James DiFrisco

Adaptationism is often taken to be the thesis that most traits are adaptations. In order to assess this thesis, it seems we must be able to establish either an exhaustive set of all traits or a representative sample of this set. Either task requires a more systematic and principled way of individuating traits than is currently available. Moreover, different trait individuation criteria can make adaptationism turn out true or false. For instance, individuation based on natural selection may render adaptationism true, but may do so by presupposing adaptationism. In this paper, we show how adaptationism depends on trait individuation and that the latter is an open and unsolved problem.

A central topic in research at the intersection of development and evolution is the origin of novel traits. Despite progress on understanding how developmental mechanisms underlie patterns of diversity in the history of life, the problem of novelty continues to challenge researchers. Here we argue that research on evolutionary novelty and the closely associated phenomenon of co-option can be reframed fruitfully by: (1) specifying a conceptual model of mechanisms that underwrite character identity, (2) providing a richer and more empirically precise notion of co-option that goes beyond common appeals to “deep homology”, and (3) attending to the nature of experimental interventions that can determine whether and how the co-option of identity mechanisms can help to explain novel character origins. This reframing has the potential to channel future investigation to make substantive progress on the problem of evolutionary novelty. To illustrate this potential, we apply our reframing to two case studies: treehopper helmets and beetle horns.

Given the pervasiveness of gene sharing in evolution and the extent of homology across the tree of life, why is everything not homologous with everything else? The continuity and overlapping genetic contributions to diverse traits across lineages seem to imply that no discrete determination of homology is possible. Although some argue that the widespread overlap in parts and processes should be acknowledged as “partial” homology, this threatens a broad base of presumed comparative morphological knowledge accepted by most biologists. Following a long scientific tradition, we advocate a strategy of “theoretical articulation” that introduces further distinctions to existing concepts to produce increased contrastive resolution among the labels used to represent biological phenomena. We pursue this strategy by drawing on successful patterns of reasoning from serial homology at the level of gene sequences to generate an enriched characterization of serial homology as a hierarchical, phylogenetic concept. Specifically, we propose that the concept of serial homology should be applied primarily to repeated but developmentally individualized body parts, such as cell types, differentiated body segments, or epidermal appendages. For these characters, a phylogenetic history can be reconstructed, similar to families of paralogous genes, endowing the notion of serial homology with a hierarchical, phylogenetic interpretation. On this basis, we propose a five-fold theoretical classification that permits a more fine-grained mapping of diverse trait-types. This facilitates answering the question of why everything is not homologous with everything else, as well as how novelty is possible given that any new character possesses evolutionary precursors. We illustrate the fecundity of our account by reference to debates over insect wing serial homologs and vertebrate paired appendages.

Advances in developmental genetics and evo-devo in the last several decades have enabled the growth of novel developmental approaches to the classic theme of homology. These approaches depart from the more standard phylogenetic view by contending that homology between morphological characters depends on developmental-genetic individuation and explanation. This article provides a systematic re-examination of the relationship between developmental and phylogenetic homology in light of current evidence from developmental and evolutionary genetics and genomics. I present a qualitative model of the processes that cause de-coupling of morphological and molecular evolution by developmental system drift (DSD), and hypothesize that DSD is a widespread and regular stochastic process that is predictable from parameters such as population size, pleiotropy, and regulatory redundancy. These theoretical findings support an integrative approach in which models of DSD aid in determining when developmental-genetic explanations of homology apply and when other explanations such as stabilizing selection are needed. I argue that a phylogenetic monism about the definition and criteria of homology supports the integration of different explanations into a theory of homology better than pluralism does.

Comparative biology builds up systematic knowledge of the diversity of life, across evolutionary lineages and levels of organization, starting with evidence from a sparse sample of model organisms. In developmental biology, a key obstacle to the growth of comparative approaches is that the concept of homology is not very well defined for levels of organization that are intermediate between individual genes and morphological characters. In this paper, we investigate what it means for ontogenetic processes to be homologous, focusing specifically on the examples of insect segmentation and vertebrate somitogenesis. These processes can be homologous without homology of the underlying genes or gene networks, since the latter can diverge over evolutionary time, while the dynamics of the process remain the same. Ontogenetic processes like these therefore constitute a dissociable level and distinctive unit of comparison requiring their own specific criteria of homology. In addition, such processes are typically complex and nonlinear, such that their rigorous description and comparison not only requires observation and experimentation, but also dynamical modeling. We propose six criteria of process homology, combining recognized indicators with novel ones derived from dynamical systems modeling. We show how these criteria apply to animal segmentation and other ontogenetic processes. We conclude by situating our proposed dynamical framework for homology of process in relation to similar research programs, such as process structuralism and developmental approaches to morphological homology.

There have been repeated attempts in the history of comparative biology to provide a mechanistic account of morphological homology. However, it is well-established that homologues can develop from diverse sets of developmental causes, appearing not to share any core causal architecture that underwrites character identity. We address this challenge with a new conceptual model of Character Identity Mechanisms. ChIMs are cohesive mechanisms with a recognizable causal profile that allows them to be traced through evolution as homologues despite having a diverse etiological organization. Our model hypothesizes that anatomical units at different levels of organization—cell types, tissues, and organs—have level-specific ChIMs with different conserved parts, activities, and organization. Relying on a methodology of conceptual engineering, we show how the ChIM concept advances our understanding of the developmental basis of morphological characters, while forging an important link between comparative and mechanistic biology.

The logic of genetic discovery has changed little over time, but the focus of biology is shifting from simple genotype–phenotype relationships to complex metabolic, physiological, developmental, and behavioral traits. In light of this, the traditional reductionist view of individual genes as privileged difference‐making causes of phenotypes is re‐examined. The scope and nature of genetic effects in complex regulatory systems, in which dynamics are driven by regulatory feedback and hierarchical interactions across levels of organization are considered. This review argues that it is appropriate to treat genes as specific actual difference‐makers for the molecular regulation of gene expression. However, they are often neither stable, proportional, nor specific as causes of the overall dynamic behavior of regulatory networks. Dynamical models, properly formulated and validated, provide the tools to probe cause‐and‐effect relationships in complex biological systems, allowing to go beyond the limitations of genetic reductionism to gain an integrative understanding of the causal processes underlying complex phenotypes.

Explanation in terms of gene regulatory networks has become standard practice in evolutionary developmental biology. In this paper, we argue that GRNs fail to provide a robust, mechanistic, and dynamic understanding of the developmental processes underlying the genotype–phenotype map. Explanations based on GRNs are limited by three main problems: the problem of genetic determinism, the problem of correspondence between network structure and function, and the problem of diachronicity, as in the unfolding of causal interactions over time. Overcoming these problems requires dynamic mechanistic explanations, which rely not only on mechanistic decomposition, but also on dynamic modeling to reconstitute the causal chain of events underlying the process of development. We illustrate the power and potential of this type of explanation with a number of biological case studies that integrate empirical investigations with mathematical modeling and analysis. We conclude with general considerations on the relation between mechanism and process in evo-devo.

The subject of this edited volume is the idea of levels of organization: roughly, the idea that the natural world is segregated into part-whole relationships of increasing spatiotemporal scale and complexity. The book comprises a collection of essays that raise the idea of levels into its own topic of analysis. Owing to the wide prominence of the idea of levels, the scope of the volume is aimed at theoreticians, philosophers, and practicing researchers of all stripes in the life sciences. The volume’s contributions reflect this diversity, and draw from fields such as developmental biology, evolutionary biology, molecular biology, ecology, cell biology, and neuroscience. The book presents wide-ranging novel insights on causation and levels, the hierarchical structure of evolution, the role of levels in biological theory, and more.

Individuality is an important concept in biology, yet there are many non-equivalent criteria of individuality expressed in different kinds of biological individuals. This article evaluates these different kinds in terms of their capacity to support explanatory generalizations over the systems they individuate. Viewing the problem of individuality from this perspective promotes a splitting strategy in which different kinds make different epistemic trade-offs that suit them for different explanatory roles. I argue that evolutionary individuals, interpreted as forming a functional kind, face difficulties of individuation and explanatory power that are mitigated by relying on more structurally based properties and non-evolutionary kinds. 1Introduction2Kinds of Biological Individuals3Evolutionary Individuals and Functional Kinds4Evolutionary versus Non-evolutionary Kinds of Individuals 4.1Physiological individuality4.2Ecological individuality4.3Developmental individuality5Conclusion

Token physicalism is often viewed as a modest and unproblematic physicalist commitment, as contrasted with type physicalism. This paper argues that the prevalence of functional individuation in biology creates serious problems for token physicalism, because the latter requires that biological entities can be individuated physically and without reference to biological functioning. After characterizing the main philosophical roles for token physicalism, I describe the distinctive uses of functional individuation in models of biological processes. I then introduce some requirements on token identity claims that arise from a position on individuation and identity known as sortalism. An examination of biological examples shows that these sortalist requirements cannot be plausibly met due to differences between individuation by functional biological criteria and by physical criteria. Even without assuming sortalism, token physicalism faces the more basic problem of excluding functionally irrelevant detail from the individuation of biological tokens. I close by suggesting that the philosophical roles for token identity are better fulfilled by a notion of token composition, which promotes a hierarchical picture of individuality.

The concept of levels of organization, despite its widespread scientific currency, has recently been criticized by a number of philosophers of science. This paper diagnoses the main source of problems facing theories of levels. On this basis, the problems with the usual criteria for distinguishing levels are evaluated: compositional relations, organizational types, and spatial scales. Drawing on some work on hierarchies in ecology, I argue in favor of an alternative conception of levels defined by the criterion of rates or time scales of processes.

This paper examines three exemplary theories of living organization with respect to their common feature of defining life in terms of metabolic closure: autopoiesis, systems, and chemoton theory. Metabolic closure is broadly understood to denote the property of organized chemical systems that each component necessary for the maintenance of the system is produced from within the system itself, except for an input of energy. It is argued that two of the theories considered—autopoiesis and systems—participate in a hylomorphist pattern of thinking which separates the “form” of the living system from its “matter.” The analysis and critique of hylomorphism found in the work of the philosopher Gilbert Simondon is then applied to these two theories, and on the basis of this critique it is argued that the chemoton model offers a superior theory of minimal life which overcomes many of the problems associated with the other two. Throughout, the relationship between hylomorphism and the understanding of living things as machines is explored. The paper concludes by considering how hylomorphism as a background ontology for theories of life fundamentally influences the way life is defined.

The received view of Bergson's philosophy of life is that it advances some form of vitalism under the heading of an “élan vital.” This paper argues against the vitalistic interpretation of Bergson's élan vital as it appears in Creative Evolution in favor of an interpretation based on his overlooked reflections on entropy and energetics. Within the interpretation developed here, the élan vital is characterized not as a spiritualistic “vital force” but as a tendency of organization opposed to the tendency of entropic degradation. It is then shown how Bergson's view of evolution and living organization resonates with more contemporary scientific approaches belonging to a “thermodynamic paradigm” in theoretical biology in different respects, including the critique of neo-Darwinism and the positing of a driving force behind evolution. Finally, the special interest that Bergson's philosophical biology bears today is considered in terms of the connection between the concepts of élan vital and duration