On random conformity bias in cultural transmission of polychotomous traits

Mathematical models of conformity and anti-conformity have commonly included a set of simplifying assumptions. For example, (1) there are m=2 cultural variants in the population, (2) naive individuals observe the cultural variants of n=3 adult "role models," and (3) individuals' levels of conformity or anti-conformity do not change over time. Three recent theoretical papers have shown that departures from each of these assumptions can produce new population dynamics. Here, we explore cases in which multiple, or all, of these assumptions are violated simultaneously: namely, in a population with m variants of a trait where conformity (or anti-conformity) occurs with respect to n role models, we study a model in which the conformity rates at each generation are random variables that are independent of the variant frequencies at that generation. For this model a class of symmetric constant equilibria exist, and it is possible that all of these equilibria are simultaneously stochastically locally stable. In such cases, the effect of initial conditions on subsequent evolutionary trajectories becomes very complicated.

Cultural transmission, competition for prey, and the evolution of cooperative hunting

Although cooperative hunting is widespread among animals, its benefits are unclear. At low frequencies, cooperative hunting may allow predators to escape competition and access bigger prey that could not be caught by a lone cooperative predator. Cooperative hunting is a more successful strategy when it is common, but its spread can result in overhunting big prey, which may have a lower per-capita growth rate than small prey. We construct a one-predator species, two-prey species model in which predators either learn to hunt small prey alone or learn to hunt big prey cooperatively. Predators first learn vertically from parents, then horizontally (i.e. socially) from random individuals or siblings. After horizontal transmission, they hunt with their learning partner if both are cooperative, and otherwise they hunt alone. Cooperative hunting cannot evolve when initially rare unless predators (a) interact with siblings, or (b) horizontally transmit the cooperative behavior to potential hunting partners. Whereas competition for small prey favors cooperative hunting when this cooperation is initially rare, the frequency of cooperative hunting cannot reach 100% unless big prey is abundant. Furthermore, a mutant that increases horizontal learning can invade if cooperative hunting is present, but not at 100%, because horizontal learning allows pairs of predators to have the same strategy. Our results reveal that the interactions between prey availability, social learning, and degree of cooperation among predators may have important effects on ecosystems.

Cultural transmission of move choice in chess

The study of cultural evolution benefits from detailed analysis of cultural transmission in specific human domains. Chess provides a platform for understanding the transmission of knowledge due to its active community of players, precise behaviours and long-term records of high-quality data. In this paper, we perform an analysis of chess in the context of cultural evolution, describing multiple cultural factors that affect move choice. We then build a population-level statistical model of move choice in chess, based on the Dirichlet-multinomial likelihood, to analyse cultural transmission over decades of recorded games played by leading players. For moves made in specific positions, we evaluate the relative effects of frequency-dependent bias, success bias and prestige bias on the dynamics of move frequencies. We observe that negative frequency-dependent bias plays a role in the dynamics of certain moves, and that other moves are compatible with transmission under prestige bias or success bias. These apparent biases may reflect recent changes, namely the introduction of computer chess engines and online tournament broadcasts. Our analysis of chess provides insights into broader questions concerning how social learning biases affect cultural evolution.

Cultural niche construction with application to fertility control: A model for education and social transmission of contraceptive use

The evolution of a cultural trait may be affected by niche construction, or changes in the selective environment of that trait due to the inheritance of other cultural traits that make up a cultural background. This study investigates the evolution of a cultural trait, such as the acceptance of the idea of contraception, that is both vertically and horizontally transmitted within a homogeneous social network. Individuals may conform to the norm, and adopters of the trait have fewer progeny than others. In addition, adoption of this trait is affected by a vertically transmitted aspect of the cultural background, such as the preference for high or low levels of education. Our model shows that such cultural niche construction can facilitate the spread of traits with low Darwinian fitness while providing an environment that counteracts conformity to norms. In addition, niche construction can facilitate the 'demographic transition' by making reduced fertility socially accepted.

Drowning in shallow causality

It has been known for decades that inference concerning genetic causes of human behavioral phenotypes cannot be legitimately made from correlations among relatives. We claim that these inferential difficulties cannot be overcome by assigning different names to causes inferred from within-family and population-level genome-wide association studies (GWASs). For educational attainment, for example, unraveling gene-environment interactions requires more than new names for causes.

Age-differentiated incentives for adaptive behavior during epidemics produce oscillatory and chaotic dynamics

Heterogeneity in contact patterns, mortality rates, and transmissibility among and between different age classes can have significant effects on epidemic outcomes. Adaptive behavior in response to the spread of an infectious pathogen may give rise to complex epidemiological dynamics. Here we model an infectious disease in which adaptive behavior incentives, and mortality rates, can vary between two and three age classes. The model indicates that age-dependent variability in infection aversion can produce more complex epidemic dynamics at lower levels of pathogen transmissibility and that those at less risk of infection can still drive complexity in the dynamics of those at higher risk of infection. Policymakers should consider the interdependence of such heterogeneous groups when making decisions.

Infectious diseases may have arrested the southward advance of microblades in Upper Palaeolithic East Asia

An unsolved archaeological puzzle of the East Asian Upper Palaeolithic is why the southward expansion of an innovative lithic technology represented by microblades stalled at the Qinling-Huaihe Line. It has been suggested that the southward migration of foragers with microblades stopped there, which is consistent with ancient DNA studies showing that populations to the north and south of this line had differentiated genetically by 19 000 years ago. Many infectious pathogens are believed to have been associated with hominins since the Palaeolithic, and zoonotic pathogens in particular are prevalent at lower latitudes, which may have produced a disease barrier. We propose a mathematical model to argue that mortality due to infectious diseases may have arrested the wave-of-advance of the technologically advantaged foragers from the north.

Evolutionary modeling suggests that addictions may be driven by competition-induced microbiome dysbiosis

Recent studies revealed mechanisms by which the microbiome affects its host's brain, behavior and wellbeing, and that dysbiosis - persistent microbiome-imbalance - is associated with the onset and progress of various chronic diseases, including addictive behaviors. Yet, understanding of the ecological and evolutionary processes that shape the host-microbiome ecosystem and affect the host state, is still limited. Here we propose that competition dynamics within the microbiome, associated with host-microbiome mutual regulation, may promote dysbiosis and aggravate addictive behaviors. We construct a mathematical framework, modeling the dynamics of the host-microbiome ecosystem in response to alterations. We find that when this ecosystem is exposed to substantial perturbations, the microbiome may shift towards a composition that reinforces the new host state. Such a positive feedback loop augments post-perturbation imbalances, hindering attempts to return to the initial equilibrium, promoting relapse episodes and prolonging addictions. We show that the initial microbiome composition is a key factor: a diverse microbiome enhances the ecosystem's resilience, whereas lower microbiome diversity is more prone to lead to dysbiosis, exacerbating addictions. This framework provides evolutionary and ecological perspectives on host-microbiome interactions and their implications for host behavior and health, while offering verifiable predictions with potential relevance to clinical treatments.

Conformity and anti-conformity in a finite population

Conformist and anti-conformist cultural transmission have been studied both empirically, in several species, and theoretically, with population genetic models. Building upon standard, infinite-population models (IPMs) of conformity, we introduce finite-population models (FPMs) and study them via simulation and a diffusion approximation. In previous IPMs of conformity, offspring observe the variants of n adult role models, where n is often three. Numerical simulations show that while the short-term behavior of the FPM with n=3 role models is well approximated by the IPM, stable polymorphic equilibria of the IPM become effective equilibria of the FPM at which the variation persists prior to fixation or loss, and which produce plateaus in curves for fixation probabilities and expected times to absorption. In the FPM with n=5 role models, the population may switch between two effective equilibria, which is not possible in the IPM, or may cycle between frequencies that are not effective equilibria, which is possible in the IPM. In all observed cases of 'equilibrium switching' and 'cycling' in the FPM, model parameters exceed O(1/N), required for the diffusion approximation, resulting in an over-estimation of the actual times to absorption. However, in those cases with n=5 role models that have one effective equilibrium and stable fixation states, even if conformity coefficients exceed O(1/N), the diffusion approximation matches closely the numerical simulations of the FPM. This suggests that the robustness of the diffusion approximation depends not only on the magnitudes of coefficients, but also on the qualitative behavior of the conformity model.

Conditions that favour cumulative cultural evolution

The emergence of human societies with complex language and cumulative culture is considered a major evolutionary transition. Why such a high degree of cumulative culture is unique to humans is perplexing given the potential fitness advantages of cultural accumulation. Here, Boyd & Richerson's (1996 Why culture is common, but cultural evolution is rare. Proc. Br. Acad. 88, 77-93) discrete-cultural-trait model is extended to incorporate arbitrarily strong selection; conformist, anti-conformist and unbiased frequency-dependent transmission; random and periodic environmental variation; finite population size; and multiple 'skill levels.' From their infinite-population-size model with success bias and a single skill level, Boyd and Richerson concluded that social learning is favoured over individual learning under a wider range of conditions when social learning is initially common than initially rare. We find that this holds only if the number n of individuals observed by a social learner is sufficiently small, but with a finite population and/or a combination of success-biased and conformist or unbiased transmission, this result holds with larger n. Assuming social learning has reached fixation, the increase in a population's mean skill level is lower if cumulative culture is initially absent than initially present, if population size is finite, or if cultural transmission has a frequency-dependent component. Hence, multiple barriers to cultural accumulation may explain its rarity. This article is part of the theme issue 'Human socio-cultural evolution in light of evolutionary transitions'.

A memetic algorithm for finding multiple subgraphs that optimally cover an input network

Finding dense subgraphs is a central problem in graph mining, with a variety of real-world application domains including biological analysis, financial market evaluation, and sociological surveys. While a series of studies have been devoted to finding subgraphs with maximum density, the problem of finding multiple subgraphs that best cover an input network has not been systematically explored. The present study discusses a variant of the densest subgraph problem and presents a mathematical model for optimizing the total coverage of an input network by extracting multiple subgraphs. A memetic algorithm that maximizes coverage is proposed and shown to be both effective and efficient. The method is applied to real-world networks. The empirical meaning of the optimal sampling method is discussed.

How Does Migration Affect HIV Sexual Risk Behaviors Among Involuntary Bachelors? The Mediating Roles of Neighborhoods and Social Networks

The sex ratio imbalance in China since the 1980s has resulted in a large number of involuntary bachelors in rural China. Previous studies have found an association between migration and HIV sexual risk behaviors among involuntary bachelors, but how migration affects these bachelors' HIV sexual risk behaviors remain poorly understood. Using data from a cross-sectional survey in 2017 (a sample of 740 male respondents who had rural household registration, had never been married, and were aged 28-49 years), we investigated the relationship between migration and HIV sexual risk behaviors. Logistic regressions show that migration, neighborhood characteristics, and social networks were significantly associated with commercial sex and multiple sex partners, whereas only neighborhood characteristics and social networks were positively correlated with sexual partnership concurrency. Neighborhood characteristics and social networks mediated the relationships of migration with commercial sex and migration with multiple sex partners. Social networks mediated the association between neighborhood characteristics and concurrency. Multiple-step mediation analysis showed that the indirect effect of migration on commercial sex and multiple sexual partners through neighborhood characteristics and social networks was significant. Our findings suggest that further interventions should address neighborhood characteristics and social networks together.

How Family Living Arrangements and Migration Distances Shape the Settlement Intentions of Rural Migrant Workers in China

Rural migrant workers and their families will decide the future of China's urbanization. Using data from the "China Migrants Dynamic Survey and Hundreds of Villages Investigation" carried out in 2018, we examine whether and how family living arrangements and migration distances shape rural migrant workers' settlement intentions in urban areas. In general, rural migrant workers' settlement intention is shown to be weak. However, individuals with children are more likely to have a stronger intention to settle permanently in urban areas. Among geographical factors, geospatial distance exerts a negative influence on migrant parents' settlement intention when the interaction effect of family living arrangements and migration distances is considered. Migrant families are increasingly concentrated in cities near their hometowns with a low entry barrier that allows them to gain access to better amenities. Socio-economic factors, especially disposable income, human resources, and housing conditions, play significant roles in migrant parents' settlement intention. The age and hometown region of migrant parents are also closely related to their intentions to settle in urban areas. Potential channels for the management of urbanization policy are also explored.

Diversity and its causes: Lewontin on racism, biological determinism and the adaptationist programme

Lewontin's 1972 paper (RC Lewontin, 1972 The apportionment of human diversity, in Evolutionary biology, vol. 6 (eds T Dobzhansky, MK Hecht, WC Steere), pp. 381–398) can be viewed as one foray in his battle against biological determinism. Our paper shows where Lewontin, The apportionment of human diversity, fits in the debate over human classification that it stimulated. We outline three assumptions inherent in the biological deterministic view of human phenotypic diversity and show how the 1972 paper, as well as Lewontin's papers in 1970 and 1974 on the problems with the heritability statistic and his 1979 criticism of naive pan-selectionism, invalidate these assumptions. These papers were crucial components of his campaign against biological determinism and the racism with which it was associated. In the current climate of widespread racism and the rise of sociogenomics, it is important to revisit Lewontin's writings and to disseminate the messages they contain.

This article is part of the theme issue ‘Celebrating 50 years since Lewontin's apportionment of human diversity’.

Hunter-gatherer genomes reveal diverse demographic trajectories during the rise of farming in Eastern Africa

The fate of hunting and gathering populations following the rise of agriculture and pastoralism remains a topic of debate in the study of human prehistory. Studies of ancient and modern genomes have found that autochthonous groups were largely replaced by expanding farmer populations with varying levels of gene flow, a characterization that is influenced by the almost universal focus on the European Neolithic.^1-5 We sought to understand the demographic impact of an ongoing cultural transition to farming in Southwest Ethiopia, one of the last regions in Africa to experience such shifts.⁶ Importantly, Southwest Ethiopia is home to several of the world's remaining hunter-gatherer groups, including the Chabu people, who are currently transitioning away from their traditional mode of subsistence.⁷ We generated genome-wide data from the Chabu and four neighboring populations, the Majang, Shekkacho, Bench, and Sheko, to characterize their genetic ancestry and estimate their effective population sizes over the last 60 generations. We show that the Chabu are a distinct population closely related to ancient people who occupied Southwest Ethiopia >4,500 years ago. Furthermore, the Chabu are undergoing a severe population bottleneck, which began approximately 1,400 years ago. By analyzing eleven Eastern African populations, we find evidence for divergent demographic trajectories among hunter-gatherer-descendant groups. Our results illustrate that although foragers respond to encroaching agriculture and pastoralism with multiple strategies, including cultural adoption of agropastoralism, gene flow, and economic specialization, they often face population decline.

Conformity and content-biased cultural transmission in the evolution of altruism

The evolution of altruism has been extensively modeled under the assumption of genetic transmission, whereas the dynamics under cultural transmission are less well understood. Previous research has shown that cultural transmission can facilitate the evolution of altruism by increasing 1) the probability of adopting the altruistic phenotype, and 2) assortment between altruists. We incorporate vertical and oblique transmission, which can be conformist or anti-conformist, into models of parental care, sibling altruism, and altruism between individuals that meet assortatively. If oblique transmission is conformist, it becomes easier for altruism to invade a population of non-altruists as the probability of vertical transmission increases. If oblique transmission is anti-conformist, decreasing vertical transmission facilitates invasion by altruism in the assortative meeting model, whereas in other models, there is a trade-off: greater vertical transmission produces greater assortment among genetically related altruists, but lowers the probability of adopting altruism via anti-conformity. Compared to conditions for invasion under genetic transmission, e.g., Hamilton's rule, we show that invasion can be easier with sufficiently strong anti-conformity, and in some models, with sufficiently high assortment even if oblique transmission is conformist. We also explore invasion by an allele A that increases individuals' content bias for altruism, in the absence of other forms of cultural transmission. If costs and benefits combine additively, A invades under previously known conditions. If costs and benefits combine multiplicatively, invasion by A and by altruism become more difficult than in the corresponding additive models.

Socioeconomic Status, Institutional Power, and Body Mass Index among Chinese Adults

Despite the vast literature on the socioeconomic status (SES) gradient of obesity among adult people, no study has investigated the relationship between institutional power and body mass index. Using national survey data from the “China Labor-force Dynamics Survey 2016” (CLDS 2016), multistage cluster-stratified probability proportional to size (PPS) sampling was employed to select cases from 29 provinces, cities, and autonomous regions in China. This study adopts an institutional approach to explore the influences of SES and institutional power on the state of being overweight or severely overweight (obese) among Chinese adults. It is shown that SES has a non-linear influence on being overweight or obese, higher education has a negative effect on being overweight or obese, income has an inverted U-shaped effect on being overweight or obese, and having a managerial or administrative job has a positive effect on being overweight but less so on obesity. These findings reveal that disparities in health outcome and risks are due to inequality in SES. The work unit is a stronger predictor of adults being overweight or obese than occupation. Working in the public sector has a positive effect on being overweight relative to working in the private sector, and only state institutions and government departments have a positive association with obesity. Our results indicate that institutional structure still has effects on individuals’ life chances in the era of China’s market transition.

Effects of cultural transmission of surnaming decisions on the sex ratio at birth

The patriarchal tradition of surnaming a child after its father in Han Chinese families may contribute to their preference for sons, a major cause of the abnormally high SRB (sex ratio at birth) in China. This high SRB can subsequently contribute to the marriage squeeze on males of marriageable age. Encouraging matrilineal surnaming has been proposed as a strategy that could potentially reduce son preference and help to adjust the imbalance in SRB. Here, we model factors that are likely to influence surnaming decisions, including cultural transmission of parents' surnaming decisions, the cultural value of a daughter, reward given to matrilineal surnaming, and awareness of current imbalance in SRB. Mathematical and computational analyses suggest that offering a significant reward and raising public awareness of the problems inherent in an excess of marriage-age males may overcome the son preference and reduce the male-biased SRB.

Sexual network and condom use among male migrants in the context of China's gender imbalance

In the context of China's gender imbalance, this study addresses the characteristics of sexual networks and their association with condom use in a sample of 713 male migrants (aged 28-64) who have rural hukou (household registration) in China. Descriptive statistics, univariate analyses, and multilevel random intercept models were used to investigate the characteristics of sexual networks and their associations with condom use. We found that age, marital status, type of sex partners, support (the main help given to each sex partner by the participant), type of sexual intercourse, and stability of sexual relationships were associated with condom use. The sexual networks were mainly composed of sex partners of similar age (58.46%), unmarried people (50.53%), and regular partners (49.38%). Married male migrants were more likely to use condoms with casual partners; unmarried male migrants were less likely to use condoms in emotional and stable relationships. Variation in individual factors, sex partners, and sexual relationship characteristics contribute to participation in condomless sex by male migrants. HIV prevention strategies should target unmarried male migrants and their casual sex partners by increasing their awareness of the risk of HIV transmission and the availability of free condoms.

Non-vertical cultural transmission, assortment and the evolution of cooperation

Cultural evolution of cooperation under vertical and non-vertical cultural transmission is studied, and conditions are found for fixation and coexistence of cooperation and defection. The evolution of cooperation is facilitated by its horizontal transmission and by an association between social interactions and horizontal transmission. The effect of oblique transmission depends on the horizontal transmission bias. Stable polymorphism of cooperation and defection can occur, and when it does, reduced association between social interactions and horizontal transmission evolves, which leads to a decreased frequency of cooperation and lower population mean fitness. The deterministic conditions are compared to outcomes of stochastic simulations of structured populations. Parallels are drawn with Hamilton’s rule incorporating relatedness and assortment.

Marriage, Health, and Old-Age Support: Risk to Rural Involuntary Bachelors' Family Development in Contemporary China

In the traditional system of Chinese families, individuals are embedded in the institution of the family with defined obligations to enhance family development. As a consequence of the male-biased sex ratio at birth in China since the 1980s, an increasing number of surplus rural males have been affected by a marriage squeeze becoming involuntary bachelors. Under China's universal heterosexual marriage tradition, family development of rural involuntary bachelors has largely been ignored, but in China's gender-imbalanced society, it is necessary to adopt a family-based approach to identify and study the plight of rural involuntary bachelors. Studies on gender imbalance indicate that these men face multiple risks from the perspectives of their life course, the family life cycle, and the family ethic. To a certain extent, these risks are caused by a conflict between the individual's family life and family ethics and are mainly reflected in problems concerning marriage, health, and old-age support. Not only do these vulnerabilities affect the individual and family development across the whole life cycle but also pose major risks to social development in the face of strong gender imbalance. In order to deal with risks faced by rural involuntary bachelors, core ethical principles, including autonomy, beneficence, and justice, need to be adopted. Through adjustments to informal support provided by the family and formal support provided by policy-makers, risk of uncertainty in family development faced by rural involuntary bachelors could be reduced.

Migration, Social Networks, and HIV Sexual Risk Behaviors Among Involuntary Bachelors in Rural China

In rural China there is an abnormally high male-biased sex ratio. The result is a large number of involuntary bachelors. This study examines how migration and social networks relate to bachelors' sexual risk behaviors. Data are from a cross-sectional questionnaire survey in which 740 male respondents who had rural household registration, had never married, and were aged 28 or older were interviewed in 2017. Logistic regression reveals that both migration and social networks place the bachelors at an especially high risk of HIV transmission by increasing the chance that they engage in commercial sex, whereas only social networks are significantly associated with sexual partnership concurrency. Additionally, social networks mediate the association between migration and commercial sex. This suggests that social networks play an important role in bachelors' risk of HIV transmission and that further intervention should address their social networks.

Designing gene drives to limit spillover to non-target populations

The prospect of utilizing CRISPR-based gene-drive technology for controlling populations has generated much excitement. However, the potential for spillovers of gene-drive alleles from the target population to non-target populations has raised concerns. Here, using mathematical models, we investigate the possibility of limiting spillovers to non-target populations by designing differential-targeting gene drives, in which the expected equilibrium gene-drive allele frequencies are high in the target population but low in the non-target population. We find that achieving differential targeting is possible with certain configurations of gene-drive parameters, but, in most cases, only under relatively low migration rates between populations. Under high migration, differential targeting is possible only in a narrow region of the parameter space. Because fixation of the gene drive in the non-target population could severely disrupt ecosystems, we outline possible ways to avoid this outcome. We apply our model to two potential applications of gene drives—field trials for malaria-vector gene drives and control of invasive species on islands. We discuss theoretical predictions of key requirements for differential targeting and their practical implications.

CRISPR-based gene drive is an emerging genetic engineering technology that enables engineered genetic variants, which are usually designed to be harmful to the organism carrying them, to be spread rapidly in populations. Although this technology is promising for controlling disease vectors and invasive species, there is a considerable risk that a gene drive could unintentionally spillover from the target population, where it was deployed, to non-target populations. We develop mathematical models of gene-drive dynamics that incorporate migration between target and non-target populations to investigate the possibility of effectively applying a gene drive in the target population while limiting its spillover to non-target populations (‘differential targeting’). We observe that the feasibility of differential targeting depends on the gene-drive design specification, as well as on the migration rates between the populations. Even when differential targeting is possible, as migration increases, the possibility for differential targeting disappears. We find that differential targeting can be effective for low migration rates, and that it is sensitive to the design of the gene drive under high migration rates. We suggest that differential targeting could be used, in combination with other mitigation measures, as an additional safeguard to limit gene drive spillovers.

Adaptive social contact rates induce complex dynamics during epidemics

Epidemics may pose a significant dilemma for governments and individuals. The personal or public health consequences of inaction may be catastrophic; but the economic consequences of drastic response may likewise be catastrophic. In the face of these trade-offs, governments and individuals must therefore strike a balance between the economic and personal health costs of reducing social contacts and the public health costs of neglecting to do so. As risk of infection increases, potentially infectious contact between people is deliberately reduced either individually or by decree. This must be balanced against the social and economic costs of having fewer people in contact, and therefore active in the labor force or enrolled in school. Although the importance of adaptive social contact on epidemic outcomes has become increasingly recognized, the most important properties of coupled human-natural epidemic systems are still not well understood. We develop a theoretical model for adaptive, optimal control of the effective social contact rate using traditional epidemic modeling tools and a utility function with delayed information. This utility function trades off the population-wide contact rate with the expected cost and risk of increasing infections. Our analytical and computational analysis of this simple discrete-time deterministic strategic model reveals the existence of an endemic equilibrium, oscillatory dynamics around this equilibrium under some parametric conditions, and complex dynamic regimes that shift under small parameter perturbations. These results support the supposition that infectious disease dynamics under adaptive behavior change may have an indifference point, may produce oscillatory dynamics without other forcing, and constitute complex adaptive systems with associated dynamics. Implications for any epidemic in which adaptive behavior influences infectious disease dynamics include an expectation of fluctuations, for a considerable time, around a quasi-equilibrium that balances public health and economic priorities, that shows multiple peaks and surges in some scenarios, and that implies a high degree of uncertainty in mathematical projections.

Loss of genetic variation in the two-locus multiallelic haploid model

In the evolutionary biology literature, it is generally assumed that for deterministic frequency-independent haploid selection models, no polymorphic equilibrium can be stable in the absence of variation-generating mechanisms such as mutation. However, mathematical analyses that corroborate this claim are scarce and almost always depend upon additional assumptions. Using ideas from game theory, we show that a monomorphism is a global attractor if one of its alleles dominates all other alleles at its locus. Further, we show that no isolated equilibrium exists, at which an unequal number of alleles from two loci is present. Under the assumption of convergence of trajectories to equilibrium points, we resolve the two-locus three-allele case for a fitness scheme formally equivalent to the classical symmetric viability model. We also provide an alternative proof for the two-locus two-allele case.

The power of randomization by sex in multilocus genetic evolution

Background

Many hypotheses have been proposed for how sexual reproduction may facilitate an increase in the population mean fitness, such as the Fisher-Muller theory, Muller’s ratchet and others. According to the recently proposed mixability theory, however, sexual recombination shifts the focus of natural selection away from favoring particular genetic combinations of high fitness towards favoring alleles that perform well across different genetic combinations. Mixability theory shows that, in finite populations, because sex essentially randomizes genetic combinations, if one allele performs better than another across the existing combinations of alleles, that allele will likely also perform better overall across a vast space of untested potential genotypes. However, this superiority has been established only for a single-locus diploid model.

Results

We show that, in both haploids and diploids, the power of randomization by sex extends to the multilocus case, and becomes substantially stronger with increasing numbers of loci. In addition, we make an explicit comparison between the sexual and asexual cases, showing that sexual recombination is the cause of the randomization effect.

Conclusions

That the randomization effect applies to the multilocus case and becomes stronger with increasing numbers of loci suggests that it holds under realistic conditions. One may expect, therefore, that in nature the ability of an allele to perform well in interaction with existing genetic combinations is indicative of how well it will perform in a far larger space of potential combinations that have not yet materialized and been tested. Randomization plays a similar role in a statistical test, where it allows one to draw an inference from the outcome of the test in a small sample about its expected outcome in a larger space of possibilities—i.e., to generalize. Our results are relevant to recent theories examining evolution as a learning process.

Reviewers

This article was reviewed by David Ardell and Brian Golding.

Evolution of transmission modifiers under frequency-dependent selection and transmission in constant or fluctuating environments

Although the Reduction Principle for rates of mutation, migration, and recombination has been proved for large populations under constant selection, the fate of modifiers of these evolutionary forces under frequency-dependent or fluctuating selection is, in general, less well understood. Here we study modifiers of transmission, which include modifiers of mutation and oblique cultural transmission, under frequency-dependent and cyclically fluctuating selection, and develop models for which the Reduction Principle fails. We show that whether increased rates of transmission can evolve from an equilibrium at which there is zero transmission (for example, no mutation) depends on the number of alleles among which transmission is occurring. In addition, properties of the null-transmission state are clarified.

Disability trajectories in activities of daily living of elderly Chinese before death

This study aims to analyze the changes in activities of daily living (ADL) of the Chinese elderly before death, and to explore the heterogeneity in this process. Using data from the Chinese Longitudinal Healthy Longevity Survey (CLHLS), we quantify disability trajectories of ADL using a group-based trajectory model and find that there are three types of disability trajectory for ADL. The elderly who differ by socioeconomic status, childhood experiences, health behaviors, ages and birth cohorts show significant differences in their disability trajectories. Long duration of disability is found to be more prevalent in older females and people with high socioeconomic status. Good and stable status of ADL is more common among males and people of low socioeconomic status, while the elderly in an early cohort who died at older ages were more likely to have experienced a long duration of disability. Selective and protective effects contribute to the observed differences in trajectories.

Marriage Aspiration, Perceived Marriage Squeeze, and Anomie Among Unmarried Rural Male Migrant Workers in China

Using data from a survey of rural–urban migrants conducted in Xiamen City, China, during 2009, this study explores determinants of anomie among unmarried rural male migrant workers in the context of China’s gender imbalance. Results indicate that the perceived marriage squeeze has exerted direct effects on anomie, and marriage aspiration has indirect effects on anomie among rural male migrant workers. The perceived marriage squeeze also has a mediating effect between marriage aspiration and anomie among unmarried rural male migrant workers. Social integration in the destination city is also a determinant of anomie among these unmarried migrant workers.

A new perspective for mitigation of SARS-CoV-2 infection: priming the innate immune system for viral attack

The course of infection by SARS-CoV-2 frequently includes a long asymptomatic period, followed in some individuals by an immune dysregulation period that may lead to complications and immunopathology-induced death. This course of disease suggests that the virus often evades detection by the innate immune system. We suggest a novel therapeutic approach to mitigate the infection's severity, probability of complications and duration. We propose that priming an individual's innate immune system for viral attack shortly before it is expected to occur may allow pre-activation of the preferable trajectory of immune response, leading to early detection of the virus. Priming can be carried out, for example, by administering a standard vaccine or another reagent that elicits a broad anti-viral innate immune response. By the time that the expected SARS-CoV-2 infection occurs, activation cascades will have been put in motion and levels of immune factors needed to combat the infection will have been elevated. The infection would thus be cleared faster and with less complication than otherwise, alleviating adverse clinical outcomes at the individual level. Moreover, priming may also mitigate population-level risk by reducing need for hospitalizations and decreasing the infectious period of individuals, thus slowing the spread and reducing the impact of the epidemic. In view of the latter consideration, our proposal may have a significant epidemiological impact even if applied primarily to low-risk individuals, such as young adults, who often show mild symptoms or none, by shortening the period during which they unknowingly infect others. The proposed view is, at this time, an unproven hypothesis. Although supported by robust bio-medical reasoning and multiple lines of evidence, carefully designed clinical trials are necessary.

Cultural evolution of conformity and anticonformity

Conformist bias occurs when the probability of adopting a more common cultural variant in a population exceeds its frequency, and anticonformist bias occurs when the reverse is true. Conformist and anticonformist bias have been widely documented in humans, and conformist bias has also been observed in many nonhuman animals. Boyd and Richerson used models of conformist and anticonformist bias to explain the evolution of large-scale cooperation, and subsequent research has extended these models. We revisit Boyd and Richerson's original analysis and show that, with conformity based on more than three role models, the evolutionary dynamics can be more complex than previously assumed. For example, we show the presence of stable cycles and chaos under strong anticonformity and the presence of new equilibria when both conformity and anticonformity act at different variant frequencies, with and without selection. We also investigate the case of population subdivision with migration and find that the common claim that conformity can maintain between-group differences is not always true. Therefore, the effect of conformity on the evolution of cooperation by group selection may be more complicated than previously stated. Finally, using Feldman and Liberman's modifier approach, we investigate the conditions under which a rare modifier of the extent of conformity or the number of role models can invade a population. Understanding the dynamics of conformist- and anticonformist-biased transmission may have implications for research on human and nonhuman animal behavior, the evolution of cooperation, and frequency-dependent transmission in general.

The sexual networks of female sex workers and potential HIV transmission risk: an entertainment venue-based study in Shaanxi, China

People involved in commercial sex are thought to be at high risk for human immunodeficiency virus/acquired immune deficiency syndrome (HIV/AIDS) transmission. To explore the characteristics of female sex workers’ (FSWs) sexual networks and how FSWs and their sex partners could serve as ‘bridges’ in HIV/AIDS transmission, egocentric sexual networks (where a subject is asked to identify his or her sexual contacts and their relationships) of 66 FSWs in Xi'an city, Shaanxi Province of China, were studied. Convenience sampling was used to collect FSWs’ socio-demographic and sexual behavior data, which we analyzed using social network and descriptive statistical methods. Results show that some egocentric sexual networks were connected by sex partners, and these were integrated into several components of a sexual network. According to centrality indicators, FSWs and their commercial sex partners (especially regular clients) served as key nodes within high-risk groups and as bridges between high-risk groups and the general population. The cluster of high-risk groups with cohesive sub-networks had larger network size (P < 0.001), more complex network structures, and more high-risk members (P < 0.05) than other isolated networks. The sexual network of FSWs was characterized by multiple sexual relations (680), unstable relationships (50.15%), and a high rate of inconsistent condom use with non-commercial sex partners (31.22%). By linking commercial and non-commercial sexual networks, the FSWs and their clients can become effective bridges for HIV/AIDS spread from high-risk groups to the general population.

The evolution of frequency-dependent cultural transmission

In a model of vertical and oblique cultural transmission of a dichotomous trait, the rates of transmission of each form of the trait are functions of the trait frequency in the population. Sufficient conditions on these functions are derived for a stable trait polymorphism to exist. If the vertical transmission rates are monotone decreasing functions of the trait frequency, a complete global stability analysis is presented. It is also shown that a unique protected polymorphism can be globally stable even though the sufficient conditions are not met. The evolution of frequency-dependent transmission is modeled using modifier theory, and exact conditions are derived for a transmission modifier to invade a population at a stable polymorphism. Finally, the interaction between frequency-dependent selection and frequency-dependent transmission is explored.

Information diffusion in signed networks

Information diffusion has been widely discussed in various disciplines including sociology, economics, physics or computer science. In this paper, we generalize the linear threshold model in signed networks consisting of both positive and negative links. We analyze the dynamics of the spread of information based on balance theory, and find that a signed network can generate path dependence while structural balance can help remove the path dependence when seeded with balanced initialized active nodes. Simulation shows that the diffusion of information based on positive links contradicts that based on negative links. More positive links in signed networks are more likely to activate nodes and remove path dependence, but they can reduce predictability that is based on active states. We also find that a balanced structure can facilitate both the magnitude and speed of information diffusion, remove the path dependence, and cause polarization.

Sex with partners met online: risky sexual behavior among bachelors in rural China

In the context of gender imbalance and marriage squeeze in China, this study identifies socio-demographic characteristics of bachelors who reported having sex with partners whom they met online and examines associations between having sex with such partners and other risky sexual behaviors. Data are from a cross-sectional survey conducted in 2017. 735 men who have rural household registration (hukou, in Chinese), and who were at least 28 years old and unmarried were interviewed. 16.5% of the sample had experienced sexual intercourse with a partner met online. After adjustment for socio-demographic characteristics, having sex with such partners was associated with a range of risky sexual behaviors: unsafe sexual intercourse (such as anal sex, group sex, not using a condom) (adjusted OR (aOR) = 5.11, 3.14-8.33, p < 0.001); commercial sex (aOR = 4.42, 2.78-7.02, p < 0.001); having sex in public places (aOR = 3.11, 1.97-4.91, p < 0.001); and multiple sexual partners (> = 6 partners) (aOR = 12.57, 6.55-24.12, p < 0.001). This suggests that bachelors who had sexual intercourse with partners whom they met online are at higher risk for HIV or other STD infections. Future interventions targeted at this population will improve the efficiency of China's HIV/STD prevention.

Clonal interference can cause wavelet-like oscillations of multilocus linkage disequilibrium

Within-host adaptation of pathogens such as human immunodeficiency virus (HIV) often occurs at more than two loci. Multiple beneficial mutations may arise simultaneously on different genetic backgrounds and interfere, affecting each other's fixation trajectories. Here, we explore how these evolutionary dynamics are mirrored in multilocus linkage disequilibrium (MLD), a measure of multi-way associations between alleles. In the parameter regime corresponding to HIV, we show that deterministic early infection models induce MLD to oscillate over time in a wavelet-like fashion. We find that the frequency of these oscillations is proportional to the rate of adaptation. This signature is robust to drift, but can be eroded by high variation in fitness effects of beneficial mutations. Our findings suggest that MLD oscillations could be used as a signature of interference among multiple equally advantageous mutations and may aid the interpretation of MLD in data.

The life history of learning: Demographic structure changes cultural outcomes

Human populations show rich cultural diversity. Underpinning this diversity of tools, rituals, and cultural norms are complex interactions between cultural evolutionary and demographic processes. Most models of cultural change assume that individuals use the same learning modes and methods throughout their lives. However, empirical data on ‘learning life histories’—the balance of dominant modes of learning (for example, learning from parents, peers, or unrelated elders) throughout an individual’s lifetime—suggest that age structure may play a crucial role in determining learning modes and cultural evolutionary trajectories. Thus, studied in isolation, demographic and cultural evolutionary models show only part of the picture. This paper describes a mathematical and computational framework that combines demographic and cultural evolutionary methods. Using this general framework, we examine interactions between the ways in which culture is spread throughout an individual’s lifetime and cultural change across generations. We show that including demographic structure alongside cultural dynamics can help to explain domain-specific patterns of cultural evolution that are a persistent feature of cultural data, and can shed new light on rare but significant demographic events.

Human populations show great cultural variety and complexity, which cultural evolutionary theory seeks to explain by applying ideas about evolution to the ways in which cultural traits change over time. We combined cultural evolutionary theory with information about how people learn over their lifetimes—changing their role models and teachers as they grow up. The result is a new theory of the interaction between life histories and learning that gives a more complete description of human cultural change. The results of our model show why different cultural traits might spread in one population compared to another and how cultural change might spark large-scale demographic changes.

Process of Decline in Activities of Daily Living of Older Chinese People Prior to Death: Evidence From Three Cohorts

Objective:

The study analyzes the decline in activities of daily living (ADL) prior to death among three cohorts of older Chinese.

Method:

With data from the Chinese Longitudinal Healthy Longevity Survey, the process of decline in ADL in older people born during the periods 1899–1908, 1909–1918, and 1919–1928 is analyzed using the hierarchical linear model with mixed effects.

Results:

The remaining survival time has a stronger effect on changes in ADL than chronological age, and there is significant heterogeneity among the older adults in ADL.

Conclusion:

Decline in ADL is delayed by extending life span. Older people with healthy behaviors, good living conditions in childhood, and age-friendly living environment have long-lasting good ADL during their remaining life span; socioeconomic resources help the older adults with ADL disabilities to survive. Selective effects of mortality and protective effects of socioeconomic resources explain the heterogeneity in ADL and its changes over time.

Evolution of hierarchy in bacterial metabolic networks

Flow hierarchy is a useful way to characterize the movement of information and matter throughout a network. Hierarchical network organizations are shown to arise when there is a cost of maintaining links in the network. A similar constraint exists in metabolic networks, where costs come from reduced efficiency of nonspecific enzymes or from producing unnecessary enzymes. Previous analyses of bacterial metabolic networks have been used to predict the minimal nutrients that a bacterium needs to grow, its mutualistic relationships with other bacteria, and its major ecological niche. We use metabolic network inference to obtain metabolite flow graphs of 2935 bacterial metabolic networks and find that flow hierarchy evolves independently of modularity and other network properties. By inferring the ancestral metabolic networks and estimating the hierarchical character of the inferred network, we show that hierarchical structure first increased and later decreased over evolutionary history. Furthermore, hierarchical structure in the network is associated with slower growth rates; bacteria with hierarchy scores above the median grow on average 2.25 times faster than those with hierarchy scores below the median.

Sex: The power of randomization

In evolutionary biology, randomness has been perceived as a force that, in and of itself, is capable of inventing: mutation creates new genetic information at random across the genome which leads to phenotypic change, which is then subject to selection. However, in science in general and in computer science in particular, the widespread use of randomness takes a different form. Here, randomization allows for the breaking of pattern, as seen for example in its removal of biases (patterns) by random sampling or random assignment to conditions. Combined with various forms of evaluation, this breaking of pattern becomes an extraordinarily powerful tool, as also seen in many randomized algorithms in computer science. Here we show that this power of randomness is harnessed in nature by sex and recombination. In a finite population, and under the assumption of interactions between genetic variants, sex and recombination allow selection to test how well an allele will perform in a sample of combinations of interacting genetic partners drawn at random from all possible such combinations; consequently, even a small number of tests of genotypes such as takes place in a finite population favors alleles that will most likely perform well in a vast number of yet unrealized genetic combinations. This power of randomization is not manifest in asexual populations.

Interpreting polygenic scores, polygenic adaptation, and human phenotypic differences

Recent analyses of polygenic scores have opened new discussions concerning the genetic basis and evolutionary significance of differences among populations in distributions of phenotypes. Here, we highlight limitations in research on polygenic scores, polygenic adaptation and population differences. We show how genetic contributions to traits, as estimated by polygenic scores, combine with environmental contributions so that differences among populations in trait distributions need not reflect corresponding differences in genetic propensity. Under a null model in which phenotypes are selectively neutral, genetic propensity differences contributing to phenotypic differences among populations are predicted to be small. We illustrate this null hypothesis in relation to health disparities between African Americans and European Americans, discussing alternative hypotheses with selective and environmental effects. Close attention to the limitations of research on polygenic phenomena is important for the interpretation of their relationship to human population differences.

Cryptic selection forces and dynamic heritability in generalized phenotypic evolution

Individuals with different phenotypes can have widely-varying responses to natural selection, yet many classical approaches to evolutionary dynamics emphasize only how a population's average phenotype increases in fitness over time. However, recent experimental results have produced examples of populations that have multiple fitness peaks, or that experience frequency-dependence that affects the direction and strength of selection on certain individuals. Here, we extend classical fitness gradient formulations of natural selection in order to describe the dynamics of a phenotype distribution in terms of its moments-such as the mean, variance, and skewness. The number of governing equations in our model can be adjusted in order to capture different degrees of detail about the population. We compare our simplified model to direct Wright-Fisher simulations of evolution in several canonical fitness landscapes, and we find that our model provides a low-dimensional description of complex dynamics not typically explained by classical theory, such as cryptic selection forces due to selection on trait ranges, time-variation of the heritability, and nonlinear responses to stabilizing or disruptive selection due to asymmetric trait distributions. In addition to providing a framework for extending general understanding of common qualitative concepts in phenotypic evolution - such as fitness gradients, selection pressures, and heritability - our approach has practical importance for studying evolution in contexts in which genetic analysis is infeasible.

Vertical and oblique cultural transmission fluctuating in time and in space

Evolutionary models for a cultural trait under vertical and oblique cultural transmission are analyzed. For a dichotomous trait, both the fitnesses of the variants and their rates of transmission are allowed to vary. In one class of models, transmission fluctuates cyclically together with fitnesses, and conditions are derived for a cultural polymorphism. A second class of models has transmission and selection fluctuating randomly with possible covariance between them. A third class of models involves two populations with migration between them and with transmission rates and fitnesses different in the two populations. Numerical analysis leads to qualitative conditions on the transmission rates and fitnesses that allow protected polymorphisms. With symmetric migration analytical conditions for protected polymorphism are derived.

The evolution of cooperation in signed networks under the impact of structural balance

Structural balance plays an important role in the dynamics of signed networks. Based on structural balance, we generalize the evolution of cooperation in signed networks. Here we develop a new simulation model to study the impact of structural balance on the evolution of cooperation in signed networks. The simulation shows that cooperation prevails when an individual has a higher probability of adjusting the signs of its relations. We also find that structural balance forces the coexistence of cooperators and defectors, while the initial attributes of networks have little impact on the evolution of cooperation in the presence of structural balance, although they have a strong effect on the evolution of structural balance.

Missing compared to what? Revisiting heritability, genes and culture

Standard models for the determination of phenotypes from genes are grounded in simple assumptions that are inherent in the modern evolutionary synthesis (MES), which was developed in the 1930s, 1940s and 1950s. The MES was framed in the context of Mendelian genetic transmission enhanced by the Fisherian view of the way discretely inherited genes determine continuously quantitative phenotypes. The statistical models that are used to estimate and interpret genetic contributions to human phenotypes-including behavioural traits-are constructed within the framework of the MES. Variance analysis constitutes the main tool and is used under this framework to characterize genetic inheritance, and hence determination of phenotypes. In this essay, we show that cultural inheritance, when incorporated into models for the determination of phenotypes, can sharply reduce estimates of the genetic contribution to these phenotypes. Recognition of the importance of non-genetic transmission of many human traits is becoming ever more necessary to prevent regression to the debates of the 1970s and 1980s concerning policies based on genetic determination of complex human phenotypes.This article is part of the theme issue 'Bridging cultural gaps: interdisciplinary studies in human cultural evolution'.

Cultural hitchhiking and competition between patrilineal kin groups explain the post-Neolithic Y-chromosome bottleneck

In human populations, changes in genetic variation are driven not only by genetic processes, but can also arise from cultural or social changes. An abrupt population bottleneck specific to human males has been inferred across several Old World (Africa, Europe, Asia) populations 5000–7000 BP. Here, bringing together anthropological theory, recent population genomic studies and mathematical models, we propose a sociocultural hypothesis, involving the formation of patrilineal kin groups and intergroup competition among these groups. Our analysis shows that this sociocultural hypothesis can explain the inference of a population bottleneck. We also show that our hypothesis is consistent with current findings from the archaeogenetics of Old World Eurasia, and is important for conceptions of cultural and social evolution in prehistory.

A population bottleneck 5000-7000 years ago in human males, but not females, has been inferred across several African, European and Asian populations. Here, Zeng and colleagues synthesize theory and data to suggest that competition among patrilineal kin groups produced the bottleneck pattern.