A mathematical model of the evolution of individual differences in developmental plasticity arising through parental bet-hedging

Individual reversible plasticity as a genotype-level bet-hedging strategy

Reversible plasticity in phenotypic traits allows organisms to cope with environmental variation within lifetimes, but costs of plasticity may limit just how well the phenotype matches the environmental optimum. An additional adaptive advantage of plasticity might be to reduce fitness variance, in other words: bet-hedging to maximize geometric (rather than simply arithmetic) mean fitness. Here, we model the evolution of plasticity in the form of reaction norm slopes, with increasing costs as the slope or degree of plasticity increases. We find that greater investment in plasticity (i.e. a steeper reaction norm slope) is favoured in scenarios promoting bet-hedging as a response to multiplicative fitness accumulation (i.e. coarser environmental grains and fewer time steps prior to reproduction), because plasticity lowers fitness variance across environmental conditions. In contrast, in scenarios with finer environmental grain and many time steps prior to reproduction, bet-hedging plays less of a role and individual-level optimization favours evolution of shallower reaction norm slopes. However, the opposite pattern holds if plasticity costs themselves result in increased fitness variation, as might be the case for production costs of plasticity that depend on how much change is made to the phenotype each time step. We discuss these contrasting predictions from this partitioning of adaptive plasticity into short-term individual benefits versus long-term genotypic (bet-hedging) benefits, and how this approach enhances our understanding of the evolution of optimum levels of plasticity in examples from thermal physiology to advances in avian lay dates.

Cohort splitting from plastic bet-hedging: insights from empirical and theoretical investigations in a wolf spider

Bet-hedging strategies help organisms to decrease variance in their fitness in unpredictably changing environments, by which way lineage fitness can be maximized in the given environment. As one strategy, diversified bet-hedging helps to achieve that by increasing phenotypic variation in fitness-related traits. For example, in diversified tracking, parents may divide the developmental phenotypes of their offspring within broods, leading to cohort splitting among the progeny. Such diversification, though, should be probabilistic and sensitive to no external stimuli. However, it was recently highlighted that plasticity in response to environmental stimuli may be part of a more dynamic case of bet-hedging. Current understanding and empirical observations of such a plastic bet-hedging remain limited. Here I use a theoretical investigation relying on empirical grounds in a specific case of cohort splitting in the wolf spider Pardosa agrestis (Westring 1861). I investigated whether cohort splitting might be a bet-hedging strategy in females of P. agrestis, and whether it would be expected to be static or plastic bet-hedging. Results show that cohort splitting is likely a bet-hedging strategy in this species, by which females maximize their lineage fitness. Also, cohort splitting appears to arise from plastic bet-hedging, as in simulated populations where both static and plastic bet-hedging females occur, the latter have considerably higher geometric mean fitness. I discuss theoretical and empirical observations in light of the current theory, and draw predictions on specific aspects of this case of plastic bet-hedging.

Niche diversity can explain cross-cultural differences in personality structure

The covariance structure of personality traits derived from statistical models (for example, Big Five) is often assumed to be a human universal. Cross-cultural studies have challenged this view, finding that less-complex societies exhibit stronger covariation among behavioural characteristics, resulting in fewer derived personality factors. To explain these results, we propose the niche diversity hypothesis, in which a greater diversity of social and ecological niches elicits a broader range of multivariate behavioural profiles and, hence, lower trait covariance in a population. We formalize this as a computational model, which reproduces empirical results from recent cross-cultural studies and also yields an additional prediction for which we find empirical support. This work provides a general explanation for population differences in personality structure in both humans and other animals and suggests a substantial reimagining of personality research: instead of reifying statistical descriptions of manifest personality structures, research should focus more on modelling their underlying causes.

How learning can change the course of evolution

The interaction between phenotypic plasticity, e.g. learning, and evolution is an important topic both in Evolutionary Biology and Machine Learning. The evolution of learning is commonly studied in Evolutionary Biology, while the use of an evolutionary process to improve learning is of interest to the field of Machine Learning. This paper takes a different point of view by studying the effect of learning on the evolutionary process, the so-called Baldwin effect. A well-studied result in the literature about the Baldwin effect is that learning affects the speed of convergence of the evolutionary process towards some genetic configuration, which corresponds to the environment-induced plastic response. This paper demonstrates that learning can change the outcome of evolution, i.e., lead to a genetic configuration that does not correspond to the plastic response. Results are obtained both analytically and experimentally by means of an agent-based model of a foraging task, in an environment where the distribution of resources follows seasonal cycles and the foraging success on different resource types is conditioned by trade-offs that can be evolved and learned. This paper attempts to answer a question that has been overlooked: whether learning has an effect on what genotypic traits are evolved, i.e. the selection of a trait that enables a plastic response changes the selection pressure on a different trait, in what could be described as co-evolution between different traits in the same genome.

The evolution of early-life effects on social behaviour-why should social adversity carry over to the future?

Numerous studies have shown that social adversity in early life can have long-lasting consequences for social behaviour in adulthood, consequences that may in turn be propagated to future generations. Given these intergenerational effects, it is puzzling why natural selection might favour such sensitivity to an individual's early social environment. To address this question, we model the evolution of social sensitivity in the development of helping behaviours, showing that natural selection indeed favours individuals whose tendency to help others is dependent on early-life social experience. In organisms with non-overlapping generations, we find that natural selection can favour positive social feedbacks, in which individuals who received more help in early life are also more likely to help others in adulthood, while individuals who received no early-life help develop low tendencies to help others later in life. This positive social sensitivity is favoured because of an intergenerational relatedness feedback: patches with many helpers tend to be more productive, leading to higher relatedness within the local group, which in turn favours higher levels of help in the next generation. In organisms with overlapping generations, this positive feedback is less likely to occur, and those who received more help may instead be less likely to help others (negative social feedback). We conclude that early-life social influences can lead to strong between-individual differences in helping behaviour, which can take different forms dependent on the life history in question. This article is part of the theme issue 'Developing differences: early-life effects and evolutionary medicine'.

Developmental Adaptation to Stress: An Evolutionary Perspective

The assumption that early stress leads to dysregulation and impairment is widespread in developmental science and informs prevailing models (e.g., toxic stress). An alternative evolutionary–developmental approach, which complements the standard emphasis on dysregulation, proposes that early stress may prompt the development of costly but adaptive strategies that promote survival and reproduction under adverse conditions. In this review, we survey this growing theoretical and empirical literature, highlighting recent developments and outstanding questions. We review concepts of adaptive plasticity and conditional adaptation, introduce the life history framework and the adaptive calibration model, and consider how physiological stress response systems and related neuroendocrine processes may function as plasticity mechanisms. We then address the evolution of individual differences in susceptibility to the environment, which engenders systematic person–environment interactions in the effects of stress on development. Finally, we discuss stress-mediated regulation of pubertal development as a case study of how an evolutionary–developmental approach can foster theoretical integration.

Prenatal influences on temperament development: The role of environmental epigenetics

This review summarizes current knowledge and outlines future directions relevant to questions concerning environmental epigenetics and the processes that contribute to temperament development. Links between prenatal adversity, epigenetic programming, and early manifestations of temperament are important in their own right, also informing our understanding of biological foundations for social-emotional development. In addition, infant temperament attributes represent key etiological factors in the onset of developmental psychopathology, and studies elucidating their prenatal foundations expand our understanding of developmental origins of health and disease. Prenatal adversity can take many forms, and this overview is focused on the environmental effects of stress, toxicants, substance use/psychotropic medication, and nutrition. Dysregulation associated with attention-deficit/hyperactivity-disruptive disorders was noted in the context of maternal substance use and toxicant exposures during gestation, as well as stress. Although these links can be made based on the existing literature, currently few studies directly connect environmental influences, epigenetic programming, and changes in brain development/behavior. The chain of events starting with environmental inputs and resulting in alterations to gene expression, physiology, and behavior of the organism is driven by epigenetics. Epigenetics provides the molecular mechanism of how environmental factors impact development and subsequent health and disease, including early brain and temperament development.

Echoes of Early Life: Recent Insights From Mathematical Modeling

In the last decades, developmental origins of health and disease (DOHaD) has emerged as a central framework for studying early‐life effects, that is, the impact of fetal and early postnatal experience on adult functioning. Apace with empirical progress, theoreticians have built mathematical models that provide novel insights for DOHaD. This article focuses on three of these insights, which show the power of environmental noise (i.e., imperfect indicators of current and future conditions) in shaping development. Such noise can produce: (a) detrimental outcomes even in ontogenetically stable environments, (b) individual differences in sensitive periods, and (c) early‐life effects tailored to predicted future somatic states. We argue that these insights extend DOHaD and offer new research directions.

Individual differences in developmental plasticity: A role for early androgens?

Developmental plasticity is a widespread property of living organisms, but different individuals in the same species can vary greatly in how susceptible they are to environmental influences. In humans, research has sought to link variation in plasticity to physiological traits such as stress reactivity, exposure to prenatal stress-related hormones such as cortisol, and specific genes involved in major neurobiological pathways. However, the determinants of individual differences in plasticity are still poorly understood. Here we present the novel hypothesis that, in both sexes, higher exposure to androgens during prenatal and early postnatal life should lead to increased plasticity in traits that display greater male variability (i.e., a majority of physical and behavioral traits). First, we review evidence of greater phenotypic variation and higher susceptibility to environmental factors in males; we then consider evolutionary models that explain greater male variability and plasticity as a result of sexual selection. These empirical and theoretical strands converge on the hypothesis that androgens may promote developmental plasticity, at least for traits that show greater male variability. We discuss a number of potential mechanisms that may mediate this effect (including upregulation of neural plasticity), and address the question of whether androgen-induced plasticity is likely to be adaptive or maladaptive. We conclude by offering suggestions for future studies in this area, and considering some research designs that could be used to empirically test our hypothesis.

Developmental plasticity: Bridging research in evolution and human health

Early life experiences can have profound and persistent effects on traits expressed throughout the life course, with consequences for later life behavior, disease risk, and mortality rates. The shaping of later life traits by early life environments, known as 'developmental plasticity', has been well-documented in humans and non-human animals, and has consequently captured the attention of both evolutionary biologists and researchers studying human health. Importantly, the parallel significance of developmental plasticity across multiple fields presents a timely opportunity to build a comprehensive understanding of this phenomenon. We aim to facilitate this goal by highlighting key outstanding questions shared by both evolutionary and health researchers, and by identifying theory and empirical work from both research traditions that is designed to address these questions. Specifically, we focus on: (i) evolutionary explanations for developmental plasticity, (ii) the genetics of developmental plasticity and (iii) the molecular mechanisms that mediate developmental plasticity. In each section, we emphasize the conceptual gains in human health and evolutionary biology that would follow from filling current knowledge gaps using interdisciplinary approaches. We encourage researchers interested in developmental plasticity to evaluate their own work in light of research from diverse fields, with the ultimate goal of establishing a cross-disciplinary understanding of developmental plasticity.

Bridging Evolutionary Biology and Developmental Psychology: Toward An Enduring Theoretical Infrastructure

Bjorklund synthesizes promising research directions in developmental psychology using an evolutionary framework. In general terms, we agree with Bjorklund: Evolutionary theory has the potential to serve as a metatheory for developmental psychology. However, as currently used in psychology, evolutionary theory is far from reaching this potential. In evolutionary biology, formal mathematical models are the norm. In developmental psychology, verbal models are the norm. In order to reach its potential, evolutionary developmental psychology needs to embrace formal modeling.

What Do Evolutionary Models Teach Us About Sensitive Periods in Psychological Development?

Abstract. Sensitive periods in development are widespread in nature. Many psychologists and biologists regard sensitive periods as byproducts of developmental processes. Although this view may be correct in some cases, it is unlikely to be the whole story. There is large variation in sensitive periods (a) between species in the same trait ( Beecher & Brenowitz, 2005 ), (b) between individuals of the same species ( Frankenhuis, Panchanathan, & Belsky, 2016 ), and (c) between different traits within a single individual ( Zeanah, Gunnar, McCall, Kreppner, & Fox, 2011 ). In this article, we discuss recent insights provided by formal models of the evolution of sensitive periods. These models help to identify the conditions in which sensitive periods are likely to evolve, and make predictions about the factors that affect their development. We conclude by discussing future directions for empirical research.

The Evolution of Interaction Shape in Differential Susceptibility

Expectations about the shape of statistical interactions play a crucial role in the study of differential susceptibility and other types of person-environment interplay. These expectations shape methodological guidelines and inform the interpretation of empirical findings; however, their logic has never been explicitly examined. This study is the first systematic exploration of the evolution of interaction shape in differential susceptibility. The model introduced here yields a number of novel insights; for example, interactions in differential susceptibility should usually be asymmetric and likely to be biased toward the prototypical shape of diathesis-stress models. This article also presents an exploratory analysis of interaction shape in recent empirical studies and ends with a discussion of the theoretical and methodological implications of the present findings.

Molecular Signatures of Natural Selection for Polymorphic Genes of the Human Dopaminergic and Serotonergic Systems: A Review

A large body of research has examined the behavioral and mental health consequences of polymorphisms in genes of the dopaminergic and serotonergic systems. Along with this, there has been considerable interest in the possibility that these polymorphisms have developed and/or been maintained due to the action of natural selection. Episodes of natural selection on a gene are expected to leave molecular “footprints” in the DNA sequences of the gene and adjacent genomic regions. Here we review the research literature investigating molecular signals of selection for genes of the dopaminergic and serotonergic systems. The gene SLC6A4, which codes for a serotonin transport protein, was the one gene for which there was consistent support from multiple studies for a selective episode. Positive selection on SLC6A4 appears to have been initiated ∼ 20–25,000 years ago in east Asia and possibly in Europe. There are scattered reports of molecular signals of selection for other neurotransmitter genes, but these have generally failed at replication across studies. In spite of speculation in the literature about selection on these genes, current evidence from population genomic analyses supports selectively neutral processes, such as genetic drift and population dynamics, as the principal drivers of recent evolution in dopaminergic and serotonergic genes other than SLC6A4.

The evolution of sensitive periods in a model of incremental development

Sensitive periods, in which experience shapes phenotypic development to a larger extent than other periods, are widespread in nature. Despite a recent focus on neural-physiological explanation, few formal models have examined the evolutionary selection pressures that result in developmental mechanisms that produce sensitive periods. Here, we present such a model. We model development as a specialization process during which individuals incrementally adapt to local environmental conditions, while receiving a constant stream of cost-free, imperfect cues to the environmental state. We compute optimal developmental programmes across a range of ecological conditions and use these programmes to simulate developmental trajectories and obtain distributions of mature phenotypes. We highlight four main results. First, matching the empirical record, sensitive periods often result from experience or from a combination of age and experience, but rarely from age alone. Second, individual differences in sensitive periods emerge as a result of stochasticity in cues: individuals who obtain more consistent cue sets lose their plasticity at faster rates. Third, in some cases, experience shapes phenotypes only at a later life stage (lagged effects). Fourth, individuals might perseverate along developmental trajectories despite accumulating evidence suggesting the alternate trajectory is more likely to match the ecology.

Adaptive explanations for sensitive windows in development

Development in many organisms appears to show evidence of sensitive windows—periods or stages in ontogeny in which individual experience has a particularly strong influence on the phenotype (compared to other periods or stages). Despite great interest in sensitive windows from both fundamental and applied perspectives, the functional (adaptive) reasons why they have evolved are unclear. Here we outline a conceptual framework for understanding when natural selection should favour changes in plasticity across development. Our approach builds on previous theory on the evolution of phenotypic plasticity, which relates individual and population differences in plasticity to two factors: the degree of uncertainty about the environmental conditions and the extent to which experiences during development (‘cues’) provide information about those conditions. We argue that systematic variation in these two factors often occurs within the lifetime of a single individual, which will select for developmental changes in plasticity. Of central importance is how informational properties of the environment interact with the life history of the organism. Phenotypes may be more or less sensitive to environmental cues at different points in development because of systematic changes in (i) the frequency of cues, (ii) the informativeness of cues, (iii) the fitness benefits of information and/or (iv) the constraints on plasticity. In relatively stable environments, a sensible null expectation is that plasticity will gradually decline with age as the developing individual gathers information. We review recent models on the evolution of developmental changes in plasticity and explain how they fit into our conceptual framework. Our aim is to encourage an adaptive perspective on sensitive windows in development.