Empirical and philosophical problems with the subspecies rank

Abstract

Species‐level taxonomy derives from empirical sources (data and techniques) that assess the existence of spatiotemporal evolutionary lineages via various species “concepts.” These concepts determine if observed lineages are independent given a particular methodology and ontology, which relates the metaphysical species concept to what “kind” of thing a species is in reality. Often, species concepts fail to link epistemology back to ontology. This lack of coherence is in part responsible for the persistence of the subspecies rank, which in modern usage often functions as a placeholder between the evolutionary events of divergence or collapse of incipient species. Thus, prospective events like lineages merging or diverging require information from unknowable future information. This is also conditioned on evidence that the lineage already has a detectably distinct evolutionary history. Ranking these lineages as subspecies can seem attractive given that many lineages do not exhibit intrinsic reproductive isolation. We argue that using subspecies is indefensible on philosophical and empirical grounds. Ontologically, the rank of subspecies is either identical to that of species or undefined in the context of evolutionary lineages representing spatiotemporally defined individuals. Some species concepts more inclined to consider subspecies, like the Biological Species Concept, are disconnected from evolutionary ontology and do not consider genealogy. Even if ontology is ignored, methods addressing reproductive isolation are often indirect and fail to capture the range of scenarios linking gene flow to species identity over space and time. The use of subspecies and reliance on reproductive isolation as a basis for an operational species concept can also conflict with ethical issues governing the protection of species. We provide a way forward for recognizing and naming species that links theoretical and operational species concepts regardless of the magnitude of reproductive isolation.

The taxonomic status of the kukri snake Oligodon arenarius Vassilieva, 2015 with a redescription of Oligodon macrurus (Angel, 1927) (Squamata, Serpentes, Colubridae)

“…However, the significant differences in body sca lation and relative tail length may lead some readers to question whether O. arenarius should be considered a subspecies of O. macrurus instead of a junior synonym. In herpetology, the contemporary application, criteria and usage of subspecies is controversial (Hawlitschek et al 2012;Torstrom et al 2014;Kindler and Fritz 2018;De Queiroz 2020;Hillis 2020;Burbrink et al 2022) and a discussion of its utility is beyond the scope of this pa per. Recent designations of subspecies generally follow the suggestions of Mayr and Ashlock (1991) and Braby et al (2012), who define subspecies as a distinct metapop ulations that are phenotypically diagnosible and are on a trajectory towards evolutionary independence.…”

Using RADseq to resolve species boundaries in a morphologically complex group of yellow-flowered shrubs (Geleznowia, Rutaceae)

“…Broadhurst (2000) suggested that their two putative taxa were distinct, but because there were additional ‘intermediate’ form(s) possibly derived from hybridisation, the two should be recognised as subspecies (however, this was not formally done). The topic of using the rank of subspecies and its evolutionary relevance has been and continues to be contentious (de Queiroz 2020, 2021; Hillis 2021; Burbrink et al 2022). It has been suggested that the rank of subspecies may be used to denote species that are incompletely separated (de Queiroz 2020).…”

Speciation rates are unrelated to the formation of population structure in Malagasy gemsnakes

“…Therefore, the continuum between microevolutionary processes and macroevolutionary patterns may be disrupted due to variance in the persistence of populations, a necessary step to be considered as distinct lineages. This variance may be associated with extinction by forces such as environmental change, competition, disease, or hybridization (Burbrink et al, 2022;Raia et al, 2016;Rhymer & Simberloff, 1996;Vonlanthen et al, 2012). It could be argued that additional processes are relevant to prevent extinction via hybridization, such as increasing genomic incompatibilities thus enhancing reproductive isolation (Mayr, 1963;Wolf & Ellegren, 2017;Wu, 2001).…”