Abstract
Species-level taxonomy is derived from methodological sources (data and
techniques) that assess the existence of spatio-temporal evolutionary
lineages via various species concepts. These concepts determine if
observed lineages are independent given a particular methodology
supposedly connected to ontology, which relates the metaphysical concept
to what “kind” of thing a species is. Often, species concepts fail to
link methodology and practice back to ontology. This lack of coherence
is in part responsible for the persistence of the rank of subspecies,
which in modern usage often functions as a placeholder between the
evolutionary events of divergence or collapse. Thus, prospective events
like lineage merger or collapse determine if a subspecies is subsumed
into an existing species, or achieves species rank given unknowable
future information. This is conditioned on evidence that the lineage
already has a detectably distinct evolutionary history. Ranking these
lineages as subspecies seems attractive given the observation that many
lineages do not exhibit intrinsic reproductive isolation. We argue that
the use of 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 spatio-temporally defined individuals. Some species
concepts more inclined to consider subspecies, like the Biological
Species Concept, are disconnected from ontology and do not consider
genealogical history. 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.