Does natural selection act primarily on individual organisms, on groups, on genes, or on whole species? The question of levels of selection - on which biologists and philosophers have long disagreed - is central to evolutionary theory and to the philosophy of biology. Samir Okasha's comprehensive analysis gives a clear account of the philosophical issues at stake in the current debate.

Hierarchical expansions of the theory of natural selection exist in two distinct bodies of thought in evolutionary biology, the group selection and the species selection traditions. Both traditions share the point of view that the principles of natural selection apply at levels of biological organization above the level of the individual organism. This leads them both to considermultilevel selection situations, where selection is occurring simultaneously at more than one level. Impeding unification of the theoretical approaches of the multilevel selection traditions (...) are the different goals of investigators in the different subdisciplines and the different types of data potentially available for analysis. We identify two alternative approaches to multilevel situations, which we termmultilevel selection [1] andmultilevel selection [2]. Of interest in the former case are the effects of group membership onindividual fitnesses, and in the latter the tendencies for the groups themselves to go extinct or to found new groups (i.e., group fitnesses). We argue that: neither represents the entire multilevel selection process; both are aspects of any multilevel selection situation; and both are legitimate approaches, suitable for answering different questions. Using this formalism, we show that: multilevel selection [2] does not require emergent group properties in order to provide an explanatory mechanism of evolutionary change; multilevel selection [1] is usually more appropriate for neontological group selection studies; and species selection is most fruitfully considered from the point of view of multilevel selection [2]. Finally we argue that the effect hypothesis of macroevolution, requiring, in selection among species, both the absence of group effects on organismic fitness (multilevel selection [1]), and the direct determination of species fitnesses by those of organisms, is untestable with paleontological data. Furthermore, the conditions for the effect hypothesis to hold are extremely restrictive and unlikely to apply to the vast majority of situations encountered in nature. (shrink)

Darwin’s classic ‘Origin of Species’ (Darwin 1859) described forces of selection acting upon individuals, but there remains a great deal of controversy about what exactly the status and definition of a biological individual is. Recently some authors have argued that the individual is dispensable – that an inability to pin it down is not problematic because little rests on it anyway. The aim of this paper is to show that there is a real problem of biological individuality, and an urgent (...) need to arbitrate over the current plethora of solutions to it. (shrink)

Biological theory demands a clear organism concept, but at present biologists cannot agree on one. They know that counting particular units, and not counting others, allows them to generate explanatory and predictive descriptions of evolutionary processes. Yet they lack a unified theory telling them which units to count. In this paper, I offer a novel account of biological individuality, which reconciles conflicting definitions of ‘organism’ by interpreting them as describing alternative realisers of a common functional role, and then defines individual (...) organisms as essentially possessing some mechanisms that play this role. (shrink)

Most philosophical accounts of emergence are incompatible with reduction. Most scientists regard a system property as emergent relative to properties of the system's parts if it depends upon their mode of organization--a view consistent with reduction. Emergence can be analyzed as a failure of aggregativity--a state in which "the whole is nothing more than the sum of its parts." Aggregativity requires four conditions, giving tools for analyzing modes of organization. Differently met for different decompositions of the system, and in different (...) degrees, these conditions provide powerful evaluation criteria for choosing decompositions, and heuristics for detecting biases of vulgar reductionisms. This analysis of emergence is compatible with reduction. (shrink)

The evolutionary problem of the units of selection has elicited a good deal of conceptual work from philosophers. We review this work to determine where the issues now stand.

We review Evolution and the Levels of Selection by Samir Okasha. This important book provides a cohesive philosophical framework for understanding levels-of-selections problems in biology. Concerning evolutionary transitions, Okasha proposes that three stages characterize the shift from a lower level of selection to a higher one. We discuss the application of Okasha’s three-stage concept to the evolutionary transition from unicellularity to multicellularity in the volvocine green algae. Okasha’s concepts are a provocative step towards a more general understanding of the major (...) evolutionary transitions; however, the application of certain ideas to the volvocine model system is not straightforward. (shrink)

Adaptations can occur at different hierarchical levels, but it can be difficult to identify the level of adaptation in specific cases. A major problem is that selection at a lower level can filter up, creating the illusion of selection at a higher level. We use optimality modeling of the volvocine algae to explore the emergence of genuine group adaptations. We find that it is helpful to develop an explicit model for what group fitness would be in the absence of group-level (...) relationships between traits and group fitness. We call this “counterfactual fitness,” because in many actual cases of interest there are group-level relationships. Once counterfactual fitness is modeled, the difference between effects that filter up and genuine group selection is explicit and so, therefore, is the distinction between apparent and genuine group adaptations. We call the latter group-specific adaptations. Recognizing group-specific adaptations is important because only group-specific adaptations would cause the lower-level units to be maladapted if they were to leave the group and enter a global cell-level population. Thus, as group-specific adaptations evolve, they create selective pressure for increased cohesiveness and individuality of groups. This article suggests that group-specific adaptations could be present in the simplest, earliest branching colonial volvocine species, which do not have distinct specialized cells. The article also makes predictions about the kind of empirical evidence needed to support or refute the hypothesis that a particular trait is a group-level adaptation. (shrink)

The concept of individuality as applied to species, an important advance in the philosophy of evolutionary biology, is nevertheless in need of refinement. Four important subparts of this concept must be recognized: spatial boundaries, temporal boundaries, integration, and cohesion. Not all species necessarily meet all of these. Two very different types of pluralism have been advocated with respect to species, only one of which is satisfactory. An often unrecognized distinction between grouping and ranking components of any species concept is necessary. (...) A phylogenetic species concept is advocated that uses a grouping criterion of monophyly in a cladistic sense, and a ranking criterion based on those causal processes that are most important in producing and maintaining lineages in a particular case. Such causal processes can include actual interbreeding, selective constraints, and developmental canalization. The widespread use of the biological species concept is flawed for two reasons: because of a failure to distinguish grouping from ranking criteria and because of an unwarranted emphasis on the importance of interbreeding as a universal causal factor controlling evolutionary diversification. The potential to interbreed is not in itself a process; it is instead a result of a diversity of processes which result in shared selective environments and common developmental programs. These types of processes act in both sexual and asexual organisms, thus the phylogenetic species concept can reflect an underlying unity that the biological species concept can not. (shrink)

The fitness of any evolutionary unit can be understood in terms of its two basic components: fecundity (reproduction) and viability (survival). Trade-offs between these fitness components drive the evolution of life-history traits in extant multicellular organisms. We argue that these trade-offs gain special significance during the transition from unicellular to multicellular life. In particular, the evolution of germ–soma specialization and the emergence of individuality at the cell group (or organism) level are also consequences of trade-offs between the two basic fitness (...) components, or so we argue using a multilevel selection approach. During the origin of multicellularity, we study how the group trade-offs between viability and fecundity are initially determined by the cell level trade-offs, but as the transition proceeds, the fitness trade-offs at the group level depart from those at the cell level. We predict that these trade-offs begin with concave curvature in single-celled organisms but become increasingly convex as group size increases in multicellular organisms. We argue that the increasingly convex curvature of the trade-off function is driven by the cost of reproduction which increases as group size increases. We consider aspects of the biology of the volvocine green algae – which contain both unicellular and multicellular members – to illustrate the principles and conclusions discussed. (shrink)

Dieser Aufsatz ist eine Besinnung auf den Begriff Individuum, wie dieser in der Biologie gebraucht wird, und er beabsichtigt, den Inhalt dieses Begriffes theoretisch zu begründen. Es ist aber unmöglich für die nähere Bestimmung des Begriffes Individuum von der etymologischen Bedeutung „unteilbar” auszugehen, weil die biologischen Individuen nicht immer unteilbar sind. Beim Suchen nach einem richtigen Untersuchungsverfahren wird zunächst dargelegt, wie diese Untersuchung wesentlich ein Teil der theoretischen Biologie, also der Wissenschaft, ist, wenn man auch von deutlichen Zusammenhängen mit der (...) Philosophie und dem spontanen Denken sprechen kann. Deshalb wird die Methode, nach der wir vorangehen müssen, die induktive Methode sein, weil diese in der Wissenschaft mit Recht hoch eingeschätzt wird.Bei den grossen Vertretern der verschiedenen Richtungen in der Biologie, beiHaeckel, von Bertalanffy, Driesch undBergson, werden in ihre Auffassungen über Individuum und Individualität derartige Übereinstimmungen festgestellt, dass darin nach den Gesetzen der Induktion die wesentlichen Momente des biologischen Individuumbegriffes erkannt werden können. Ausserdem stimmen diese Momente mit dem Inhalt des Begriffs zusammen, wie der sich aus dem spontanen Denken ergibt.Diese Momente sind: Subsistenz, Einheit und Totalität, Tendenz nach einem geschlossenen System und schliesslich die Unmöglichkeit einer erschöpfenden Definition. In dieser Reihenfolge werden sie kritisch erörtert, und daraufhin werden folgende biologische Begriffsbestimmungen vorgeschlagen : Individualität ist die Tendenz des einzelnen Organismus nach ungeteilter subsistenter Totalität. Etwas ist Individuum in dem Masse, wo diese Tendenz verwirklicht ist.Durch einige Beispiele aus dem System und der Ontogenie wird die Anwendung dieses dynamischen Begriffes erläutert.Dans cette étude nous avons soumis à un examen l'idée d'individu, telle qu'elle est d'un usage courant en biologie. Nous avons cherché à fonder la théorie du contenu de cette idée. Pour en donner une définition plus exacte, nous ne pouvons pas prendre pour point de départ le sens étymologique d'„indivisible”, puisque les individus biologiques ne le sont pas toujours. En essayant d'abord d'établir la vraie méthode de notre recherche, nous avons expliqué que celle-ci est une branche de la biologie théorique, par conséquent de la science positive, bien qu'il y ait des rapports avec la philosophie et avec la simple pensée spontanée. Il nous faudra donc procéder d'après la méthode qui est surtout demandée par les sciences exactes, à savoir la méthode inductive.Chez les grands représentants des différents courants de la pensée en biologie, notamment chezHaeckel, von Bertalanffy, Driesch etBergson, nous rencontrons dans leurs conceptions sur l'individu et sur l'individualité des correspondances tellement frappantes que nous sommes autorisé à y voir, d'après les lois mêmes de l'induction, les éléments essentiels de l'idée d'individu biologique. Ces éléments s'accordent d'ailleurs avec le contenu de l'idée que nous nous en formons spontanément.Ces éléments sont les suivants: subsistance, unité et totalité, tendance à devenir un système clos, et enfin l'impossibilité d'une définition adéquate. Nous en avons fait la critique dans cet ordre. En suite nous avons proposé les définitions biologiques suivantes : l'individualité consiste en la tendance que possède un organisme naturellement isolé à devenir une totalité subsistante et non-divisée. On aura donc un individu dans la mesure que cette tendance a été réalisée.Enfin, nous avons illustré l'application de cette notion par quelques exemples empruntés au système et à l'ontogénie. (shrink)

The book presents a new way of understanding Darwinism and evolution by natural selection, combining work in biology, philosophy, and other fields.

Alfred I. Tauber (ed.), Organism and the Origins of Self. Dordrecht, Netherlands: Kluwer Academic Publishers, 1991. xix + 384 pp., US$ 110.00 (US$ 25.00 paperback). This is a fascinating book based on a 1990 symposium at Boston University. It promises to change the way one conceives of the organism. The authors start from different specializations but provide a most tantalizing feast of ideas. Richard Lewontin commences the book with a strange foreword. Lewontin submits that the concern with the "self and (...) private identity in modern biology is ... only one manifestation of the ideology of individualism that is a shibboleth of our particular [bourgeois] form of social organization" (p. xix). If this is the case, the reader should stop reading the book and ask for her money back. But I advise patience. Tauber's introduction is a mixture of the history of the medical notion of disease and the advocacy of a dialectical concept of the self as the interpenetration of part and whole. Tauber highlights Elie Metchnikoff's appealing idea of phagocytosis as just one form of the active organism striving to attain harmony among competing cells. (shrink)

this requirement for adaptations. Emergent characters are always potential adaptations. Not all selection processes produce adaptations, however. The key issue, in delineating a selection process, is the relationship between a character and fitness. The emergent character approach is more restrictive than alternative schemas that delineate selection..