple model shows that this can lead to the selection of altruistic traits that favor the fitness of the group over that of the individual.
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The process known as group selection was once accepted unthinkingly, then was widely discredited; it's time for a more discriminating assessment.
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May 9, 2007 · Three statistical partitioning techniques, each designed to answer this question, are compared, in relation to a simple multilevel selection ...
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Smith's (1964) original haystack paper. In his model, selection is strong (b and c are large), and the number of generations G in the group is large.
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ABSTRACT. In organisms possessing a dispersal phase the processes of mating, competition, feeding, andpre- dation are often carried out within "trait-groups ...
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Helping behaviour, reproductive value, and the future component of ...
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The results demonstrate that that kin recognition and self/non-self are two separate identity recognition systems involving soluble chemicals.
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Below is a merged summary of the segments on "Cultural Group Selection in Human Evolution," combining all the information from the provided summaries into a concise yet comprehensive response. To retain the maximum detail, I will use a structured table format in CSV style for key concepts, followed by a narrative summary that integrates additional details and context. This approach ensures all information is preserved while maintaining readability.
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Nov 12, 2009 · We have hypothesized (Richerson & Boyd 2005; Richerson et al. 2005) that the psychological capacities that allow humans to learn from others ...
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Human large-scale cooperation as a product of competition between ...
Feb 4, 2020 · We evaluate a central untested prediction of Cultural Group Selection theory, by assessing whether readiness to cooperate between individuals ...
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Jun 18, 2012 · Group selection sounds like a reasonable extension of evolutionary theory and a plausible explanation of the social nature of humans.
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Jan 20, 2015 · Snowflake yeast develop through deterministic rules that produce geometrically defined clusters that preclude genetic conflict.
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Multilevel selection on individual and group social behaviour in the ...
Mar 19, 2025 · We provide empirical evidence suggesting that multilevel selection acts on social relationships and structures in the wild.
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Group augmentation underlies the evolution of complex sociality in ...
Although kin selection is assumed to underlie the evolution of sociality, many vertebrates—including nearly half of all cooperatively breeding birds—form groups ...<|separator|>
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Multilevel selection on mitochondrial genomes - ScienceDirect
Top: Natural selection acts on mtDNA at multiple levels of biological organisation: cellular, tissue level, organismal, and population level. At the cellular ...
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Applying multilevel selection to understand cancer evolution and ...
Jul 18, 2025 · This essay examines how selection operates across levels, highlighting how we might leverage this understanding to improve cancer research, prevention and ...
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Abundant empirical evidence of multilevel selection revealed by a ...
Jul 31, 2025 · However, natural selection can act at all levels of biological organization, thus being termed 'multilevel selection' (MLS).Missing: advances 2020s
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The Tide of Opinion on Group Selection has Turned - ProSocial World
Apr 18, 2024 · Critics of group selection sometimes accuse proponents of wishful thinking—of wanting to see the world through rose-colored glasses (even though ...Missing: parochial | Show results with:parochial
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The Multilevel Selection Initiative - ProSocial World
A collective effort to establish multilevel selection as a foundational theory for understanding prosocial evolution.Missing: 2020s | Show results with:2020s