Major Transitions (Evolution)

Le theme de l’individuation et de la persistance des entites vivantes constitue l’une des problematiques centrales de la philosophie de la biologie. Dans les sciences du vivant, l’organisme represente traditionnellement l’exemple paradigmatique de l’individu biologique. Toutefois, malgre le lien intuitif entre l’organisme et l’individu, la biologie contemporaine a su faire un usage plus large du concept d’individu. Ainsi, la circonscription de ce qui etait apprehende comme appartenant au vivant a ete elargie par la reconnaissance de plusieurs types d’individus biologiques, parmi lesquels nous trouvons non seulement les organismes dits « traditionnels », mais egalement toute une variete d’entites telles que des protistes, des molecules d’ARN, des prions, des virus et des bacteries de toutes sortes. Par ailleurs, cette nouvelle vision de l’individu biologique a ete utilisee afin d’illustrer les differents niveaux de l’organisation biologique. En ce sens, la discussion porte principale...

Since its inception, the universe has evolved towards increasing complexity due to entropy and natural forces. Once the universe was cooled sufficiently by its expansion, “free-living” particles united to form hydrogen atoms. Gravity united matter into compact, high-energy formations (stars). Conditions within the first generation of stars drove the formation of more complex elements. The existence of these elements allowed the creation of planetary systems. Favourable conditions on Earth and the diversity of elements enabled the creation of further complexity in the form of life. This complexity has not evolved in a linear manner but in leaps towards higher levels. Emergence of each new level, or “universal state”, depends on a universal qualitative change: a “universal transition”. On Earth, life inherited this tendency of evolution towards complexity by uniting matter in increasing levels of individuality, from organic molecules to molecular complexes, cells, multicellularity, and complex populations. The emergence of a new level is allowed by “scale constraints”, which prevent eternal accumulation of complexity at lower levels, forcing the systems towards a new higher-level, i.e., the occurrence of an evolutionary transition in individuality (ETI). In humans, sociocultural and technological evolution may be guiding our species to a transition towards a global superorganism.

abstract: The evolution of human sociality is a field in ferment, with writers struggling to isolate elementary causal forces and organize them systematically. The elements of a usable model for evolved human sociality have become available only within the past few years. Those elements are scattered throughout the books here under review and a small set of articles. None of the books or articles fully exemplifies the whole model. After laying out the model, I use it to evaluate the books, describing how each contributes to it, and measuring each against it. The central idea in a usable model of human sociality is that the identity of the social group is integral to individual identity. In addition to that one central idea, a minimum of seven concepts is necessary to construct a model of sociality that includes the complex forms of organization in post-agricultural societies: (1) dominance, (2) egalitarianism or reverse dominance, (3) leadership, (4) internalized norms, (5) strong reciprocity or third-party enforcement of norms, (6) legal institutions, and (7) legitimacy in the exercise of power. These seven concepts can be reduced to four components: power, values, individuals, and groups. This model of evolved human sociality moves beyond the inconclusive debate between proponents of inclusive fitness and proponents of group selection. It also offers a distinct alternative to the identity politics that currently pervade literary and cultural study.

The theme about individuation and persistence of living entities is one of the central issues in the philosophy of biology. Embrace the different types of entities inhabiting the biological world, as the tentative to define their limits well, question biological ontology directly. In this perspective, it is important to focus first on the place occupied by the organism in biology. In the life sciences, the organism represents traditionally the paradigmatic example of the biological individual. However, it is important to emphasize that, despite the intuitive connection between the organism and the individual, contemporary biology has been able to make wider use of the concept of the individual. Thus, the contours of what was apprehended as belonging to the living world has been enlarged by the recognition of several types of biological individuals, among which we find not only so-called "traditional" organisms, but also a variety of entities such as protists, RNA molecules, prions, viruses and bacteria of all kinds. In addition, this new vision of the biological individual has been used to illustrate the different levels of biological organization. This idea is confirmed by the logical structure of the principle of natural selection, which is sufficiently abstract to be applied at all levels of biological organization. Thus the notion of “unit of selection” has become the most common criterion as defining a biological individual, emphasizing its evolutionary character. An individual corresponds, according to this vision, to an entity subject to the force of natural selection, whose causal power acts on this unity as a whole. In this sense, the discussion focuses on the level - genes, cells, organisms, superorganisms, species, ecosystems - where selection acts, in which the organism can be considered a simple level between a variety of levels of selection.
Consequently, the extension of the limits of biological ontology allowed, on the one hand, to question the concept of the individual by releasing it from the reference to the organism, and on the other hand, to relativize the importance of the organism within the biological discourse. From there, philosophers and biologists question the necessity of erecting organisms as an indispensable category for thinking about life. However, we argue that, to contribute to the development of biological individuality theme, we must give voice to another discourse on individuality, which emerges from discussions on the ontology of organisms. It is towards the systemic tradition that we must turn our attention, in order to extend research on the nature of biological individuality in the light of a renewed definition of the organism. Thus, we assumes an important distinction between two non-exclusive biological tendencies about the problem of the biological individual: the Darwinian individual on one side, and the ontogenetic individual on the other.
In an ontogenetic approach, an individual, far from being the entity that undergoes the action of selection during evolutionary time, corresponds to the entity that self-determines during its individual life time. Thus, this approach is not based on a hierarchical evolutionary vision in which the organism represents a simple level of analysis between a plurality of biological individuals. On the contrary, it is based on a more physiological conception, centered on biological organization. The question of the ontology of organisms then takes the form of an inquiry into the peculiarity of this form of biological individuality which is, since Kant, characterized by natural objects made of things related into “the unity of a whole by being reciprocally cause and effect of their form”.

This article presents a general theory of the evolution of the evolutionary mechanisms that discover and perpetuate adaptations in living systems. I propose that new evolutionary mechanisms arise because they overcome the limitation in the ability of evolutionary mechanisms that operate at the level of individual entities to systematically discover beneficial cooperative arrangements between the individuals. Evolutionary mechanisms that arise to overcome this limitation are hierarchical in structure. This is because the limitation can be comprehensively overcome by a form of organization in which arrangements intervene across a dynamic of individuals to sustain beneficial cooperative adaptations. These intervening arrangements must be in hierarchical relationship with the dynamic of interacting individuals if they are to escape the limitations of the evolutionary processes that operate at the level of individuals in the dynamic. Variation in these interventions, and sorting of this variation on the basis of its benefit to the organization, will allow beneficial cooperative adaptations to be discovered and sustained. The repeated arising of evolutionary mechanisms in this way produces the familiar nested hierarchies of living processes, and, as each new level of organization is formed, extends the scope in space and time of cooperation among living processes. The paper identifies other key metaevolutionary trends within and across levels of organization of living processes, and considers the evolution of human organization from this metaevolutionary perspective.

Within the past few years, theoretical biology, evolutionary social science, and evolutionary literary study have been correcting basic mistakes, producing new concepts, and reaching a more complete and adequate understanding of human behavior, including cultural and literary behavior. This chapter integrates these new concepts and demonstrates how they can be used to understand specific literary works. Separate sections are devoted to ideas from theoretical biology about the sources of cooperation in all living things, to ideas from the social sciences about human social organization, to ideas from evolutionary aesthetics about the adaptive function of the arts, and to ideas from literary theory about how meaning works in fiction. A section discussing specific literary examples illustrates the way the arts help generate group identity, integrate individuals into groups, and mediate between the needs of individuals and the claims of groups. Seven concepts from the research on evolved human sociality are used to organize the literary examples: dominance, reverse dominance, internalized norms, leadership, strong reciprocity, legal institutions, and legitimacy.

What are the prospects for a monistic view of biological individuality given the multiple epistemic roles the concept must satisfy? In this paper, I examine the epistemic adequacy of two recent accounts based on the capacity to undergo natural selection. One is from Ellen Clarke, and the other is by Peter Godfrey- Smith. Clarke’s position reflects a strong monism, in that she aims to characterize individuality in purely functional terms and refrains from privileging any specific material properties as important in their own right. I argue that Clarke’s functionalism impairs the epistemic adequacy of her account compared to a middle-ground position taken by Godfrey-Smith. In comparing Clarke and Godfrey-Smith’s account, two pathways emerge to pluralism about biological individuality. The first develops from the contrast between functionalist and materialist approaches, and the second from an underlying temporal structure involved in using evolutionary processes to define individuality.

For decades Darwinian processes were framed in the form of the Lewontin conditions: reproduction, variation and reproductive success taken to be sufficient and necessary. Since Buss (1987) and the work of Maynard Smith and Szathmáry (1995) biologists were eager to explain the major transitions from individuals to groups forming new individuals subject to Darwinian mechanisms themselves. Explanations that seek to explain the emergence of a new level of selection, however, cannot employ properties that would already have to exist on that level for selection to take place. Hammerschmidt et al. (2014) provided an experiment corroborating much of the theoretical work Paul Rainey has done since 2003 on how new Darwinian individuals on a multicellular level can occur with a relaxed version of the Lewontin conditions. In this paper I will evaluate the significance of their results for future research and the debates surrounding multi-level selection.

The transition to multicellular life was one of a few major events in the history of life that created new opportunities for more complex biological systems to evolve. Indeed, multicellularity is a prerequisite for the evolution of large, complex organisms such as plants and animals. An understanding of the ecological conditions and evolutionary mechanisms that favor this key innovation is necessary to determine where and with what likelihood complex life may have arisen beyond Earth. Using the model unicellular alga Chlamydomonas reinhardtii, we have experimentally generated de novo origins of simple (undifferentiated) multicellularity in two separate experiments. We subjected genetically diverse populations of C. reinhardtii to two different selective pressures, both of which favor increased size. The first experiment used low-speed centrifugation to select for an increased rate of settling out of liquid medium. In a subset of independent populations this type of selection results in heritable multicellularity: amorphous clusters consisting of daughter cells that fail to separate develop from single cells that have dispersed from other clusters. The second experiment used a predator, the ciliate Paramecium tetraurelia, as the agent of selection. Two of five experimental populations evolved multicellular structures not observed in any of the three control populations. Furthermore, the form of multicellularity observed in the predation experiment differs substantially from that in the centrifugation experiment, with highly structured, spheroidal colonies reminiscent of volvocine algae such as Eudorina.

Among biologists, there is no general agreement on exactly what entities qualify as ‘organisms’. Instead, there are multiple competing organism concepts and definitions. While some authors think this is a problem that should be corrected, others have suggested that biology does not actually need an organism concept. We argue that the organism concept is central to biology and should not be abandoned. Both organism concepts and operational definitions are useful. We review criteria used for recognizing organisms and conclude that they are not categorical but rather continuously variable. Different organism concepts are useful for addressing different questions, and it is important to be explicit about which is being used. Finally, we examine the origins of the derived state of organismality, and suggest that it may result from positive feedback between natural selection and functional integration in biological entities.

The volvocine green algal genus Volvox includes ∼20 species with diverse sizes (in terms of both diameter and cell number), morphologies, and developmental programs. Two suites of characters are shared among distantly related lineages within Volvox. The traits characteristic of all species of Volvox—large (>500) numbers of small somatic cells, much smaller numbers of reproductive cells, and oogamy in sexual reproduction—have three or possibly four separate origins. In addition, some species have evolved a suite of developmental characters that differs from the ancestral developmental program. Most multicellular volvocine algae, including some species of Volvox, share an unusual pattern of cell division known as palintomy or multiple fission. Asexual reproductive cells (gonidia) grow up to many times their initial size and then divide several times in rapid succession, with little or no growth between divisions. Three separate Volvox lineages have evolved a reduced form of palintomy in which reproductive cells are small and grow between cell divisions. In each case, these changes are accompanied by a reduction in the rate of cell division and by a requirement of light for cell division to occur. Thus, two suites of characters—those characteristic of all Volvox species and those related to reduced palintomy—have each evolved convergently or in parallel in lineages that diverged at least 175 million years ago (mya).

Evolution's Arrow argues that evolution is directional and progressive, and that this has major consequences for humanity. Without resort to teleology, the book demonstrates that evolution moves in the direction of producing cooperative organisations of greater scale and evolvability - evolution has organised molecular processes into cells, cells into organisms, and organisms into societies. The book founds this position on a new theory of the evolution of cooperation. It shows that self-interest at the level of the genes does not prevent cooperation from increasing as evolution unfolds. Evolution progresses by discovering ways to build cooperative organisations out of self-interested individuals. The book also shows that evolution itself has evolved. Evolution has progressively improved the ability of evolutionary mechanisms to discover effective adaptations. And it has produced new and better mechanisms. Evolution's Arrow uses this understanding of the direction of evolution to identify the next great steps in the evolution of life on earth - the steps that humanity must take if we are to continue to be successful in evolutionary terms. A key step for humanity is to increase the scale and evolvability of our societies, eventually forming a unified and cooperative society on the scale of the planet. We must also transform ourselves psychologically to become self-evolving organisms - organisms that are able to escape their biological and cultural past by adapting in whatever directions are necessary to achieve future evolutionary success.

The Evolutionary Manifesto shows that evolution is directional and demonstrates that this has major implications for humanity. The Manifesto reveals that humanity must align its social syatems and behaviour with the trajectory of evolution if we are to survive and thrive into the future. The Manifesto goes on to show that humanity has an essential role to play in the future evolution of life on this planet. It demonstrates that life on Earth has reached a critical stage in evolution’s trajectory. Up to now evolution has moved along its trajectory blindly and unconsciously. But if evolution on Earth is to continue to advance successfully in the future, the process will have to be driven intentionally and consciously by humanity. In this way, a proper understanding of evolution shows us how we can live a life that matters in a larger scheme of things. An evolutionary worldview answers the fundamental existential question that faces us all: “What should I do with my life?” The Manifesto outlines in broad terms what humanity must do to advance the evolutionary process—how we will intentionally have to re-make our political, social and economic systems, and how we will need to develop our cognition and consciousness. As such, the Manifesto presents a detailed case for evolutionary activism--coordinated action directed at advancing the evolutionary process intentionally.

Two great trends are evident in the evolution of life on Earth: towards increasing diversification and towards increasing integration. Diversification has spread living processes across the planet, progressively increasing the range of environments and free energy sources exploited by life. Integration has proceeded through a stepwise process in which living entities at one level are integrated into cooperative groups that become larger-scale entities at the next level, and so on, producing cooperative organizations of increasing scale (for example, cooperative groups of simple cells gave rise to the more complex eukaryote cells, groups of these gave rise to multi-cellular organisms, and cooperative groups of these organisms produced animal societies). The trend towards increasing integration has continued during human evolution with the progressive increase in the scale of human groups and societies. The trends towards increasing diversification and integration are both driven by selection. An understanding of the trajectory and causal drivers of the trends suggests that they are likely to culminate in the emergence of a global entity. This entity would emerge from the integration of the living processes, matter, energy and technology of the planet into a global cooperative organization. Such an integration of the results of previous diversifications would enable the global entity to exploit the widest possible range of resources across the varied circumstances of the planet. This paper demonstrates that it's case for directionality meets the tests and criticisms that have proven fatal to previous claims for directionality in evolution.

This thesis is about the complicated and multifaceted problem of the evolution of the eumetazoan body plan and its key role in the emergence of organismal complexity in the animal kingdom. As such, it draws on a very broad range of topics and discussions including empirical research programmes in palaeontology, comparative morphology, classical and genetic developmental biology, and morphological and molecular phylogenetics; as well as theoretical/philosophical research around the conceptual bases of phylogenetics, homology, scientific integration, and biological complexity and individuality. In the following chapters, I first provide background on and an outline of the thesis (chapter 1); I then propose an overarching conceptual framework for the integration of different kinds of evidence in macroevolutionary biology (chapter 2), with a special emphasis on the integration of morphological and developmental genetic evidence in inferring morphological homology (chapter 3); followed by providing a phylogeny of the major animal groups incorporating the two Ediacaran fossils Dickinsonia and Yorgia (chapter 4) and building on this phylogenetic placement to propose a novel evolutionary scenario for the evolution of key features of the eumetazoan body plan—namely the gastric cavity and bilateral symmetry—in light of Dickinsonia and other Ediacaran fossils (chapter 5). I finish with a discussion on the evolution of complexity in the animal kingdom in light of preceding chapters and the multilevel selection literature (chapter 6), and a brief final conclusion.

Functional specialization, or division of labour (DOL), of parts within organisms and colonies is common in most multi-cellular, colonial and social organisms, but it is far from ubiquitous. Several mechanisms have been proposed to explain the evolutionary origins of DOL; the basic feature common to all of them is that functional differences can arise easily. These mechanisms cannot explain the many groups of colonial and social animals that exhibit no DOL despite up to 500 million years of evolution. Here, I propose a new hypothesis, based on a multi-level selection theory, which predicts that a reproductive DOL is required to evolve prior to subsequent functional specialization. I test this hypothesis using a dataset consisting of the type of DOL for living and extinct colonial and social animals. The frequency distribution of DOL and the sequence of its acquisition confirm that reproductive specialization evolves prior to functional specialization. A corollary of this hypothesis is observed in colonial, social and also within multi-cellular organisms; those species without a reproductive DOL have a smaller range of internal variation, in terms of the number of polymorphs or cell types, than species with a reproductive DOL.

Most of us grow up learning that cheating is bad. We should not steal. We should not lie. And above all: we should not exploit the groups we are part of, whether this is our family, our circle of friends, or even humanity itself. The implication is that cooperation and trust would be driven to extinction if such behavior were to spread – leading the downfall of not only the group, but also the individual cheats themselves [Kant sends his regards].

In this paper I argue that discussion about the technosphere and the Earth’s possible current geological transition needs to be situated within wider reflection about how technospheres might arise on other worlds. Engaging with astrobiological speculation about ‘exo-technospheres’ can help us to understand whether technospheres are likely, what their preconditions might be, and whether they endure.  Engaging with science fiction can help us to avoid observer biases that encourage linear assumptions about the preconditions and emergence of technospheres.  Exploring earlier major transitions in Earth’s evolution can shed light on the shifting distribution of metabolic and reproductive powers between the human and technological parts of the contemporary technosphere.  The long-term evolution of technical objects also suggests that they have shown a tendency to pass through their own major transitions in their relation to animality.  Such reflection can shed new light on the nature and likely future development of the Earth’s technosphere.

This paper presents a modified random network
model to illustrate how groups can form in the absence of
evolutionary forces, assuming groups are collections of
entities at any level of organization. This model is inspired
by the Zero Force Evolutionary Law, which states that
there is always a tendency for diversity and complexity to
increase in any evolutionary system containing variation
and heredity. That is, in the absence of evolutionary forces,
the expectation is a continual increase in diversity and
complexity at any level of biological hierarchy. I show that,
when modeled, this expectation of increasing variation
results not only in the formation of groups, but also in a
higher probability of group formation than is found in a
model that is purely random.

The transition from unicellular to differentiated multicellular organisms constitutes an increase in the level complexity, because previously existing individuals are combined to form a new, higher-level individual. The volvocine algae represent a unique opportunity to study this transition because they diverged relatively recently from unicellular relatives and because extant species display a range of intermediate grades between unicellular and multicellular, with functional specialization of cells. Following the approach Darwin used to understand "organs of extreme perfection" such as the vertebrate eye, this jump in complexity can be reduced to a series of small steps that cumulatively describe a gradual transition between the two levels. We use phylogenetic reconstructions of ancestral character states to trace the evolution of steps involved in this transition in volvocine algae. The history of these characters includes several well-supported instances of multiple origins and reversals. The inferred changes can be understood as components of cooperation–conflict–conflict mediation cycles as predicted by multilevel selection theory. One such cycle may have taken place early in volvocine evolution, leading to the highly integrated colonies seen in extant volvocine algae. A second cycle, in which the defection of somatic cells must be prevented, may still be in progress.

Per Ahlberg of Upsalla Universitet, Upsalla Sweden has reconstructed what is proposed by evolutionists as a major example of the supposed evolution of fishes into a clade of land-dwelling amphibian tetrapods. It seems the inspiration for this reconstruction was based upon a single piece of bone less than 3cm long which Alhberg found while sifting through bone fragments in a museum drawer. That a single bone or bone fragment can ignite the imagination of an evolutionist to imagine an entire creature which is presumedly evidence of evolutionary transition brings Nebraska Man to mind. Ahlber's fossil reconstruction of Icthyostega seems to be highly imaginative but not scientific because it does not conform to the known skeletal features of Icthyostegids and other fossil vertebrates of similar homology. I debated this matter with Mr. Ahlberg, but was not able to get straight answers from him about how his reconstruction is scientifically sound. Evolutionist have made much effort to marry the earth's fossil record of life with Darwinian theory. The proposed transition from the fossils of Celocanth, Sauripteris, Osteolepis, Panderichthys. Eusthenopteron, and Tiktaalik to Ichthyostega, especially Ahlberg's reconstruction, requires the transition of features which is not apparent in these fossils. Such transition would require countless changes to anatomy. Amongst the major transitions necessary but which are not present in any fossil are: 1) transition of the rib cage of Osteichthyes (bony fishes) to the bicipital ribs of amphibian tetrapods-development of tuberculum and capitulum in the ribs and parapophyses and diapophysis in the vertebrae 2) transition of the barrel-shaped rib cage of Ichthyostegids to one that is congested and heavily overlapping 3) migration of the shoulder girdle (Clavicle, Acromion, and Scapula) of Osteichthyes to that of Ichthyostegids 4) increase in rigidity of the spine and ribs from fish to Icthyostega, then to greater lateral flexion of the spine and ribs of reptiles 5) transition of lobe fins into limbs with digits 6) gradual dissapearance of ray fins 7) disappearance of gills and appearance of lungs 8) rearangement of caudal and sacral vertabrae 9) transition of "ashtray-shaped" vertabrae to "hokey-puck" shaped vertabrae It is quite interesting that so many prominent evolutionist scientists have acknowledged in their own books that the fossil record of life does not possess fossils which are clearly transitional, and that species appear fully-formed without intermediates. It seems that when evolutionists create works intended for their peers, they admit transitional forms are not known, but when speaking to the public, suddenly transitional forms are abundant. The lineage proposed by evolutionist scientists between fishes and tetrapods centers upon the species Eusthenopteron, Panderichthys, Tiktallik Rosea, Acanthostega, and Icthyostega. Unarguably, Eusthenopteron, Pandericthys, and Tiktaalik are fish. The transition of fish to amphibian tetrapods would require an intermediate form which crawls upon it's stomach on land but is able to swim well. While the morphology of Acanthostega seems to fit this need, it appears that the creature is simply a giant salamander which evolutionists have given a different name and placed in their imagined lineage to fill a critical gap.

Since Darwin, multilevel selection has been the key concept of the hierarchical approach to evolution. The debate around the significance of group selection as an evolutionary phenomenon (in both an early controversial  version and subsequent mathematical definition) has been the main entrance to multilevel selection theories. We present here a historical sketch of the debate prior to the formalization proposed by Samir Okasha. We also consider two extensions of Okasha’s multilevel theory: the diachronic perspective made possible by the study of major evolutionary transitions; the cross-level exaptive by-products. The double hierarchy, first proposed by Niles Eldredge and Stanley Salthe is a different kind of multilevel approach to evolution that avoids some theoretical impasses produced by a strictly selection-centered approach to extend the levels of evolutionary change. We argue that a dual hierarchical approach has a major heuristic power in order to embrace the complexity of evolutionary phenomena, from molecules to ecosystems, and is a candidate for an updated and unifying meta-theory of evolutionary patterns.

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 get precise about how such processes can happen, especially how they
can get started.

The evolution of cooperation often depends upon population structure, yet nearly all models of cooperation implicitly assume that this structure remains static. This is a simplifying assumption, because most organisms possess genetic traits that affect their population structure to some degree. These traits, such as a group size preference, affect the relatedness of interacting individuals and hence the opportunity for kin or group selection. We argue that models that do not explicitly consider their evolution cannot provide a satisfactory account of the origin of cooperation, because they cannot explain how the prerequisite population structures arise. Here, we consider the concurrent evolution of genetic traits that affect population structure, with those that affect social behavior. We show that not only does population structure drive social evolution, as in previous models, but that the opportunity for cooperation can in turn drive the creation of population structures that support it. This occurs through the generation of linkage disequilibrium between socio-behavioral and population-structuring traits, such that direct kin selection on social behavior creates indirect selection pressure on population structure. We illustrate our argument with a model of the concurrent evolution of group size preference and social behavior.

Cooperation received much less attention 30 years ago than other forms of ecological interaction, such as competition and predation. Workers generally viewed cooperation as being of limited interest, of special relevance to certain species (e.g. social insects, birds, humans and our primate relatives) but not of general significance to life on earth. This view has changed, due in large part to the study of evolutionary transitions in individuality (ETIs). What began as the study of animal social behaviour some 40 years ago has now embraced the study of social interactions at all levels in the hierarchy of life. Instead of being seen as a special characteristic clustered in certain lineages of social animals, cooperation is now seen as the primary creative force behind ever greater levels of complexity through the creation of new kinds of individuals. Cooperation plays this central role in ETIs because it exports fitness from the lower level (its costs) to the new higher level (its benefits).

For decades Darwinian processes were framed in the form of the Lewontin conditions: reproduction, variation and reproductive success taken to be sufficient and necessary. Since Buss (1987) and the work of Maynard Smith and Szathmáry (1995) biologists were eager to explain the major transitions from individuals to groups forming new individuals subject to Darwinian mechanisms themselves. Explanations that seek to explain the emergence of a new level of selection, however, cannot employ properties that would already have to exist on that level for selection to take place...

The four billion or so years during which life has existed on Earth have seen some millions of speciation events and innumerable instances of evolutionary change within populations. In contrast, only a few dozen examples are known in which such changes have led to the reorganization of the very units of adaptation and evolution. These reorganizations, in which groups of previously existing evolutionary individuals form a new kind of individual, include the transitions from groups of replicating molecules to prokaryotic cells, from associations ...

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, ...

Selection on the level of loosely associated groups has been suggested as a route towards the evolution of cooperation between individuals and the subsequent formation of higher-level biological entities. Such group selection explanations remain problematic, however, due to the narrow range of parameters under which they can overturn within-group selection that favours selfish behaviour. In principle, individual selection could act on such parameters so as to strengthen the force of between-group selection and hence increase cooperation and individual fitness, as illustrated in our previous work. However, such a process cannot operate in parameter regions where group selection effects are totally absent, since there would be no selective gradient to follow. One key parameter, which when increased often rapidly causes group selection effects to tend to zero, is initial group size, for when groups are formed randomly then even moderately sized groups lack significant variance in their composition. However, the consequent restriction of any group selection effect to small sized groups is derived from models that assume selfish types will competitively exclude their more cooperative counterparts at within-group equilibrium. In such cases, diversity in the migrant pool can tend to zero and accordingly variance in group composition cannot be generated. In contrast, we show that if within-group dynamics lead to a stable coexistence of selfish and cooperative types, then the range of group sizes showing some effect of group selection is much larger.

The volvocine green algae are a model system for the evolution of multicellularity and cellular differentiation. A combination of molecular genetic and phylogenetic comparative approaches has resulted in a detailed picture of the transition from single cells to differentiated, multicellular organisms in this group. To be useful as a model system, the volvocine algae should provide information that is relevant to other groups. Here I discuss recent advances in understanding the origins of multicellularity and cellular differentiation in the volvocine algae and consider the implications for such transitions in general. Several general principles emerge that are relevant to the origins of major multicellular groups, such as animals, plants, fungi, and red and brown algae. First, if the lessons learned from the volvocine algae can be generalized to other origins of multicellularity, we should expect these transitions to be understandable as a series of small changes, each potentially adaptive in itself. In addition, cooperation, conflict and mediation of conflicts among cells are likely to have played central roles. Finally, we should expect the histories of these transitions to include parallel evolution of some traits, periods of relatively rapid change interspersed with long periods of stasis, and simpler forms coexisting with more complex forms for long periods of time as in the evolution of the volvocine algae.