Evolutionary Transitions In Individuality Research Articles

An Organismal Perspective on the Evolution of Insect Societies

Insect societies, i.e. the colonies of eusocial ants, bees, wasps and termites, have been likened to multicellular organisms for more than a century. This framework of “superorganisms” has to date largely been used as a mechanistic description of colony functioning, or as an example of an evolutionary transition in individuality. Here I take the superorganismal view a step further, and explore what can potentially be gained if we truly accept insect societies as organisms. I suggest ways to test evolutionary theories about organismal features originally derived for solitary organisms using traits of insect societies as analogies. I explore examples such as evolution of anisogamy, sex allocation and fertilization strategies and life histories, and point out promising directions for comparative work, and potential confounding factors in such analyses, derived from social insect studies.

Major problems in evolutionary transitions: how a metabolic perspective can enrich our understanding of macroevolution

The model of major transitions in evolution (MTE) devised by Maynard Smith and Szathmary has exerted tremendous influence over evolutionary theorists. Although MTE has been criticized for inconsistently combining different types of event, its ongoing appeal lies in depicting hierarchical increases in complexity by means of evolutionary transitions in individuality (ETIs). In this paper, we consider the implications of major evolutionary events overlooked by MTE and its ETI-oriented successors, specifically the biological oxygenation of Earth, and the acquisitions of mitochondria and plastids. By reflecting on these missed events, we reveal a central philosophical disagreement over the explanatory goals of major transitions theory that has yet to be made explicit in the literature. We go on to argue that this philosophical disagreement is only reinforced by Szathmary’s recent revisions of MTE in the form of MTE 2.0. This finding motivates us to propose an alternative explanatory strategy: specifically, an interactionist metabolic perspective on major transitions. A metabolic framework not only avoids many of the criticisms that beset classic and revised MTE models, but also accommodates missing events and provides crucial explanatory components for standard major transitions. Although we do not provide a full-blown alternative theory and do not claim to achieve unity, we explain why foregrounding metabolism is crucial for any attempt to capture the major turning points in evolution, and why it does not lead to unmanageable pluralism.

Social niche construction and evolutionary transitions in individuality.

Social evolution theory conventionally takes an externalist explanatory stance, treating observed cooperation as explanandum and the positive assortment of cooperative behaviour as explanans. We ask how the circumstances bringing about this positive assortment arose in the first place. Rather than merely push the explanatory problem back a step, we move from an externalist to an interactionist explanatory stance, in the spirit of Lewontin and the Niche Construction theorists. We develop a theory of ‘social niche construction’ in which we consider biological entities to be both the subject and object of their own social evolution. Some important cases of the evolution of cooperation have the side-effect of causing changes in the hierarchical level at which the evolutionary process acts. This is because the traits (e.g. life-history bottlenecks) that act to align the fitness interests of particles (e.g. cells) in a collective can also act to diminish the extent to which those particles are bearers of heritable fitness variance, while augmenting the extent to which collectives of such particles (e.g. multicellular organisms) are bearers of heritable fitness variance. In this way, we can explain upward transitions in the hierarchical level at which the Darwinian machine operates in terms of particle-level selection, even though the outcome of the process is a collective-level selection regime. Our theory avoids the logical and metaphysical paradoxes faced by other attempts to explain evolutionary transitions.

Major evolutionary transitions in individuality

The evolution of life on earth has been driven by a small number of major evolutionary transitions. These transitions have been characterized by individuals that could previously replicate independently, cooperating to form a new, more complex life form. For example, archaea and eubacteria formed eukaryotic cells, and cells formed multicellular organisms. However, not all cooperative groups are en route to major transitions. How can we explain why major evolutionary transitions have or haven’t taken place on different branches of the tree of life? We break down major transitions into two steps: the formation of a cooperative group and the transformation of that group into an integrated entity. We show how these steps require cooperation, division of labor, communication, mutual dependence, and negligible within-group conflict. We find that certain ecological conditions and the ways in which groups form have played recurrent roles in driving multiple transitions. In contrast, we find that other factors have played relatively minor roles at many key points, such as within-group kin discrimination and mechanisms to actively repress competition. More generally, by identifying the small number of factors that have driven major transitions, we provide a simpler and more unified description of how life on earth has evolved.

Levels, Time and Fitness in Evolutionary Transitions in Individuality

In this paper, I identify two major problems with the model of evolutionary transitions in individuality (ETIs) developed by Michod and colleagues, and extended by Okasha, commonly referred to as the “export-of-fitness view”. First, it applies the concepts of viability and fertility inconsistently across levels of selection. This leads Michod to claim that once an ETI is complete, lower-level entities composing higher-level individuals have nil fitness. I argue that this claim is mistaken, propose a correct way to translate the concepts of viability and fertility from one level to the other and show that once an ETI is complete, neither viability nor fertility of the lower level entities is nil. Second, the exportof-fitness view does not sufficiently take the parameter of time into account when estimating fitness across levels of selection. As a result fitness is measured over different periods of time at each level. This ultimately means that fitness is measured in different environmental conditions at each level and misleads Okasha into making the claim that the two levels are ontologically distinct levels of selection. I show that once fitness is measured over the same period of time across levels, the claim about two levels of selection can only be an epistemic one.

Eukaryogenesis, how special really?

Eukaryogenesis is widely viewed as an improbable evolutionary transition uniquely affecting the evolution of life on this planet. However, scientific and popular rhetoric extolling this event as a singularity lacks rigorous evidential and statistical support. Here, we question several of the usual claims about the specialness of eukaryogenesis, focusing on both eukaryogenesis as a process and its outcome, the eukaryotic cell. We argue in favor of four ideas. First, the criteria by which we judge eukaryogenesis to have required a genuinely unlikely series of events 2 billion years in the making are being eroded by discoveries that fill in the gaps of the prokaryote:eukaryote “discontinuity.” Second, eukaryogenesis confronts evolutionary theory in ways not different from other evolutionary transitions in individuality; parallel systems can be found at several hierarchical levels. Third, identifying which of several complex cellular features confer on eukaryotes a putative richer evolutionary potential remains an area of speculation: various keys to success have been proposed and rejected over the five-decade history of research in this area. Fourth, and perhaps most importantly, it is difficult and may be impossible to eliminate eukaryocentric bias from the measures by which eukaryotes as a whole are judged to have achieved greater success than prokaryotes as a whole. Overall, we question whether premises of existing theories about the uniqueness of eukaryogenesis and the greater evolutionary potential of eukaryotes have been objectively formulated and whether, despite widespread acceptance that eukaryogenesis was “special,” any such notion has more than rhetorical value.

A Darwinian approach to the origin of life cycles with group properties

A selective explanation for the evolution of multicellular organisms from unicellular ones requires knowledge of both selective pressures and factors affecting the response to selection. Understanding the response to selection is particularly challenging in the case of evolutionary transitions in individuality, because these transitions involve a shift in the very units of selection. We develop a conceptual framework in which three fundamental processes (growth, division, and splitting) are the scaffold for unicellular and multicellular life cycles alike. We (i) enumerate the possible ways in which these processes can be linked to create more complex life cycles, (ii) introduce three genes based on growth, division and splitting that, acting in concert, determine the architecture of the life cycles, and finally, (iii) study the evolution of the simplest five life cycles using a heuristic model of coupled ordinary differential equations in which mutations are allowed in the three genes. We demonstrate how changes in the regulation of three fundamental aspects of colonial form (cell size, colony size, and colony cell number) could lead unicellular life cycles to evolve into primitive multicellular life cycles with group properties. One interesting prediction of the model is that selection generally favors cycles with group level properties when intermediate body size is associated with lowest mortality. That is, a universal requirement for the evolution of group cycles in the model is that the size-mortality curve be U-shaped. Furthermore, growth must decelerate with size.

Geometry shapes evolution of early multicellularity.

Organisms have increased in complexity through a series of major evolutionary transitions, in which formerly autonomous entities become parts of a novel higher-level entity. One intriguing feature of the higher-level entity after some major transitions is a division of reproductive labor among its lower-level units in which reproduction is the sole responsibility of a subset of units. Although it can have clear benefits once established, it is unknown how such reproductive division of labor originates. We consider a recent evolution experiment on the yeast Saccharomyces cerevisiae as a unique platform to address the issue of reproductive differentiation during an evolutionary transition in individuality. In the experiment, independent yeast lineages evolved a multicellular “snowflake-like” cluster formed in response to gravity selection. Shortly after the evolution of clusters, the yeast evolved higher rates of cell death. While cell death enables clusters to split apart and form new groups, it also reduces their performance in the face of gravity selection. To understand the selective value of increased cell death, we create a mathematical model of the cellular arrangement within snowflake yeast clusters. The model reveals that the mechanism of cell death and the geometry of the snowflake interact in complex, evolutionarily important ways. We find that the organization of snowflake yeast imposes powerful limitations on the available space for new cell growth. By dying more frequently, cells in clusters avoid encountering space limitations, and, paradoxically, reach higher numbers. In addition, selection for particular group sizes can explain the increased rate of apoptosis both in terms of total cell number and total numbers of collectives. Thus, by considering the geometry of a primitive multicellular organism we can gain insight into the initial emergence of reproductive division of labor during an evolutionary transition in individuality.

Are ant supercolonies crucibles of a new major transition in evolution?

The biological hierarchy of genes, cells, organisms and societies is a fundamental reality in the living world. This hierarchy of entities did not arise ex nihilo at the origin of life, but rather has been serially generated by a succession of critical events known as 'evolutionary transitions in individuality' (ETIs). Given the sequential nature of ETIs, it is natural to look for candidates to form the next hierarchical tier. We analyse claims that these candidates are found among 'supercolonies', ant populations in which discrete nests cooperate as part of a wider collective, in ways redolent of cells in a multicellular organism. Examining earlier empirical work and new data within the recently proposed 'Darwinian space' framework, we offer a novel analysis of the evolutionary status of supercolonies and show how certain key conditions might be satisfied in any future process transforming these collaborative networks into true Darwinian individuals.

Origins of evolutionary transitions.

An 'evolutionary transition in individuality' or 'major transition' is a transformation in the hierarchical level at which natural selection operates on a population. In this article I give an abstract (i.e. level-neutral and substrate-neutral) articulation of the transition process in order to precisely understand how such processes can happen, especially how they can get started.

Behavioral Responses in Structured Populations Pave the Way to Group Optimality

An unresolved controversy regarding social behaviors is exemplified when natural selection might lead to behaviors that maximize fitness at the social-group level but are costly at the individual level. Except for the special case of groups of clones, we do not have a general understanding of how and when group-optimal behaviors evolve, especially when the behaviors in question are flexible. To address this question, we develop a general model that integrates behavioral plasticity in social interactions with the action of natural selection in structured populations. We find that group-optimal behaviors can evolve, even without clonal groups, if individuals exhibit appropriate behavioral responses to each other's actions. The evolution of such behavioral responses, in turn, is predicated on the nature of the proximate behavioral mechanisms. We model a particular class of proximate mechanisms, prosocial preferences, and find that such preferences evolve to sustain maximum group benefit under certain levels of relatedness and certain ecological conditions. Thus, our model demonstrates the fundamental interplay between behavioral responses and relatedness in determining the course of social evolution. We also highlight the crucial role of proximate mechanisms such as prosocial preferences in the evolution of behavioral responses and in facilitating evolutionary transitions in individuality.

Editorial

Next article No AccessEditorialEditorialHenry S. RichardsonHenry S. Richardson Search for more articles by this author PDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by Ethics Volume 121, Number 1October 2010 Article DOIhttps://doi.org/10.1086/656906 Views: 44Total views on this site Citations: 1Citations are reported from Crossref © 2010 by The University of Chicago. All rights reserved.PDF download Crossref reports the following articles citing this article:Pierrick Bourrat, Guilhem Doulcier, Caroline J Rose, Paul B Rainey, Katrin Hammerschmidt Tradeoff breaking as a model of evolutionary transitions in individuality and limits of the fitness-decoupling metaphor, eLife 11 (Aug 2022).https://doi.org/10.7554/eLife.73715

GENOMICS IN THE LIGHT OF EVOLUTIONARY TRANSITIONS

Molecular biology has entrenched the gene as the basic hereditary unit and genomes are often considered little more than collections of genes. However, new concepts and genomic data have emerged, which suggest that the genome has a unique place in the hierarchy of life. Despite this, a framework for the genome as a major evolutionary transition has not been fully developed. Instead, genome origin and evolution are frequently considered as a series of neutral or nonadaptive events. In this article, we argue for a Darwinian multilevel selection interpretation for the origin of the genome. We base our arguments on the multilevel selection theory of hypercycles of cooperative interacting genes and predictions that gene-level trade-offs in viability and reproduction can help drive evolutionary transitions. We consider genomic data involving mobile genetic elements as a test case of our view. A new concept of the genome as a discrete evolutionary unit emerges and the gene-genome juncture is positioned as a major evolutionary transition in individuality. This framework offers a fresh perspective on the origin of macromolecular life and sets the scene for a new, emerging line of inquiry--the evolutionary ecology of the genome.

Triassic origin and early radiation of multicellular volvocine algae

Evolutionary transitions in individuality (ETIs) underlie the watershed events in the history of life on Earth, including the origins of cells, eukaryotes, plants, animals, and fungi. Each of these events constitutes an increase in the level of complexity, as groups of individuals become individuals in their own right. Among the best-studied ETIs is the origin of multicellularity in the green alga Volvox, a model system for the evolution of multicellularity and cellular differentiation. Since its divergence from unicellular ancestors, Volvox has evolved into a highly integrated multicellular organism with cellular specialization, a complex developmental program, and a high degree of coordination among cells. Remarkably, all of these changes were previously thought to have occurred in the last 50-75 million years. Here we estimate divergence times using a multigene data set with multiple fossil calibrations and use these estimates to infer the times of developmental changes relevant to the evolution of multicellularity. Our results show that Volvox diverged from unicellular ancestors at least 200 million years ago. Two key innovations resulting from an early cycle of cooperation, conflict and conflict mediation led to a rapid integration and radiation of multicellular forms in this group. This is the only ETI for which a detailed timeline has been established, but multilevel selection theory predicts that similar changes must have occurred during other ETIs.

Cooperation and conflict during evolutionary transitions in individuality

Cooperation received much less attention 30 years agothan other forms of ecological interaction, such ascompetition and predation. Workers generally viewedcooperation as being of limited interest, of specialrelevance to certain species (e.g. social insects, birds,humans and our primate relatives) but not of generalsignificance to life on earth. This view has changed, duein large part to the study of evolutionary transitions inindividuality (ETIs). What began as the study of animalsocial behaviour some 40 years ago has now embracedthe study of social interactions at all levels in thehierarchy of life. Instead of being seen as a specialcharacteristic clustered in certain lineages of socialanimals, cooperation is now seen as the primary creativeforce behind ever greater levels of complexity throughthe creation of new kinds of individuals. Cooperationplays this central role in ETIs because it exports fitnessfrom the lower level (its costs) to the new higher level (itsbenefits).How did this shift in understanding the importance ofcooperation come about? Darwin (1859), Wilson (1975)and Hamilton (1963, 1964a,b) all understood the import-ance of cooperation for social organisms. There waspioneering work done as early as 1902 on the importanceof cooperation in the struggle for existence (Kropotkin,1902), and there was the now widely accepted theory ofMargulis (1970, 1981) and others on the endosymbioticorigins of mitochondria and chloroplasts in the eukary-otic cell. However, cooperation was also viewed as adestabilizing force in ecological communities and likely oflimited significance because of the positive feedbackloops it creates (May, 1973). Sociobiology had definedaltruism as its core problem (Wilson, 1975), but thealtruism problem was not viewed as general to life onearth until workers began applying cooperation thinkingto the evolution of interactions at other levels in thehierarchy of life in addition to social organisms, such asto the level of genes within gene groups (e.g. Eigen SPrice, 1970, 1972; e.g. Hamilton, 1975; Wade, 1978;Wilson, 1980). The evolutionary transitions problem(Maynard Smith, 1988, 1991; Maynard Smith & Szathm-a´ry, 1995) grew out of these two developments which, ineffect, extended the sociobiology revolution to all kindsof replicating units in the hierarchy of life.Lehmann & Keller (2006) propose a four-way classi-fication scheme for population models of the evolution ofcooperation, according to the issue of whether thebenefits are direct (individual selection) or indirect (kinselection). Within the first category of direct benefits, adistinction is made according to whether the benefits aremediated through the behaviour of another individual(as through learning in reciprocation) or not. Within thesecond category of kin-selected indirect effects, a distinc-tion is made as to whether there are many genesinvolved in the traits or a few (as in the ‘green-beard’effect).The distinction between direct and indirect effects iswidely used to describe social behaviour and the evolu-tionoffitnesseffects associated withinteractions betweenindividuals. This distinction seems less helpful, however,when one’s interest concerns the origin of the individualsthemselves, i.e. ETIs. Indeed the direct-indirect distinc-tion presumes that one knows what the individual is.Direct or indirect with regard to what? The individual,of course.Transforming our understanding of life is the realiza-tion that evolution occurs not only through evolutionwithin populations but also during ETIs – when groupsbecome so integrated they evolve into a new higher-levelindividual. The major landmarks in the diversification oflife and the hierarchical organization of the living worldare consequences of a series of ETIs: from nonlife to life,from networks of cooperating genes to the first prokary-otic-like cell, from prokaryotic to eukaryotic cells, fromunicellular to multicellular organisms, from asexual tosexual populations, and from solitary to social organisms.It is a major challenge to understand why (environmen-tal selective pressures) and how (underlying genetics,physiology and development) the basic features of anevolutionary individual, such as fitness heritability,indivisibility, and evolvability, shift their reference fromthe old to the new level. Classifying the many factorsinvolved in the evolution of cooperation into a fewgeneral categories as Lehmann & Keller (2006) havedone will certainly help in meeting this challenge.Individuals often associate in groups, and under certainconditions these groups evolve into a new kind ofindividual. Cooperation is fundamental to this processbecause it transfers fitness from the lower-level indivi-duals (in terms of its costs) up to the level of the group(the benefits of cooperation), thereby serving to create anew level of fitness and possibly, under certain condi-tions, a new higher-level individual (Michod, 1999).Indeed, as already mentioned, the major levels in thehierarchy of life (genes, gene networks, cells, eukaryoticcells, multicellular organisms) are thought to haveevolved from this process of individuation of groups(Maynard Smith & Szathma´ry, 1995; Michod, 1999).

The group covariance effect and fitness trade-offs during evolutionary transitions in individuality

Transforming our understanding of life is the realization that evolution occurs not only among individuals within populations but also through the integration of groups of preexisting individuals into a new higher-level individual, that is, through evolutionary transitions in individuality. During evolutionary transitions (such as during the origin of gene networks, bacteria-like cells, eukaryotic cells, multicellular organisms, and societies), fitness must be reorganized; specifically, it must be transferred from the lower- to the higher-level units and partitioned among the lower-level units that specialize in the fitness components of the new higher-level individual. This paper studies the role of fitness trade-offs in fitness reorganization, the evolution of cooperation, and the conversion of a group into a new individual during the origin of multicellular life. Specifically, this study shows that the fitness of the group is augmented over the average fitness of its members according to a covariance effect. This covariance effect appears to be one of the first emergent properties of the group and a general aspect of groups with multiplicative properties that are themselves averages of properties of lower-level units. The covariance effect allows groups to break through the constraints that govern their members, and this effect likely applies to group dynamics in other fields.

On the Reorganization of Fitness During Evolutionary Transitions in Individuality

The basic problem in an evolutionary transition is to understand how a group of individuals becomes a new kind of individual, possessing the property of heritable variation in fitness at the new level of organization. During an evolutionary transition, for example, from single cells to multicellular organisms, the new higher-level evolutionary unit (multicellular organism) gains its emergent properties by virtue of the interactions among lower-level units (cells). We see the formation of cooperative interactions among lower-level units as a necessary step in evolutionary transitions; only cooperation transfers fitness from lower levels (costs to group members) to higher levels (benefits to the group). As cooperation creates new levels of fitness, it creates the opportunity for conflict between levels as deleterious mutants arise and spread within the group. Fundamental to the emergence of a new higher-level unit is the mediation of conflict among lower-level units in favor of the higher-level unit. The acquisition of heritable variation in fitness at the new level, via conflict mediation, requires the reorganization of the basic components of fitness (survival and reproduction) and life-properties (such as immortality and totipotency) as well as the co-option of lower-level processes for new functions at the higher level. The way in which the conflicts associated with the transition in individuality have been mediated, and fitness and general life-traits have been re-organized, can influence the potential for further evolution (i.e., evolvability) of the newly emerged evolutionary individual. We use the volvocalean green algal group as a model-system to understand evolutionary transitions in individuality and to apply and test the theoretical principles presented above. Lastly, we discuss how the different notions of individuality stem from the basic properties of fitness in a multilevel selection context.

Cooperation and conflict in the evolution of individuality. II. Conflict mediation.

Evolutionary transitions in the units of selection require the promotion of cooperation and the regulation of conflict among the lower level units. For multicellular organisms to emerge as a new unit of selection, the selfish tendencies of their component cells had to be controlled. Theoretical results indicate organisms may regulate this internal conflict and competition in several ways: by reducing the somatic mutation rate, by sequestering cells in a germ line and by directly reducing the benefits to cell of defecting.

Locating Synthetic Lethals

Previous articleNext article No AccessLetters to the EditorsLocating Synthetic LethalsJ.M. ThodayJ.M. Thoday Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The American Naturalist Volume 97, Number 896Sep. - Oct., 1963 Published for The American Society of Naturalists Article DOIhttps://doi.org/10.1086/282285 Views: 5Total views on this site Citations: 32Citations are reported from Crossref PDF download Crossref reports the following articles citing this article:Lutz Fromhage, Hanna Kokko Monogamy and haplodiploidy act in synergy to promote the evolution of eusociality, Nature Communications 2, no.11 (Jul 2011).https://doi.org/10.1038/ncomms1410John S Chuang, Olivier Rivoire, Stanislas Leibler Cooperation and Hamilton's rule in a simple synthetic microbial system, Molecular Systems Biology 6, no.11 (Aug 2010): 398.https://doi.org/10.1038/msb.2010.57P. A. Araúz, F. Mestres, C. Pegueroles, C. Arenas, G. Tzannidakis, C. B. 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