Species Concepts and Species Delimitation

TLDR

Species delimitation has been historically conflated with species conceptualization, causing controversy over the definition and methods for identifying species, though most concepts agree that a separately evolving metapopulation lineage is the primary defining property and differ only on secondary criteria such as reproductive isolation, diagnosability, or monophyly. The study proposes a unified species concept that treats existence as the sole necessary property and regards former secondary criteria as evidence lines for assessing lineage separation. The unified concept separates species conceptualization from delimitation, limits secondary criteria to delimitation evidence, treats any single property as evidence of a species (with more properties increasing corroboration), and encourages development of new delimitation methods independent of traditional criteria.

Abstract

The issue of species delimitation has long been confused with that of species conceptualization, leading to a half century of controversy concerning both the definition of the species category and methods for inferring the boundaries and numbers of species. Alternative species concepts agree in treating existence as a separately evolving metapopulation lineage as the primary defining property of the species category, but they disagree in adopting different properties acquired by lineages during the course of divergence (e.g., intrinsic reproductive isolation, diagnosability, monophyly) as secondary defining properties (secondary species criteria). A unified species concept can be achieved by treating existence as a separately evolving metapopulation lineage as the only necessary property of species and the former secondary species criteria as different lines of evidence (operational criteria) relevant to assessing lineage separation. This unified concept of species has several consequences for species delimitation, including the following: First, the issues of species conceptualization and species delimitation are clearly separated; the former secondary species criteria are no longer considered relevant to species conceptualization but only to species delimitation. Second, all of the properties formerly treated as secondary species criteria are relevant to species delimitation to the extent that they provide evidence of lineage separation. Third, the presence of any one of the properties (if appropriately interpreted) is evidence for the existence of a species, though more properties and thus more lines of evidence are associated with a higher degree of corroboration. Fourth, and perhaps most significantly, a unified species concept shifts emphasis away from the traditional species criteria, encouraging biologists to develop new methods of species delimitation that are not tied to those properties.