What is a species? Theoretical musings over this question have generated a multitude of species concepts, however the ‘nitty-gritty’ business of species delimitation has been relatively neglected. Species delimitation is the process of deciding how individuals and populations fit into natural groups. The importance of defining species within an evidence-based framework cannot be overemphasized, because species are not simply constructs of classification, but should reflect potential for gene flow and recent evolutionary processes. In light of rapid human-induced environmental change, it is even more important to have accurately defined biological systems. The first step in understanding how human activities and interventions are affecting biodiversity is to accurately document species.
Historically species delimitation was purposely theory neutral so as not to ‘bias’ the process. Taxonomy was largely morphology based and there was no easy way to distinguish biological forms that look the same, but are actually independent evolutionary lineages, i.e. convergent form. Modern taxonomists generally use DNA-based methods to infer relationships, and incorporate empirical evidence to understand reproductive isolation and species boundaries. Even when these tools are employed, various evolutionary processes can make species delimitation non-trivial including hybridization and clinal variation. These confounding processes are especially prevalent in plant evolution.
In an effort to define species as accurately as possible and account for future environmental change, a few species delimitation studies have incorporated ecological niche modeling. Two of the papers, Rissler and Apodaca (2007) and Bond and Stockman (2008), use similar GIS based approaches to provide information about likelihood of gene flow among potential species lineages. The method outlined in Rissler & Apodaca (2007) involves first using DNA-based methods to identify potential species lineages, and then ecological divergence of lineages is assessed to infer future potential for divergence. The most exciting part of this study is that the authors develop a method to assess the ecological suitability of the contact zone between lineages. Is the area between the taxa appropriate habitat for one or both of the taxa? If it is, then range expansion could result in admixture of the populations in the near future.
Bond and Stockman (2008) use ecological data similarly, but estimate selective divergence using proxies of genetic exchangeability and ecological exchangeability. Genetic exchangeability can best be understood as the Euclidian distance between populations and is relevant to the potential ability of migrants to move between populations or lineages. Ecological exchangeability is a fitness parameter that can be used to estimate the likelihood of survival of one lineage in the actual environment of the other lineage. These approaches assess the likelihood of future introgression between lineages by evaluating genetic, morphological, and ecological evidence. Consider the scenario of recently diverged lineages: if the contact zone is suitable habitat for one or both of the lineages, then range expansion could easily cause hybridization. In this case, subspecific taxonomic designation might be the appropriate circumscription these taxonomic units.
Incorporating ecological models into species delimitation will allow species delimitation to have a predictive component. Additionally, DNA-based methods allows morphologically similar, but evolutionarily independent lineages to be identified. This combined method is an effective way to rapidly and accurately describe species and document biodiversity.