Olivier Rieppel

Comparative morphologists, developmental biologists, as well as paleontologists, recognize the existence of ‘types,’ or ‘Baupläne,’ in nature, marked out by a certain ‘sameness’ of structure that prevails through all variation in shape and function. The ‘sameness’ that marks out ‘types’ is one of structural correspondence (topology and connectivity), which is believed to be causally rooted in ontogeny (developmental constraints). In an evolutionary context, the structural relations that mark out ‘types’ are explained as relations of homology. The use of concepts such as ‘type’ or ‘Bauplan’ has been criticized from a ‘populational thinking’ point of view as being incompatible with current evolutionary theory. The present article explores the contrasting viewpoints, and concludes that current evolutionary theory can accommodate the concept of a ‘type’ in the sense of a ‘homeostatic property cluster natural kind.’ The ‘homeostatic property cluster natural kind’ is a nonessentialistic concept that allows the kind to be historically delimited.

The concept that renders morphology a tool for phylogeny reconstruction is homology. The concept of homology is rooted in pre-evolutionary idealistic morphology. The claim that the goal of idealistic morphology was the seriability of form may sound paradoxical given that this discipline proceeded within a framework of strictly delimited types. But the types only demarcate where seriability starts and where it comes to an end. Carl Gegenbaur’s was recognized as a milestone in idealistic morphology. A comparison with the second edition of 1870 illustrates Gegenbaur’s turn to evolutionary morphology. The methodology remained the same–seriability of form–but the series was no longer merely descriptive or conceptual but now a historical, evolutionary one. Gegenbaur emphasized that seriability of form was possible not only between species of the same type, but also between parts of organisms of the same type. Pursuing this project, he found that different parts of organisms evolve at different rates, resulting in an incongruence between the series of parts relative to the series of species under comparison. This incongrence was called chevauchement des spécialisations by Louis Dollo, Spezialisationskreuzungen by Othenio Abel, and heterobathmy of characters by Armen Takhtajan. Willi Hennig, the founder of modern methods in phylogenetic systematics, discovered that the heterobathmy of characters was a precondition for the establishment of the phylogenetic relationships based on shared derived characters. The result was a replacement of the search for ancestors by a search for relative degrees of phylogenetic relationships.

The relatively late acceptance of Darwinism in German biology and paleontology is frequently attributed to a lingering of Lamarckism, a persisting influence of German idealistic Naturphilosophie and Goethean romanticism. These factors are largely held responsible for the vitalism underlying theories of saltational and orthogenetic evolutionary change that characterize the writings of many German paleontologists during the first half of the 20th century. A prominent exponent of that tradition was Karl Beurlen, who is credited with having been the first German paleontologist to present a full-fledged theory of saltational evolution and orthogenetic change. A review of Beurlen's writings reveals motives and concerns far more complex, however, and firmly rooted in contemporary völkisch thought and Aryan Science. Beurlen's mature theory of evolution can indeed be understood as his own contribution to Aryan Geology and Biology, tainted as it was with National-Socialist ideology. Evolutionary biologists of the time who opposed the theories of Beurlen and likeminded authors, i.e., idealistic morphology, typology, saltational change, orthogenesis and cyclism did so on Darwinian principles, which ultimately prevailed. But at the time when the battle was fought, their adherence to the principle of natural selection was likewise ideologically tainted, namely in terms of racial theory. National-Socialist ideology was unable to forge a unity of evolutionary theory in Germany even amongst those of its proponents who endorsed this ideology

In a recent contribution, Ereshefsky (2007a) maintained the following points against Nixon and Carpenter (2000), Keller et al. (2003), and Rieppel (2005a, 2006a,b): (1) that species and taxa are individuals, not natural kinds; (2) that “origin essentialism” conflates qualitative essentialism with genealogical connectedness; and (3) that rigid designation theory applies to taxon names. Here I argue that: (1) the conception of species as individuals or natural kinds is not mutually exclusive but rather context sensitive; species are best seen as spatio-temporally located processual systems that instantiate an HPC natural kind; (2) “origin essentialism” is problematic because of the fuzzy boundaries of species and taxa, yet required if rigid designation theory is to apply to taxon names; and (3) the theories of direct reference and rigid designation do not apply to taxon names.

Taking its clues from Popperian philosophy of science, cladistics adopted a number of assumptions of the empiricist tradition. These include the identification of a dichotomy between observation reports and theoretical statements and its subsequent abandonment on the basis of the insight that all observation reports are theory-laden. The neglect of the ‘context of discovery’, which is the step of theory (hypothesis) generation. The emphasis on coherentism in the ‘context of justification’, which is the step of evaluation of the relative merits of alternative theories. The appeal to a total evidence approach in phylogenetic inference. And finally, a silence about causation, which results in an instrumentalist approach to phylogeny reconstruction. This paper explores how these empiricist assumptions are embedded in phylogenetic systematics, and why these assumptions are problematic for cladists (or any taxonomists).

This paper explores the use of Popper's philosophy of science by cladists in their battle against evolutionary and numerical taxonomy. Three schools of biological systematics fiercely debated each other from the late 1960s: evolutionary taxonomy, phenetics or numerical taxonomy, and phylogenetic systematics or cladistics. The outcome of that debate was the victory of phylogenetic systematics/cladistics over the competing schools of thought. To bring about this "cladistic turn" in systematics, the cladists drew heavily on the philosopher K.R. Popper in order to dress up phylogenetic systematics as a hypothetico-deductivist, indeed falsificationist, research program that would put an end to authoritarianism. As the case of the "cladistic revolution" demonstrates, scientists who turn to philosophy in defense of a research program read philosophers with an agenda in mind. That agenda is likely to distort the philosophical picture, as happened to Popper's philosophy of science at the hands of cladists.

This paper takes a hierarchical approach to the question whether species are individuals or natural kinds. The thesis defended here is that species are spatiotemporally located complex wholes (individuals), that are composed of (i.e., include) causally interdependent parts, which collectively also instantiate a homeostatic property cluster (HPC) natural kind. Species may form open or closed genetic systems that are dynamic in nature, that have fuzzy boundaries due to the processual nature of speciation, that may have leaky boundaries as is manifest in lateral gene transfer and introgression, that may be of multiple origins through hybridization, and that may split and merge and split again over time. The identity conditions of species qua individuals will have to be anchored in their history, rather than in their unique evolutionary origin. Species qua historically conditioned HPC natural kinds requires the kind to be mereologically structured, subject to the part-whole relation rather than the membership relation. This implies that there can be more than one kind of natural kinds

The history of biological systematics documents a continuing tension between classifications in terms of nested hierarchies congruent with branching diagrams (the ‘Tree of Life’) versus reticulated relations. The recognition of conflicting character distribution led to the dissolution of the scala naturae into reticulated systems, which were then transformed into phylogenetic trees by the addition of a vertical axis. The cladistic revolution in systematics resulted in a representation of phylogeny as a strictly bifurcating pattern (cladogram). Due to the ubiquity of character conflict—at the genetic or morphological level, or at any level in between—some characters will necessarily have to be discarded ( qua noise) in favor of others in support of a strictly bifurcating phylogenetic tree. Pattern analysts will seek maximal congruence in the distribution of characters (ultimately of any kind) relative to a branching tree-topology; process explainers will call such tree-topologies into question by reference to incompatible evolutionary processes. Pattern analysts will argue that process explanations must not be brought to bear on pattern reconstruction; process explainers will insist that the reconstructed pattern requires a process explanation to become scientifically relevant, i.e., relevant to evolutionary theory. The core question driving the current debate about the adequacy of the ‘Tree of Life’ metaphor seems to be whether the systematic dichotomization of the living world is an adequate representation of the complex evolutionary history of global biodiversity. In ‘Questioning the Tree of Life’, it seems beneficial to draw at least four conceptual distinctions: pattern reconstruction versus process explanation as different epistemological approaches to the study of phylogeny; open versus closed systems as expressions of different kinds of population (species) structures; phylogenetic trees versus cladograms as representations of evolutionary processes versus patterns of relationships; and genes versus species as expressions of different levels of causal integration and evolutionary transformation.

The “species-as-individuals” thesis takes species, or taxa, to be individuals. On grounds of spatiotemporal boundedness, any biological entity at any level of complexity subject to evolutionary processes is an individual. From evolutionary theory flows an ontology that does not countenance universal properties shared by evolving entities. If austere nominalism were applied to evolving entities, however, nature would be reduced to a mere flow of passing events, each one a blob in space–time and hence of passing interest only. Yet if there is genuine biodiversity in nature, if nature is genuinely carved into species, and taxa, then these evolutionary entities will be genuinely differentiated into specific kinds, each species being one of its kind. Given the fact that evolving entities have un-sharp boundaries, an appropriately weak, “non-essentialist” concept of natural kind has to be invoked that does not allow for strong identity conditions. The thesis of this paper is that species are not either individuals, or natural kinds. Instead, species are complex wholes (particulars, individuals) that instantiate a specific natural kind.

The definition of taxon names as formalized by the PhyloCode is based on Kripke's thesis of “rigid designation” that applies to Millian proper names. Accepting the thesis of “rigid designation” into systematics in turn is based on the thesis that species, and taxa, are individuals. These largely semantic and metaphysical issues are here contrasted with an epistemological approach to taxonomy. It is shown that the thesis of “rigid designation” if deployed in taxonomy introduces a new essentialism into systematics, which is exactly what the PhyloCode was designed to avoid. Rigidly designating names are not supposed to change their meaning, but if the shifting constitution of a clade is thought to cause a shift of meaning of the taxon name, then the taxon name is not a “rigid designator”. Phylogenetic nomenclature either fails to preserve the stability of meaning of taxon names that it propagates, or it is rendered inconsistent with its own philosophical background. The alternative explored here is to conceptualize taxa as natural kinds, and to replace the analytic definition of taxon names by their explanatory definition. Such conceptualization of taxa allows taxon names to better track the results of ongoing empirical research. The semantic as well as epistemic gain is that if taxon names are associated with natural kind terms instead of being proper names, the composition of the taxon will naturally determine the meaning of its name.

Species are generally considered to be the basic units of evolution, and hence to constitute spatio-temporally bounded entities. In addition, it has been argued that species also instantiate a natural kind. Evolution is fundamentally about change. The question then is how species can remain the same through evolutionary change. Proponents of the species qua individuals thesis individuate species through their unique evolutionary origin. Individuals, or spatio-temporally located particulars in general, can be bodies, objects, events, or processes, or a combination of these. It is here argued that species are best understood as open or closed, causally integrated processual systems that also instantiate an historically conditioned homeostatic property cluster natural kind.

A long-standing debate has dominated systematic biology and the ontological commitments made by its theories. The debate has contrasted individuals and the part – whole relationship with classes and the membership relation. This essay proposes to conceptualize the hierarchy of higher taxa is terms of a hierarchy of homeostatic property cluster natural kinds (biological species remain largely excluded from the present discussion). The reference of natural kind terms that apply to supraspecific taxa is initially fixed descriptively; the extension of those natural kind terms is subsequently established by empirical investigation. In that sense, classification precedes generalization, and description provides guidance to empirical investigation. The reconstruction of a hierarchy of (homeostatic property cluster) natural kinds is discussed in the light of cladistic methods of phylogeny reconstruction.

In biological systematics, as well as in the philosophy of biology, species and higher taxa are individuated through their unique evolutionary origin. This is taken by some authors to mean that monophyly is a (relational) property not only of higher taxa, but also of species. A species is said to originate through speciation, and to go extinct when it splits into two daughter species (or through terminal extinction). Its unique evolutionary origin is said to bestow identity on a species through time and change, and to render species names rigid designators. Species names are thus believed to function just like names of supraspecific taxa. However, large parts of the Web of Life are composed of species that do not have a unique evolutionary origin from a single population, lineage or stem-species. Further, monophyly is an ambiguous concept if it is defined simply in terms of ‘unique evolutionary origin’. Disambiguating the concept by defining a monophyletic taxon as ‘a taxon that includes the ancestor and all, and only, its descendant’ renders monophyly inapplicable to species. At the heart of the problem lies a fundamental distinction between species and monophyletic taxa, where species form mutually exclusive reticulated systems, while higher taxa form inclusive hierarchical systems. Examples are given both at the species level and below to illustrate the problems that result from the application of the monophyly criterion to species. The conclusion is that the concepts of exclusivity and monophyly should be treated as non-overlapping: exclusivity marks out a species synchronistically, i.e. in the present time. Monophyly marks out clades (groups of species) diachronistically, i.e. within an historical dimension.

The German synthesis of evolutionary theory that grew out of opposition to idealistic morphology has been anchored in the systematic work at the species level and below pursued by the Berlin School around Erwin Stresemann (involving Bernhard Rensch and Ernst Mayr), in the 1939 German translation of Dobzhansky’s Genetics and the Origin of Species, and in a 1943 anthology on evolution edited by Gerhard Heberer. The latter volume opened with a philosophical essay written by Hugo Dingler that was intended to provide the theoretical foundation for the theory of descent. Dingler and Heberer not only shared a commitment to the Darwinian evolutionary mechanisms of random mutation and natural selection, but also drew from Darwinism the justification for racial hygiene and eugenics. Dingler’s “unequivocal-methodological system” is a meta-scientific construct that offers no ontological grounding of evolutionary theory. Dingler’s voluntarism is subjectivist and stipulative and for those reasons ineffective in a critique of idealistic morphology. Dingler claimed descent with modification as a historical fact, but dealt only marginally with evolutionary mechanisms, and did not touch on issues of phylogeny reconstruction.