Species Concepts and Species Delimitation

— 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. [Species concept; species criteria; species delimitation.] Readers of Systematic Biology hardly need to be reminded of the importance of species in biology. According to various authors, species are one of the fundamental units of biology, making them comparable in importance to genes, cells, and organisms, some of the fundamental units at lower levels of biological organization (e.g., Mayr, 1982; see also de Queiroz, 2005a). However, because species exist at a higher level of organization than the humans observing them, species also are generally much larger and longer lived than their human observers. Moreover, the connections among their parts (i.e., organisms) are ephemeral. This makes it more or less impossible for humans to perceive entire species simply by looking at them, as they do for cells and organisms, which is why biologists have symposia devoted to the topic of species delimitation. To complicate matters, for roughly the past half century, the issue of species delimitation has been confused by a problem involving the concept of species itself. The problem is that currently different subgroups of biologists advocate different and at least partially incompatible species concepts (reviewed by Mayden, 1997; de Queiroz, 1998; Harrison, 1998). Mayden (1997) listed 24 different named species concepts, and there are even more alternative definitions (where a definition is a concise description of a concept, so that any given species concept may be associated with definitions that differ in minor details of wording). Many of these concepts and their associated definitions are incompatible in that they can lead to different conclusions concerning the boundaries and numbers of species. Thus, the species concept problem—that is, current disagreements about the theoretical concept of the species—is closely tied to the issue of species delimitation—that is, how to determine the boundaries and numbers of species from emperical data. Fortunately, this species concept problem is not as serious as it appears. Despite the obvious differences among contemporary alternative species concepts and definitions, they exhibit an underlying conceptual unity, which provides the basis for a unified concept of species. As a consequence, biologists are now in a position to free ourselves from seemingly endless debates about the concept of species and thus also the definition of the species category. One of the most significant benefits of a unified species concept is that it allows biologists to approach the problem of species delimitation in a more straightforward way. In this paper, I will review the species concept problem and a proposal about how diverse species concepts can be unified, which I have published previously (de Queiroz, 1998, 1999, 2005a, 2005b, 2005c). I will then examine some of the consequences of a unified species concept for the problem of species delimitation. ALTERNATIVE SPECIES CONCEPTS Table 1 is a list of alternative species concepts. The list consists of major categories of alternative species concepts advocated by contemporary biologists, with the categories defined in terms of the properties upon which they are based. Most readers of this journal are likely knowledgeable about at least some of these proposed concepts, which include the familiar biological, ecological, evolutionary, and phylogenetic concepts, among others. Importantly, all of these concepts have advocates among contemporary biologists. In addition, many of the concepts are at least partially incompatible. For example, several authors have called attention to situations in which adoption of the biological species concept leads to the recognition of fewer species taxa than adoption of one of the alternative species concepts, such as the