The Brown Bear and The Polar Bear:
Species Individuality and Monophyly
Taking a stand against the origin essentialism that underlies the thesis of species individuality, the philosopher of biology Marjorie Grene argued that species “have a nature”, i.e., they are identifiable and re-identifiable through their intrinsic and extrinsic biological properties. In that sense, species are (variable cluster) natural kinds. This perspective she contrasted with purely ostensively defined species-individuals tagged by rigidly designating species (proper) names, a thesis which in her view reduces evolution to a mere succession of “just one darned thing after another.” There can be little doubt – on phenotypical, physiological, behavioral, and ecological, but also on intuitive grounds – that the brown bear (Ursus arctos) and the polar bear (Ursus maritimus) have a different ‘nature’, i.e., represent different species. And yet, the two species are known (or inferred) to have given rise to viable offspring through hybridization, both in the geological past (late Pleistocene), as well as in the present. Phylogenetic analysis of matrilineally inherited mitochondrial DNA finds the polar bear nested within the brown bear clade; bisexually inherited nuclear DNA shows the polar bear and the brown bear to form sister groups (sister clades). Analysis of nuclear DNA allows the interpretation of the polar bear and brown bear as two monophyletic species, or clades that split in the middle Pleistocene. Analysis of mitochondrial DNA renders the brown bear species paraphyletic, or alternatively the polar bear must be considered a subspecies of the brown bear. This example raises a number of issues relevant for a philosophical perspective on the ‘species problem’. These are: the recognition of monophyly and paraphyly as relational concepts; the distinction of tokogenetic and cladogenetic relationships and the consequent applicability or lack thereof of the concept of monophyly to species; the theoretical foundation of origin essentialism with respect to species, and the problem it faces in the age of discordance. Monophyly, paraphyly, and polyphyly are concepts related to hierarchical structure: what is para/polyphyletic at a lesser level of inclusiveness becomes monophyletic at a more inclusive level. The relevant distinction is thus simply one of monophyly versus non-monophyly. Monophyly, defined as applying to a clade that includes the ancestor and all, and only, its descendants, is a hierarchically structured cladogenetic (one – many) relation, whereby in current phylogenetic analysis the application of Hennig’s ‘principle of dichotomy’ enforces a strictly dichotomous hierarchy (the cladogenetic relation being one over two). Tied to such a hierarchical structure, the relation of monophyly is not applicable to species of sexually reproducing organisms, which instantiate tokogenetic (many – many) genealogical relations, i.e., constitute reticulated systems. The origin essentialism underlying the thesis of species individuality is based on Hennig’s ‘deviation rule’ and thus on strictly cladogenetic relations, which require the stem species to go extinct when it splits into two daughter (sister) species. Hennig’s ‘deviation rule’ clashes with the concept of a surviving stem-species. According to Hennig, the phylogenetic system must be built on phylogenetic relations alone, not on phenotypical, genotypical, physiological, ecological, behavioral etc. properties. So if the stem-species A gives rise to the daughter species B and C in a lineage-splitting (speciation) event, it is biologically possible that only the daughter species B deviates distinctly from the stem species A in phenotypical, genotypical, physiological, ecological, behavioral etc. properties, whereas the daughter species C remains indistinguishable from the stem species A in phenotypical, genotypical, physiological, ecological, behavioral etc. properties (this is Mayr’s founder principle). According to Hennig, the daughter species C cannot, however, be considered to be identical with the stem species A, while the daughter species B is considered a distinct species, because genealogically, the same (identical) cladogenetic relation of descent (the same spatiotemporally unique speciation event) that ties A to B also ties A to C. In other words, on purely genealogical grounds A cannot be considered identical with C, while not also being considered identical with B. Hence the conclusion that the stem-species A goes extinct as it splits into the two daughter species B and C. That way, species are not only spatially, but also temporally bounded, the species thus individuated through its unique origin and consequent death. In that sense, origin essentialism appears to be too strong, or restrictive, in species delimitation as it cannot accommodate introgression through hybridization or horizontal (lateral) gene transfer. But according to Hennig, who considered species to be individuals, the phylogenetic system must be governed by a unifying perspective, i.e., a single, all-pervading measure of relationship, which he chose to be recency of common ancestry. If crocodiles share a more recent common ancestry with birds than with lizards, crocodiles and birds form a monophyletic clade excluding lizards irrespective of how many more similarities crocodiles may seem to share with the reptilian grade of evolution than with birds. To think otherwise, in Hennig’s view, would violate what he called the ‘metabase’ of the phylogenetic system, as the latter would become subject to different and potentially conflicting measures of relationship. Along the same line of argumentation, a change of genotypical properties of a clade of bisexually reproducing organisms through introgression as a consequence of hybridization or horizontal (lateral) gene transfer does not necessarily violate the individuation of a clade through its origin as a consequence of speciation. But this is exactly what is claimed to happen in the case of asexually reproducing organisms like bacteria or protozoans, when a lineage or species is delimited and consequently individuated through its genotypical constitution. This claim reveals, however, an underlying transition from a metaphysical to an epistemological context. The individuation of species through origin essentialism is a metaphysical claim: if in a metaphysical context everything that is not a universal is considered to be a particular, then spatio-temporally located species must be individuals. In the biological (empirical epistemic) context where Marjorie Grene located the ‘nature’ of a species, species must be considered cluster natural kinds, marked out by their varied phenotypical, genotypical, physiological, ecological, behavioral etc. properties, both intrinsic and extrinsic.