Major evolutionary transitions in individuality

Fundamental constraints to the logic of living systems

It has been argued that the historical nature of evolution makes it a highly path-dependent process. Under this view, the outcome of evolutionary dynamics could have resulted in organisms with different forms and functions. At the same time, there is ample evidence that convergence and constraints strongly limit the domain of the potential design principles that evolution can achieve. Are these limitations relevant in shaping the fabric of the possible? Here, we argue that fundamental constraints are associated with the logic of living matter. We illustrate this idea by considering the thermodynamic properties of living systems, the linear nature of molecular information, the cellular nature of the building blocks of life, multicellularity and development, the threshold nature of computations in cognitive systems and the discrete nature of the architecture of ecosystems. In all these examples, we present available evidence and suggest potential avenues towards a well-defined theoretical formulation.

Reciprocal signaling between quorum sensing mutants: A model for division of labor

Division of labor, the specialization of subsets of individuals in complementary tasks, increases population efficiency and fitness. We explored swarming motility in Pseudomonas aeruginosa quorum sensing mutants as a model for studying the division of labor. Deletion of the signal synthesis genes lasI or rhlI disrupts swarming, but co-culturing ΔlasI and ΔrhlI restores it in a density-dependent manner. This indicates a successful division of labor where ΔrhlI produces the signal necessary for the ΔlasI mutant, and the ΔlasI reciprocates. We used RNA sequencing to identify additional genes potentially involved in division of labor. Our findings underscore P. aeruginosa swarming as a tractable bacterial model for the division of labor among cells-a hallmark of differentiated multicellularity.

Evolution of labor division in reproduction and multiple group tasks

Labor division is a phenomenon observed across various biological contexts, including examples such as the differentiation between germ/somatic cells in multicellular organisms and the division between reproductive/worker individuals within social animal groups. In such cases, certain members contribute to tasks that enhance the viability of the entire group, even if this requires a reduction in their individual reproductive efforts. Given that group members have the potential to adopt varying contribution levels, a comprehensive analysis of the evolution becomes intricate due to the problem's high dimensionality. In this paper, I introduce a novel method for analyzing the evolution of the distribution of contribution levels to group viability, with a particular formulation centered on the success of clonal strains. The analysis demonstrates that the curvature of the fecundity function in relation to contributions to the group plays a pivotal role in determining the occurrence of labor division between reproductive and non-reproductive tasks, aligning in part with results from prior research. Furthermore, I extend this analysis to encompass contributions to multiple categories of tasks for group viability. My findings indicate that investments in non-reproductive tasks are selected based on the average contributions for each task, with individual variation playing a less significant role as long as average values remain consistent. Additionally, I explore the impact of group size and relatedness within the group on labor division. The results highlight that increases in group size and relatedness have a positive influence on the evolution of cooperation, although their effects are not directly tied to labor division itself.

Larger colony sizes favoured the evolution of more worker castes in ants

The size-complexity hypothesis is a leading explanation for the evolution of complex life on earth. It predicts that in lineages that have undergone a major transition in organismality, larger numbers of lower-level subunits select for increased division of labour. Current data from multicellular organisms and social insects support a positive correlation between the number of cells and number of cell types and between colony size and the number of castes. However, the implication of these results is unclear, because colony size and number of cells are correlated with other variables which may also influence selection for division of labour, and causality could be in either direction. Here, to resolve this problem, we tested multiple causal hypotheses using data from 794 ant species. We found that larger colony sizes favoured the evolution of increased division of labour, resulting in more worker castes and greater variation in worker size. By contrast, our results did not provide consistent support for alternative hypotheses regarding either queen mating frequency or number of queens per colony explaining variation in division of labour. Overall, our results provide strong support for the size-complexity hypothesis.

Organ Evolution: Emergence of Multicellular Function

Instances of multicellularity across the tree of life have fostered the evolution of complex organs composed of distinct cell types that cooperate, producing emergent biological functions. How organs originate is a fundamental evolutionary problem that has eluded deep mechanistic and conceptual understanding. Here I propose a cell- to organ-level transitions framework, whereby cooperative division of labor originates and becomes entrenched between cell types through a process of functional niche creation, cell-type subfunctionalization, and irreversible ratcheting of cell interdependencies. Comprehending this transition hinges on explaining how these processes unfold molecularly in evolving populations. Recent single-cell transcriptomic studies and analyses of terminal fate specification indicate that cellular functions are conferred by modular gene expression programs. These discrete components of functional variation may be deployed or combined within cells to introduce new properties into multicellular niches, or partitioned across cells to establish division of labor. Tracing gene expression program evolution at the level of single cells in populations may reveal transitions toward organ complexity.

Multiple distinct evolutionary mechanisms govern the dynamics of selfish mitochondrial genomes in Caenorhabditis elegans

Cells possess multiple mitochondrial DNA (mtDNA) copies, which undergo semi-autonomous replication and stochastic inheritance. This enables mutant mtDNA variants to arise and selfishly compete with cooperative (wildtype) mtDNA. Selfish mitochondrial genomes are subject to selection at different levels: they compete against wildtype mtDNA directly within hosts and indirectly through organism-level selection. However, determining the relative contributions of selection at different levels has proven challenging. We overcome this challenge by combining mathematical modeling with experiments designed to isolate the levels of selection. Applying this approach to many selfish mitochondrial genotypes in Caenorhabditis elegans reveals an unexpected diversity of evolutionary mechanisms. Some mutant genomes persist at high frequency for many generations, despite a host fitness cost, by aggressively outcompeting cooperative genomes within hosts. Conversely, some mutant genomes persist by evading inter-organismal selection. Strikingly, the mutant genomes vary dramatically in their susceptibility to genetic drift. Although different mechanisms can cause high frequency of selfish mtDNA, we show how they give rise to characteristically different distributions of mutant frequency among individuals. Given that heteroplasmic frequency represents a key determinant of phenotypic severity, this work outlines an evolutionary theoretic framework for predicting the distribution of phenotypic consequences among individuals carrying a selfish mitochondrial genome.

Unconditional versus condition-dependent social immunity

Socially living animals can counteract disease through cooperative defences, leading to social immunity that collectively exceeds the sum of individual defences. In superorganismal colonies of social insects with permanent caste separation between reproductive queen(s) and nonreproducing workers, workers are obligate altruists and thus engage in unconditional social immunity, including highly specialised and self-sacrificial hygiene behaviours. Contrastingly, cooperation is facultative in cooperatively breeding families, where all members are reproductively totipotent but offspring transiently forgo reproduction to help their parents rear more siblings. Here, helpers should either express condition-dependent social immunity or disperse to pursue independent reproduction. We advocate inclusive fitness theory as a framework to predict when and how indirect fitness gains may outweigh direct fitness costs, thus favouring conditional social immunity.

Enhanced metabolic entanglement emerges during the evolution of an interkingdom microbial community

While different stages of mutualism can be observed in natural communities, the dynamics and mechanisms underlying the gradual erosion of independence of the initially autonomous organisms are not yet fully understood. In this study, by conducting the laboratory evolution on an engineered microbial community, we reproduce and molecularly track the stepwise progression towards enhanced partner entanglement. We observe that the evolution of the community both strengthens the existing metabolic interactions and leads to the emergence of de novo interdependence between partners for nitrogen metabolism, which is a common feature of natural symbiotic interactions. Selection for enhanced metabolic entanglement during the community evolution repeatedly occurred indirectly, via pleiotropies and trade-offs within cellular regulatory networks, and with no evidence of group selection. The indirect positive selection of metabolic dependencies between microbial community members, which results from the direct selection of other coupled traits in the same regulatory network, may therefore be a common but underappreciated driving force guiding the evolution of natural mutualistic communities.

Stability of ecologically scaffolded traits during evolutionary transitions in individuality

Evolutionary transitions in individuality are events in the history of life leading to the emergence of new levels of individuality. Recent studies have described an ecological scaffolding scenario of such transitions focused on the evolutionary consequences of an externally imposed renewing meta-population structure with limited dispersal. One difficulty for such a scenario has been explaining the stability of collective-level traits when scaffolding conditions no longer apply. Here, we show that the stability of scaffolded traits can rely on evolutionary hysteresis: even if the environment is reverted to an ancestral state, collectives do not return to ancestral phenotypes. We describe this phenomenon using a stochastic meta-population model and adaptive dynamics. Further, we show that ecological scaffolding may be limited to Goldilocks zones of the environment. We conjecture that Goldilocks zones-even if they might be rare-could act as initiators of evolutionary transitions and help to explain the near ubiquity of collective-level individuality.

Naturalizing relevance realization: why agency and cognition are fundamentally not computational

The way organismic agents come to know the world, and the way algorithms solve problems, are fundamentally different. The most sensible course of action for an organism does not simply follow from logical rules of inference. Before it can even use such rules, the organism must tackle the problem of relevance. It must turn ill-defined problems into well-defined ones, turn semantics into syntax. This ability to realize relevance is present in all organisms, from bacteria to humans. It lies at the root of organismic agency, cognition, and consciousness, arising from the particular autopoietic, anticipatory, and adaptive organization of living beings. In this article, we show that the process of relevance realization is beyond formalization. It cannot be captured completely by algorithmic approaches. This implies that organismic agency (and hence cognition as well as consciousness) are at heart not computational in nature. Instead, we show how the process of relevance is realized by an adaptive and emergent triadic dialectic (a trialectic), which manifests as a metabolic and ecological-evolutionary co-constructive dynamic. This results in a meliorative process that enables an agent to continuously keep a grip on its arena, its reality. To be alive means to make sense of one's world. This kind of embodied ecological rationality is a fundamental aspect of life, and a key characteristic that sets it apart from non-living matter.

Metabolically-driven flows enable exponential growth in macroscopic multicellular yeast

The ecological and evolutionary success of multicellular lineages is due in no small part to their increased size relative to unicellular ancestors. However, large size also poses biophysical challenges, especially regarding the transport of nutrients to all cells; these constraints are typically overcome through multicellular innovations (e.g., a circulatory system). Here we show that an emergent biophysical mechanism - spontaneous fluid flows arising from metabolically-generated density gradients - can alleviate constraints on nutrient transport, enabling exponential growth in nascent multicellular clusters of yeast lacking any multicellular adaptations for nutrient transport or fluid flow. Surprisingly, beyond a threshold size, the metabolic activity of experimentally-evolved snowflake yeast clusters drives large-scale fluid flows that transport nutrients throughout the cluster at speeds comparable to those generated by the cilia of extant multicellular organisms. These flows support exponential growth at macroscopic sizes that theory predicts should be diffusion limited. This work demonstrates how simple physical mechanisms can act as a 'biophysical scaffold' to support the evolution of multicellularity by opening up phenotypic possibilities prior to genetically-encoded innovations. More broadly, our findings highlight how co-option of conserved physical processes is a crucial but underappreciated facet of evolutionary innovation across scales.

The bubble theory: exploring the transition from first replicators to cells and viruses in a landscape-based scenario

This study proposes a landscape-based scenario for the origin of viruses and cells, focusing on the adaptability of preexisting replicons from the RNP (ribonucleoprotein) world. The scenario postulates that life emerged in a subterranean "warm little pond" where organic matter accumulated, resulting in a prebiotic soup rich in nucleotides, amino acids, and lipids, which served as nutrients for the first self-replicating entities. Over time, the RNA world, followed by the RNP world, came into existence. Replicators/replicons, along with the nutritious soup from the pond, were washed out into the river and diluted. Lipid bubbles, enclosing organic matter, provided the last suitable environment for replicons to replicate. Two survival strategies emerged under these conditions: cell-like structures that obtained nutrients by merging with new bubbles, and virus-like entities that developed various techniques to transmit themselves to fresh bubbles. The presented hypothesis provides the possibility for the common origin of cells and viruses on rocky worlds hosting liquid water, like Earth.

The diversity of social complexity in termites

Sociality underpins major evolutionary transitions and significantly influences the structure and function of complex ecosystems. Social insects, seen as the pinnacle of sociality, have traits like obligate sterility that are considered 'master traits', used as single phenotypic measures of this complexity. However, evidence is mounting that completely aligning both phenotypic and evolutionary social complexity, and having obligate sterility central to both, is erroneous. We hypothesize that obligate and functional sterility are insufficient in explaining the diversity of phenotypic social complexity in social insects. To test this, we explore the relative importance of these sterility traits in an understudied but diverse taxon: the termites. We compile the largest termite social complexity dataset to date, using specimen and literature data. We find that although functional and obligate sterility explain a significant proportion of variance, neither trait is an adequate singular proxy for the phenotypic social complexity of termites. Further, we show both traits have only a weak association with the other social complexity traits within termites. These findings have ramifications for our general comprehension of the frameworks of phenotypic and evolutionary social complexity, and their relationship with sterility.

Empirically testing a relationship between cooperation and the prime numbers

Theoretical models suggest a relationship between cooperation and the prime numbers. In environments where agents play multiple one-shot prisoner's dilemma games per generation, cooperators evolve to fixation more frequently when cooperating on a cyclical schedule with a prime-number period length. This finding parrots classic predator-prey models showing selection for prime-number prey life cycles. Here, I report an empirical test of the former models using previously published data concerning humans playing one-shot public goods games across multiple time points-i.e. an analogue to multiple one-shot prisoner's dilemma games. I find very modest evidence of cyclicality at prime-numbered time intervals, though results indicate rough agreement between theoretical predictions and observed rates of full cooperation across time points. Analyses of individual decisions find increased contributions to the public good at prime-number time points and separate placebo tests indicate a 4-in-1000 chance of spuriously estimating this effect. However, when exploratory analyses exclude low-value prime-numbered time points, the magnitude of the estimated effect decreases and the hypothesis of no effect cannot be rejected, implying that low-value, prime-number time points drive estimates, contrary to theoretical model predictions. These findings cast doubt on the hypothesis of increased cooperation at prime-number time points-at least among humans playing public goods games.

Intracellularity, extracellularity, and squeezing in the symbiotic organ underpin nurturing and functioning of bacterial symbiont in leaf beetles

Cassidine leaf beetles are associated with genome-reduced symbiotic bacteria Stammera involved in pectin digestion. Stammera cells appear to be harbored in paired symbiotic organs located at the foregut-midgut junction either intracellularly or extracellularly, whereas the symbiont is extracellular in the ovary-accessory glands of adult females and during caplet transmission in eggs. However, using fluorescence and electron microscopy, an intracellular symbiotic configuration of Stammera was observed in Notosacantha species. Detailed inspection of other cassidine species revealed fragmented cell membrane and cytoplasm of the symbiotic organs, wherein Stammera cells are in an intermediate status between intracellularity and extracellularity. We also identified a mitochondria-rich region adjacent to the symbiont-filled region and well-developed muscle fibers surrounding the whole symbiotic organ. Based on these observations, we discuss why the Stammera genome has been reduced so drastically and how symbiont-derived pectinases are produced and supplied to the host's alimentary tract for plant cell wall digestion.

Social restructuring during harsh environmental conditions promotes cooperative behaviour in a songbird

Cooperation may emerge from intrinsic factors such as social structure and extrinsic factors such as environmental conditions. Although these factors might reinforce or counteract each other, their interaction remains unexplored in animal populations. Studies on multilevel societies suggest a link between social structure, environmental conditions and individual investment in cooperative behaviours. These societies exhibit flexible social configurations, with stable groups that overlap and associate hierarchically. Structure can be seasonal, with upper-level units appearing only during specific seasons, and lower-level units persisting year-round. This offers an opportunity to investigate how cooperation relates to social structure and environmental conditions. Here, we study the seasonal multilevel society of superb fairy-wrens (Malurus cyaneus), observing individual responses to experimental playback of conspecific distress calls. Individuals engaged more in helping behaviour and less in aggressive/territorial song during the harsher non-breeding season compared to the breeding season. The increase in cooperation was greater for breeding group members than for members of the same community, the upper social unit, comprised of distinct breeding groups in association. Results suggest that the interaction between social structure and environmental conditions drives the seasonal switch in cooperation, supporting the hypothesis that multilevel societies can emerge to increase cooperation during harsh environmental conditions.

Modeling endosymbioses: Insights and hypotheses from theoretical approaches

Endosymbiotic relationships are pervasive across diverse taxa of life, offering key avenues for eco-evolutionary dynamics. Although a variety of experimental and empirical frameworks have shed light on critical aspects of endosymbiosis, theoretical frameworks (mathematical models) are especially well-suited for certain tasks. Mathematical models can integrate multiple factors to determine the net outcome of endosymbiotic relationships, identify broad patterns that connect endosymbioses with other systems, simplify biological complexity, generate hypotheses for underlying mechanisms, evaluate different hypotheses, identify constraints that limit certain biological interactions, and open new lines of inquiry. This Essay highlights the utility of mathematical models in endosymbiosis research, particularly in generating relevant hypotheses. Despite their limitations, mathematical models can be used to address known unknowns and discover unknown unknowns.

Cell-in-cell phenomena across the tree of life

Cells in obligately multicellular organisms by definition have aligned fitness interests, minimum conflict, and cannot reproduce independently. However, some cells eat other cells within the same body, sometimes called cell cannibalism. Such cell-in-cell events have not been thoroughly discussed in the framework of major transitions to multicellularity. We performed a systematic screening of 508 articles, from which we chose 115 relevant articles in a search for cell-in-cell events across the tree of life, the age of cell-in-cell-related genes, and whether cell-in-cell events are associated with normal multicellular development or cancer. Cell-in-cell events are found across the tree of life, from some unicellular to many multicellular organisms, including non-neoplastic and neoplastic tissue. Additionally, out of the 38 cell-in-cell-related genes found in the literature, 14 genes were over 2.2 billion years old, i.e., older than the common ancestor of some facultatively multicellular taxa. All of this suggests that cell-in-cell events may have originated before the origins of obligate multicellularity. Thus, our results show that cell-in-cell events exist in obligate multicellular organisms, but are not a defining feature of them. The idea of eradicating cell-in-cell events from obligate multicellular organisms as a way of treating cancer, without considering that cell-in-cell events are also part of normal development, should be abandoned.

Conflict-reducing innovations in development enable increased multicellular complexity

Obligately multicellular organisms, where cells can only reproduce as part of the group, have evolved multiple times across the tree of life. Obligate multicellularity has only evolved when clonal groups form by cell division, rather than by cells aggregating, as clonality prevents internal conflict. Yet obligately multicellular organisms still vary greatly in 'multicellular complexity' (the number of cells and cell types): some comprise a few cells and cell types, while others have billions of cells and thousands of types. Here, we test whether variation in multicellular complexity is explained by two conflict-suppressing mechanisms, namely a single-cell bottleneck at the start of development, and a strict separation of germline and somatic cells. Examining the life cycles of 129 lineages of plants, animals, fungi and algae, we show using phylogenetic comparative analyses that an early segregation of the germline stem-cell lineage is key to the evolution of more cell types, driven by a strong correlation in the Metazoa. By contrast, the presence of a strict single-cell bottleneck was not related to either the number of cells or the number of cell types, but was associated with early germline segregation. Our results suggest that segregating the germline earlier in development enabled greater evolutionary innovation, although whether this is a consequence of conflict reduction or other non-conflict effects, such as developmental flexibility, is unclear.

Resource adaptation drives the size-complexity rule in termites

The size-complexity rule posits that the evolution of larger cooperative groups should favour more division of labour. Examples include more cell types in larger multicellular organisms, and more polymorphic castes in larger eusocial colonies. However, a correlation between division of labour and group size may reflect a shared response of both traits to resource availability and/or profitability. Here, this possibility was addressed by investigating the evolution of sterile caste number (worker and soldier morphotypes) in termites, a major clade of eusocial insects in which the drivers of caste polymorphism are poorly understood. A novel dataset on 90 termite species was compiled from the published literature. The analysis showed that sterile caste number did increase markedly with colony size. However, after controlling for resource adaptations and phylogeny, there was no evidence for this relationship. Rather, sterile caste number increased with increasing nest-food separation and decreased with soil-feeding, through changes in worker (but not soldier) morphotype number. Further, colony size increased with nest-food separation, thus driving the false correlation between sterile caste number and colony size. These findings support adaptation to higher energy acquisition as key to the rise of complex insect societies, with larger size being a by-product.

Adaptive advantages of restorative RNA editing in fungi for resolving survival-reproduction trade-offs

RNA editing in various organisms commonly restores RNA sequences to their ancestral state, but its adaptive advantages are debated. In fungi, restorative editing corrects premature stop codons in pseudogenes specifically during sexual reproduction. We characterized 71 pseudogenes and their restorative editing in Fusarium graminearum, demonstrating that restorative editing of 16 pseudogenes is crucial for germ tissue development in fruiting bodies. Our results also revealed that the emergence of premature stop codons is facilitated by restorative editing and that premature stop codons corrected by restorative editing are selectively favored over ancestral amino acid codons. Furthermore, we found that ancestral versions of pseudogenes have antagonistic effects on reproduction and survival. Restorative editing eliminates the survival costs of reproduction caused by antagonistic pleiotropy and provides a selective advantage in fungi. Our findings highlight the importance of restorative editing in the evolution of fungal complex multicellularity and provide empirical evidence that restorative editing serves as an adaptive mechanism enabling the resolution of genetic trade-offs.

Phenoptosis and the Various Types of Natural Selection

In the first description of evolution, the fundamental mechanism is the natural selection favoring the individuals best suited for survival and reproduction (selection at the individual level or classical Darwinian selection). However, this is a very reductive description of natural selection that does not consider or explain a long series of known phenomena, including those in which an individual sacrifices or jeopardizes his life on the basis of genetically determined mechanisms (i.e., phenoptosis). In fact, in addition to (i) selection at the individual level, it is essential to consider other types of natural selection such as those concerning: (ii) kin selection and some related forms of group selection; (iii) the interactions between the innumerable species that constitute a holobiont; (iv) the origin of the eukaryotic cell from prokaryotic organisms; (v) the origin of multicellular eukaryotic organisms from unicellular organisms; (vi) eusociality (e.g., in many species of ants, bees, termites); (vii) selection at the level of single genes, or groups of genes; (viii) the interactions between individuals (or more precisely their holobionts) of the innumerable species that make up an ecosystem. These forms of natural selection, which are all effects and not violations of the classical Darwinian selection, also show how concepts as life, species, individual, and phenoptosis are somewhat not entirely defined and somehow arbitrary. Furthermore, the idea of organisms selected on the basis of their survival and reproduction capabilities is intertwined with that of organisms also selected on the basis of their ability to cooperate and interact, even by losing their lives or their distinct identities.

Agential autonomy and biological individuality

What is a biological individual? How are biological individuals individuated? How can we tell how many individuals there are in a given assemblage of biological entities? The individuation and differentiation of biological individuals are central to the scientific understanding of living beings. I propose a novel criterion of biological individuality according to which biological individuals are autonomous agents. First, I articulate an ecological-dynamical account of natural agency according to which, agency is the gross dynamical capacity of a goal-directed system to bias its repertoire to respond to its conditions as affordances. Then, I argue that agents or agential dynamical systems can be agentially dependent on, or agentially autonomous from, other agents and that this agential dependence/autonomy can be symmetrical or asymmetrical, strong or weak. Biological individuals, I propose, are all and only those agential dynamical systems that are strongly agentially autonomous. So, to determine how many individuals there are in a given multiagent aggregate, such as multicellular organism, a colony, symbiosis, or a swarm, we first have to identify how many agential dynamical systems there are, and then what their relations of agential dependence/autonomy are. I argue that this criterion is adequate to the extent that it vindicates the paradigmatic cases, and explains why the paradigmatic cases are paradigmatic, and why the problematic cases are problematic. Finally, I argue for the importance of distinguishing between agential and causal dependence and show the relevance of agential autonomy for understanding the explanatory structure of evolutionary developmental biology.

Open questions in the social lives of viruses

Social interactions among viruses occur whenever multiple viral genomes infect the same cells, hosts, or populations of hosts. Viral social interactions range from cooperation to conflict, occur throughout the viral world, and affect every stage of the viral lifecycle. The ubiquity of these social interactions means that they can determine the population dynamics, evolutionary trajectory, and clinical progression of viral infections. At the same time, social interactions in viruses raise new questions for evolutionary theory, providing opportunities to test and extend existing frameworks within social evolution. Many opportunities exist at this interface: Insights into the evolution of viral social interactions have immediate implications for our understanding of the fundamental biology and clinical manifestation of viral diseases. However, these opportunities are currently limited because evolutionary biologists only rarely study social evolution in viruses. Here, we bridge this gap by (1) summarizing the ways in which viruses can interact socially, including consequences for social evolution and evolvability; (2) outlining some open questions raised by viruses that could challenge concepts within social evolution theory; and (3) providing some illustrative examples, data sources, and conceptual questions, for studying the natural history of social viruses.

Evolutionary consequences of nascent multicellular life cycles

A key step in the evolutionary transition to multicellularity is the origin of multicellular groups as biological individuals capable of adaptation. Comparative work, supported by theory, suggests clonal development should facilitate this transition, although this hypothesis has never been tested in a single model system. We evolved 20 replicate populations of otherwise isogenic clonally reproducing 'snowflake' yeast (Δace2/∆ace2) and aggregative 'floc' yeast (GAL1p::FLO1 /GAL1p::FLO1) with daily selection for rapid growth in liquid media, which favors faster cell division, followed by selection for rapid sedimentation, which favors larger multicellular groups. While both genotypes adapted to this regime, growing faster and having higher survival during the group-selection phase, there was a stark difference in evolutionary dynamics. Aggregative floc yeast obtained nearly all their increased fitness from faster growth, not improved group survival; indicating that selection acted primarily at the level of cells. In contrast, clonal snowflake yeast mainly benefited from higher group-dependent fitness, indicating a shift in the level of Darwinian individuality from cells to groups. Through genome sequencing and mathematical modeling, we show that the genetic bottlenecks in a clonal life cycle also drive much higher rates of genetic drift-a result with complex implications for this evolutionary transition. Our results highlight the central role that early multicellular life cycles play in the process of multicellular adaptation.

Conflict and conflict resolution in the major transitions

Conflict and conflict resolution have been argued to be fundamental to the major transitions in evolution. These were key events in life's history in which previously independently living individuals cooperatively formed a higher-level individual, such as a multicellular organism or eusocial colony. Conflict has its central role because, to proceed stably, the evolution of individuality in each major transition required within-individual conflict to be held in check. This review revisits the role of conflict and conflict resolution in the major transitions, addressing recent work arguing for a minor role. Inclusive fitness logic suggests that differences between the kin structures of clones and sexual families support the absence of conflict at the origin of multicellularity but, by contrast, suggest that key conflicts existed at the origin of eusociality. A principal example is conflict over replacing the founding queen (queen replacement). Following the origin of each transition, conflict remained important, because within-individual conflict potentially disrupts the attainment of maximal individuality (organismality) in the system. The conclusion is that conflict remains central to understanding the major transitions, essentially because conflict arises from differences in inclusive fitness optima while conflict resolution can help the system attain a high degree of coincidence of inclusive fitness interests.

Adaptation in the face of internal conflict: the paradox of the organism revisited

The paradox of the organism refers to the observation that organisms appear to function as coherent purposeful entities, despite the potential for within-organismal components like selfish genetic elements and cancer cells to erode them from within. While it is commonly accepted that organisms may pursue fitness maximisation and can be thought to hold particular agendas, there is a growing recognition that genes and cells do so as well. This can lead to evolutionary conflicts between an organism and the parts that reside within it. Here, we revisit the paradox of the organism. We first outline its conception and relationship to debates about adaptation in evolutionary biology. Second, we review the ways selfish elements may exploit organisms, and the extent to which this threatens organismal integrity. To this end, we introduce a novel classification scheme that distinguishes between selfish elements that seek to distort transmission versus those that seek to distort phenotypic traits. Our classification scheme also highlights how some selfish elements elude a multi-level selection decomposition using the Price equation. Third, we discuss how the organism can retain its status as the primary fitness-maximising agent in the face of selfish elements. The success of selfish elements is often constrained by their strategy and further limited by a combination of fitness alignment and enforcement mechanisms controlled by the organism. Finally, we argue for the need for quantitative measures of both internal conflicts and organismality.

Multispecies interactions shape the transition to multicellularity

The origin of multicellularity transformed the adaptive landscape on Earth, opening diverse avenues for further innovation. The transition to multicellular life is understood as the evolution of cooperative groups which form a new level of individuality. Despite the potential for community-level interactions, most studies have not addressed the competitive context of this transition, such as competition between species. Here, we explore how interspecific competition shapes the emergence of multicellularity in an experimental system with two yeast species, Saccharomyces cerevisiae and Kluyveromyces lactis, where multicellularity evolves in response to selection for faster settling ability. We find that the multispecies context slows the rate of the transition to multicellularity, and the transition to multicellularity significantly impacts community composition. Multicellular K. lactis emerges first and sweeps through populations in monocultures faster than in cocultures with S. cerevisiae. Following the transition, the between-species competitive dynamics shift, likely in part to intraspecific cooperation in K. lactis. Hence, we document an eco-evolutionary feedback across the transition to multicellularity, underscoring how ecological context is critical for understanding the causes and consequences of innovation. By including two species, we demonstrate that cooperation and competition across several biological scales shapes the origin and persistence of multicellularity.

Seasonal wild dance of dual endosymbionts in the pear psyllid Cacopsylla pyricola (Hemiptera: Psylloidea)

Most sap-feeding insects maintain obligate relationships with endosymbiotic bacteria that provide their hosts with essential nutrients. However, knowledge about the dynamics of endosymbiont titers across seasons in natural host populations is scarce. Here, we used quantitative PCR to investigate the seasonal dynamics of the dual endosymbionts "Candidatus Carsonella ruddii" and "Ca. Psyllophila symbiotica" in a natural population of the pear psyllid Cacopsylla pyricola (Hemiptera: Psylloidea: Psyllidae). Psyllid individuals were collected across an entire year, covering both summer and overwintering generations. Immatures harboured the highest titers of both endosymbionts, while the lowest endosymbiont density was observed in males. The density of Carsonella remained high and relatively stable across the vegetative period of the pear trees, but significantly dropped during the non-vegetative period, overlapping with C. pyricola's reproductive diapause. In contrast, the titer of Psyllophila was consistently higher than Carsonella's and exhibited fluctuations throughout the sampling year, which might be related to host age. Despite a tightly integrated metabolic complementarity between Carsonella and Psyllophila, our findings highlight differences in their density dynamics throughout the year, that might be linked to their metabolic roles at different life stages of the host.

Multicellularity and the Need for Communication-A Systematic Overview on (Algal) Plasmodesmata and Other Types of Symplasmic Cell Connections

In the evolution of eukaryotes, the transition from unicellular to simple multicellular organisms has happened multiple times. For the development of complex multicellularity, characterized by sophisticated body plans and division of labor between specialized cells, symplasmic intercellular communication is supposed to be indispensable. We review the diversity of symplasmic connectivity among the eukaryotes and distinguish between distinct types of non-plasmodesmatal connections, plasmodesmata-like structures, and 'canonical' plasmodesmata on the basis of developmental, structural, and functional criteria. Focusing on the occurrence of plasmodesmata (-like) structures in extant taxa of fungi, brown algae (Phaeophyceae), green algae (Chlorophyta), and streptophyte algae, we present a detailed critical update on the available literature which is adapted to the present classification of these taxa and may serve as a tool for future work. From the data, we conclude that, actually, development of complex multicellularity correlates with symplasmic connectivity in many algal taxa, but there might be alternative routes. Furthermore, we deduce a four-step process towards the evolution of canonical plasmodesmata and demonstrate similarity of plasmodesmata in streptophyte algae and land plants with respect to the occurrence of an ER component. Finally, we discuss the urgent need for functional investigations and molecular work on cell connections in algal organisms.

Distinct types of multicellular aggregates in Pseudomonas aeruginosa liquid cultures

Pseudomonas aeruginosa forms suspended multicellular aggregates when cultured in liquid media. These aggregates may be important in disease, and/or as a pathway to biofilm formation. The polysaccharide Psl and extracellular DNA (eDNA) have both been implicated in aggregation, but previous results depend strongly on the experimental conditions. Here we develop a quantitative microscopy-based method for assessing changes in the size distribution of suspended aggregates over time in growing cultures. For exponentially growing cultures of P. aeruginosa PAO1, we find that aggregation is mediated by cell-associated Psl, rather than by either eDNA or secreted Psl. These aggregates arise de novo within the culture via a growth process that involves both collisions and clonal growth, and Psl non-producing cells do not aggregate with producers. In contrast, we find that stationary phase (overnight) cultures contain a different type of multicellular aggregate, in which both eDNA and Psl mediate cohesion. Our findings suggest that the physical and biological properties of multicellular aggregates may be very different in early-stage vs late-stage bacterial cultures.

Is chimerism associated with cancer across the tree of life?

Chimerism is a widespread phenomenon across the tree of life. It is defined as a multicellular organism composed of cells from other genetically distinct entities. This ability to 'tolerate' non-self cells may be linked to susceptibility to diseases like cancer. Here we test whether chimerism is associated with cancers across obligately multicellular organisms in the tree of life. We classified 12 obligately multicellular taxa from lowest to highest chimerism levels based on the existing literature on the presence of chimerism in these species. We then tested for associations of chimerism with tumour invasiveness, neoplasia (benign or malignant) prevalence and malignancy prevalence in 11 terrestrial mammalian species. We found that taxa with higher levels of chimerism have higher tumour invasiveness, though there was no association between malignancy or neoplasia and chimerism among mammals. This suggests that there may be an important biological relationship between chimerism and susceptibility to tissue invasion by cancerous cells. Studying chimerism might help us identify mechanisms underlying invasive cancers and also could provide insights into the detection and management of emerging transmissible cancers.

The symbiotic origin of the eukaryotic cell

Eukaryogenesis represented a major evolutionary transition that led to the emergence of complex cells from simpler ancestors. For several decades, the most accepted scenario involved the evolution of an independent lineage of proto-eukaryotes endowed with an endomembrane system, including a nuclear compartment, a developed cytoskeleton and phagocytosis, which engulfed the alphaproteobacterial ancestor of mitochondria. However, the recent discovery by metagenomic and cultural approaches of Asgard archaea, which harbour many genes in common with eukaryotes and are their closest relatives in phylogenomic trees, rather supports scenarios based on the symbiosis of one Asgard-like archaeon and one or more bacteria at the origin of the eukaryotic cell. Here, we review the recent discoveries that led to this conceptual shift, briefly evoking current models of eukaryogenesis and the challenges ahead to discriminate between them and to establish a detailed, plausible scenario that accounts for the evolution of eukaryotic traits from those of their prokaryotic ancestors.

Molecular patterns and processes in evolving sociality: lessons from insects

Social insects have provided some of the clearest insights into the origins and evolution of collective behaviour. Over 20 years ago, Maynard Smith and Szathmáry defined the most complex form of insect social behaviour-superorganismality-among the eight major transitions in evolution that explain the emergence of biological complexity. However, the mechanistic processes underlying the transition from solitary life to superorganismal living in insects remain rather elusive. An overlooked question is whether this major transition arose via incremental or step-wise modes of evolution. We suggest that examination of the molecular processes underpinning different levels of social complexity represented across the major transition from solitary to complex sociality can help address this question. We present a framework for using molecular data to assess to what extent the mechanistic processes that take place in the major transition to complex sociality and superorganismality involve nonlinear (implying step-wise evolution) or linear (implying incremental evolution) changes in the underlying molecular mechanisms. We assess the evidence for these two modes using data from social insects and discuss how this framework can be used to test the generality of molecular patterns and processes across other major transitions. This article is part of a discussion meeting issue 'Collective behaviour through time'.

Four reasons for scepticism about a human major transition in social individuality

The 'major transitions in evolution' are mainly about the rise of hierarchy, new individuals arising at ever higher levels of nestedness, in particular the eukaryotic cell arising from prokaryotes, multicellular individuals from solitary protists and individuated societies from multicellular individuals. Some lists include human societies as a major transition, but based on a comparison with the non-human transitions, there are reasons for scepticism. (i) The foundation of the major transitions is hierarchy, but the cross-cutting interactions in human societies undermine hierarchical structure. (ii) Natural selection operates in three modes-stability, growth and reproductive success-and only the third produces the complex adaptations seen in fully individuated higher levels. But human societies probably evolve mainly in the stability and growth modes. (iii) Highly individuated entities are marked by division of labour and commitment to morphological differentiation, but in humans differentiation is mostly behavioural and mostly reversible. (iv) As higher-level individuals arise, selection drains complexity, drains parts, from lower-level individuals. But there is little evidence of a drain in humans. In sum, a comparison with the other transitions gives reasons to doubt that human social individuation has proceeded very far, or if it has, to doubt that it is a transition of the same sort. This article is part of the theme issue 'Human socio-cultural evolution in light of evolutionary transitions'.

Human cooperation and evolutionary transitions in individuality

A major evolutionary transition in individuality involves the formation of a cooperative group and the transformation of that group into an evolutionary entity. Human cooperation shares principles with those of multicellular organisms that have undergone transitions in individuality: division of labour, communication, and fitness interdependence. After the split from the last common ancestor of hominoids, early hominins adapted to an increasingly terrestrial niche for several million years. We posit that new challenges in this niche set in motion a positive feedback loop in selection pressure for cooperation that ratcheted coevolutionary changes in sociality, communication, brains, cognition, kin relations and technology, eventually resulting in egalitarian societies with suppressed competition and rapid cumulative culture. The increasing pace of information innovation and transmission became a key aspect of the evolutionary niche that enabled humans to become formidable cooperators with explosive population growth, the ability to cooperate and compete in groups of millions, and emergent social norms, e.g. private property. Despite considerable fitness interdependence, the rise of private property, in concert with population explosion and socioeconomic inequality, subverts potential transition of human groups into evolutionary entities due to resurgence of latent competition and conflict. This article is part of the theme issue 'Human socio-cultural evolution in light of evolutionary transitions'.

Can models of evolutionary transition clarify the debates over the Neolithic Revolution?

The 'Neolithic Revolution,' sometimes referred to as the emergence of agriculture at its earliest in the southern Levant, is the most significant shift in human history, shaping the world we live in today. Yet, after 100 years of study, its major cause, tempo (gradual or revolutionary), and impact of human intentionality remain disputed. Here, we examine the research potential of an evolutionary transition in individuality (ETI) to clarify this dramatic shift. Applying an ETI research perspective reveals how different causes and conditions lead to the same result, enabling a holistic view rather than a reduction of 'Neolithic' to 'agriculture,' or to one major climatic condition, inheritance system or standard evolutionary model, thus allowing us to clarify and bypass some of these heated, unresolved disputes. Additionally, unlike current archaeological emphasis on 'where,' 'when,' 'why' and 'how' questions, the ETI perspective offers a productive path for resolving a fundamental preliminary anomaly: why and how could the Neolithic lifeway evolve at all, given the selfish interest of individuals in a hunter-gatherer group? We do not intend to solve the shift to Neolithic lifeways, only to offer a fresh lens for examining it, emphasizing the relevance of tracking within and between group differences. This article is part of the theme issue 'Human socio-cultural evolution in light of evolutionary transitions'.

Philopatry yields higher fitness than dispersal in a cooperative breeder with sex-specific life history trajectories

Social evolution is tightly linked to dispersal decisions, but the ecological and social factors selecting for philopatry or dispersal often remain obscure. Elucidating selection mechanisms underlying alternative life histories requires measurement of fitness effects in the wild. We report on a long-term field study of 496 individually marked cooperatively breeding fish, showing that philopatry is beneficial as it increases breeding tenure and lifetime reproductive success in both sexes. Dispersers predominantly join established groups and end up in smaller groups when they ascend to dominance. Life history trajectories are sex specific, with males growing faster, dying earlier, and dispersing more, whereas females more likely inherit a breeding position. Increased male dispersal does not seem to reflect an adaptive preference but rather sex-specific differences in intrasexual competition. Cooperative groups may thus be maintained because of inherent benefits of philopatry, of which females seem to get the greater share in social cichlids.

Epithelial-to-mesenchymal transition as a learning paradigm of cell biology

Epithelial-to-mesenchymal transition (EMT) is a complex biological process that occurs during normal embryogenesis and in certain pathological conditions, particularly in cancer. EMT can be viewed as a cell biology-based process, since it involves all the cellular components, including the plasma membrane, cytoskeleton and extracellular matrix, endoplasmic reticulum, Golgi apparatus, lysosomes, and mitochondria, as well as cellular processes, such as regulation of gene expression and cell cycle, adhesion, migration, signaling, differentiation, and death. Therefore, we propose that EMT could be used to motivate undergraduate medical students to learn and understand cell biology. Here, we describe and discuss the involvement of each cellular component and process during EMT. To investigate the density with which different cell biology concepts are used in EMT research, we apply a bibliometric approach. The most frequent cell biology topics in EMT studies were regulation of gene expression, cell signaling, cell cycle, cell adhesion, cell death, cell differentiation, and cell migration. Finally, we suggest that the study of EMT could be incorporated into undergraduate disciplines to improve cell biology understanding among premedical, medical and biomedical students.

Co-expression Gene Networks and Machine-learning Algorithms Unveil a Core Genetic Toolkit for Reproductive Division of Labour in Rudimentary Insect Societies

The evolution of eusociality requires that individuals forgo some or all their own reproduction to assist the reproduction of others in their group, such as a primary egg-laying queen. A major open question is how genes and genetic pathways sculpt the evolution of eusociality, especially in rudimentary forms of sociality-those with smaller cooperative nests when compared with species such as honeybees that possess large societies. We lack comprehensive comparative studies examining shared patterns and processes across multiple social lineages. Here we examine the mechanisms of molecular convergence across two lineages of bees and wasps exhibiting such rudimentary societies. These societies consist of few individuals and their life histories range from facultative to obligately social. Using six species across four independent origins of sociality, we conduct a comparative meta-analysis of publicly available transcriptomes. Standard methods detected little similarity in patterns of differential gene expression in brain transcriptomes among reproductive and non-reproductive individuals across species. By contrast, both supervised machine learning and consensus co-expression network approaches uncovered sets of genes with conserved expression patterns among reproductive and non-reproductive phenotypes across species. These sets overlap substantially, and may comprise a shared genetic "toolkit" for sociality across the distantly related taxa of bees and wasps and independently evolved lineages of sociality. We also found many lineage-specific genes and co-expression modules associated with social phenotypes and possible signatures of shared life-history traits. These results reveal how taxon-specific molecular mechanisms complement a core toolkit of molecular processes in sculpting traits related to the evolution of eusociality.

The social evolution of individual differences: Future directions for a comparative science of personality in social behavior

Personality is essential for understanding the evolution of cooperation and conflict in behavior. However, personality science remains disconnected from the field of social evolution, limiting our ability to explain how personality and plasticity shape phenotypic adaptation in social behavior. Researchers also lack an integrative framework for comparing personality in the contextualized and multifaceted behaviors central to social interactions among humans and other animals. Here we address these challenges by developing a social evolutionary approach to personality, synthesizing theory, methods, and organizing questions in the study of individuality and sociality in behavior. We critically review current measurement practices and introduce social reaction norm models for comparative research on the evolution of personality in social environments. These models demonstrate that social plasticity affects the heritable variance of personality, and that individual differences in social plasticity can further modify the rate and direction of adaptive social evolution. Future empirical studies of frequency- and density-dependent social selection on personality are crucial for further developing this framework and testing adaptive theory of social niche specialization.

Genetic conflicts and the case for licensed anthropomorphizing

The use of intentional language in biology is controversial. It has been commonly applied by researchers in behavioral ecology, who have not shied away from employing agential thinking or even anthropomorphisms, but has been rarer among researchers from more mechanistic corners of the discipline, such as population genetics. One research area where these traditions come into contact—and occasionally clash—is the study of genetic conflicts, and its history offers a good window to the debate over the use of intentional language in biology. We review this debate, paying particular attention to how this interaction has played out in work on genomic imprinting and sex chromosomes. In light of this, we advocate for a synthesis of the two approaches, a form of licensed anthropomorphizing. Here, agential thinking’s creative potential and its ability to identify the fulcrum of evolutionary pressure are combined with the rigidity of formal mathematical modeling.

Microbial warfare and the evolution of symbiosis

Cooperative symbionts enable their hosts to exploit a diversity of environments. A low genetic diversity (high relatedness) between the symbionts within a host is thought to favour cooperation by reducing conflict within the host. However, hosts will not be favoured to transmit their symbionts (or commensals) in costly ways that increase relatedness, unless this also provides an immediate fitness benefit to the host. We suggest that conditionally expressed costly competitive traits, such as antimicrobial warfare with bacteriocins, could provide a relatively universal reason for why hosts would gain an immediate benefit from increasing the relatedness between symbionts. We theoretically test this hypothesis with a simple illustrative model that examines whether hosts should manipulate relatedness, and an individual-based simulation, where host control evolves in a structured population. We find that hosts can be favoured to manipulate relatedness, to reduce conflict between commensals via this immediate reduction in warfare. Furthermore, this manipulation evolves to extremes of high or low vertical transmission and only in a narrow range is partly vertical transmission stable.

Resource sharing is sufficient for the emergence of division of labour

Division of labour occurs in a broad range of organisms. Yet, how division of labour can emerge in the absence of pre-existing interindividual differences is poorly understood. Using a simple but realistic model, we show that in a group of initially identical individuals, division of labour emerges spontaneously if returning foragers share part of their resources with other group members. In the absence of resource sharing, individuals follow an activity schedule of alternating between foraging and other tasks. If non-foraging individuals are fed by other individuals, their alternating activity schedule becomes interrupted, leading to task specialisation and the emergence of division of labour. Furthermore, nutritional differences between individuals reinforce division of labour. Such differences can be caused by increased metabolic rates during foraging or by dominance interactions during resource sharing. Our model proposes a plausible mechanism for the self-organised emergence of division of labour in animal groups of initially identical individuals. This mechanism could also play a role for the emergence of division of labour during the major evolutionary transitions to eusociality and multicellularity.

New opportunities in plant microbiome engineering for increasing agricultural sustainability under stressful conditions

Plant microbiome (or phytomicrobiome) engineering (PME) is an anticipated untapped alternative strategy that could be exploited for plant growth, health and productivity under different environmental conditions. It has been proven that the phytomicrobiome has crucial contributions to plant health, pathogen control and tolerance under drastic environmental (a)biotic constraints. Consistent with plant health and safety, in this article we address the fundamental role of plant microbiome and its insights in plant health and productivity. We also explore the potential of plant microbiome under environmental restrictions and the proposition of improving microbial functions that can be supportive for better plant growth and production. Understanding the crucial role of plant associated microbial communities, we propose how the associated microbial actions could be enhanced to improve plant growth-promoting mechanisms, with a particular emphasis on plant beneficial fungi. Additionally, we suggest the possible plant strategies to adapt to a harsh environment by manipulating plant microbiomes. However, our current understanding of the microbiome is still in its infancy, and the major perturbations, such as anthropocentric actions, are not fully understood. Therefore, this work highlights the importance of manipulating the beneficial plant microbiome to create more sustainable agriculture, particularly under different environmental stressors.

Rapid acquisition of microorganisms and microbial genes can help explain punctuated evolution

The punctuated mode of evolution posits that evolution occurs in rare bursts of rapid evolutionary change followed by long periods of genetic stability (stasis). The accepted cause for the rapid changes in punctuated evolution is special ecological circumstances – selection forces brought about by changes in the environment. This article presents a complementary explanation for punctuated evolution by the rapid formation of genetic variants in animals and plants by the acquisition of microorganisms from the environment into microbiomes and microbial genes into host genomes by horizontal gene transfer. Several examples of major evolutionary events driven by microorganisms are discussed, including the formation of the first eukaryotic cell, the ability of some animals to digest cellulose and other plant cell-wall complex polysaccharides, dynamics of root system architecture, and the formation of placental mammals. These changes by cooperation were quantum leaps in the evolutionary development of complex bilolgical systems and can contribute to an understanding of the mechanisms underlying punctuated evolution.

Tradeoff breaking as model of evolutionary transitions in individuality and the limits of the fitness-decoupling metaphor

Evolutionary transitions in individuality (ETIs) involve the formation of Darwinian collectives from Darwinian particles. The transition from cells to multicellular life is a prime example. During an ETI, collectives become units of selection in their own right. However, the underlying processes are poorly understood. One observation used to identify the completion of an ETI is an increase in collective-level performance accompanied by a decrease in particle-level performance, for example measured by growth rate. This seemingly counterintuitive dynamic has been referred to as fitness decoupling and has been used to interpret both models and experimental data. Extending and unifying results from the literature, we show that fitness of particles and collectives can never decouple because calculations of fitness performed over appropriate and equivalent time intervals are necessarily the same provided the population reaches a stable collective size distribution. By way of solution, we draw attention to the value of mechanistic approaches that emphasise traits, and tradeoffs among traits, as opposed to fitness. This trait-based approach is sufficient to capture dynamics that underpin evolutionary transitions. In addition, drawing upon both experimental and theoretical studies, we show that while early stages of transitions might often involve tradeoffs among particle traits, later—and critical—stages are likely to involve the rupture of such tradeoffs. Thus, when observed in the context of ETIs, tradeoff-breaking events stand as a useful marker of these transitions.

Multicellularity in animals: The potential for within-organism conflict

Metazoans function as individual organisms but also as “colonies” of cells whose single-celled ancestors lived and reproduced independently. Insights from evolutionary biology about multicellular group formation help us understand the behavior of cells: why they cooperate, and why cooperation sometimes breaks down. Current explanations for multicellularity focus on two aspects of development which promote cooperation and limit conflict among cells: a single-cell bottleneck, which creates organisms composed of clones, and a separation of somatic and germ cell lineages, which reduces the selective advantage of cheating. However, many obligately multicellular organisms thrive with neither, creating the potential for within-organism conflict. Here, we argue that the prevalence of such organisms throughout the Metazoa requires us to refine our preconceptions of conflict-free multicellularity. Evolutionary theory must incorporate developmental mechanisms across a broad range of organisms—such as unusual reproductive strategies, totipotency, and cell competition—while developmental biology must incorporate evolutionary principles. To facilitate this cross-disciplinary approach, we provide a conceptual overview from evolutionary biology for developmental biologists, using analogous examples in the well-studied social insects.

The emergence of group fitness

Whether and how selection can act on collectives rather than single entities has been a tumultuous issue in evolutionary biology for decades. Despite examples of multilevel selection, a simple framework is needed that makes explicit the constraints that lead to the emergence of a "group fitness function." We use evolutionary game theory to show that two constraints are sufficient for the emergence of a well-defined group fitness, which could even apply to multispecies groups. First, different parts of the group contribute to one another's growth via resources produced proportionally to the density of each resource producer (not the density of the population receiving benefits). Second, invading groups do not share these resources with resident groups. Jointly, these two constraints lead to the "entanglement" of invading individuals' outcomes such that individual fitness can no longer be defined and group fitness predicts evolutionary dynamics through the emergence of a higher level evolutionary individual. Group fitness is an emergent property, irreducible to the fitness of the group's parts and exhibiting downward causality on the parts. By formalizing group fitness as a model for evolutionary transitions in individuality, these results open up a broad class of models under the multilevel-selection framework.