Delimiting species: a Renaissance issue in systematic biology

Abstract

The literature about species concepts might be larger than that about any other subject in evolutionary biology, but the issue of empirically testing species boundaries has been given little attention relative to seemingly endless debates over what species are. The practical issue of delimiting species boundaries is nevertheless of central importance to many areas of evolutionary biology. The number of recently described methods for delimiting species suggests renewed interest in the topic, and some methods are explicitly quantitative. Here, we review nine of these methods by summarizing the relevant biological properties of species amenable to empirical evaluation, the classes of data required and some of the strengths and limitations of each.Systematic biology rests on an extensive literature about the theory and methodology of phylogenetic inference and the theory of species concepts [1-3], but on a relatively small literature about the methods of delimiting species [4]. This state of affairs is rather odd given that two frequently stated empirical goals of systematic biology are to: (1) discover MONOPHYLETIC (see Glossary) groups at higher levels; and (2) discover lineages (i.e. species [5]) at lower levels [6]. Interest in delimiting species and inferring speciation patterns and mechanisms was high during the mid-20th century era of the 'New Systematics' [7], after which activity declined [4], but there are now signs of a Renaissance, and some novel methods have recently been proposed for testing species boundaries in a statistically rigorous framework [8][9][10][11]. From the broader perspective of evolutionary theory, delimiting species is important in the context of understanding many evolutionary mechanisms and processes. Demographic structure within a species is frequently extensive [12], and this intraspecific structure will probably influence the rates at which novel adaptations originate and spread among demes [13,14], whereas the species boundary will define the limits within or across which evolutionary processes operate [15]. Over-or under-resolving species boundaries will obviously confound studies aimed at understanding these population-level processes. Species are also routinely used as fundamental units of analysis in biogeography, ecology, macroevolution and conservation biology [16 -20], and a better understanding of these larger scale processes requires that systematists employ methods to delimit objectively and rigorously what species are in nature.Here, we review nine methods of delimiting species chosen to show a range of differences with respect to the biological properties of species that can be empirically tested, the types of data needed (DNA, morphology, etc.), the density of population sampling required, and the generality of implementation (bisexual taxa only versus bisexuals and asexuals). Our review is incomplete, as other methods [9,21-24] could not be included owing to space limitations. To place operational methods into context, we briefly make a distinction between the issu...

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