Species concepts

Conservation strategies are often directed at individual species or at habitats that have been identified as species-rich and they therefore tend to assume that most individuals have been assigned correctly to a particular species. But is this necessarily the case? Although generally supportive of conservation initiatives, most biologists would argue that the identity of species is far from straightforward. Historically, researchers have often relied on the biological species concept (BSC), which defines species as '... groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups' (Mayr, 1942). Although conceptually straightforward, the BSC does have several shortcoming, for example a literal interpretation does not allow for hybridization and few can agree on how this dilemma should be solved. In addition, the BSC cannot accommodate species that reproduce asexually or by self-fertilization.

More than 20 different species concepts can be found in the literature (Hey et al., 2003). One alternative to the BSC that has been gaining support in recent years is the phylogenetic species concept (PSC). This defines species as groups of individuals that share at least one uniquely derived characteristic, and is often interpreted to mean that a species is the smallest identifiable monophyletic group of organisms within which there is a parental pattern of ancestry and descent (Cracraft, 1983). The PSC circumvents to some extent the problem of asexual reproduction, but it has been criticized for dividing organisms on the basis of characteristics that may have little biological relevance, and also for creating an overwhelmingly large number of species. Furthermore, two groups that are identified as separate species under the PSC may retain the potential to reproduce with one another. If reproduction between these two groups did occur, they would no longer be monophyletic and therefore would have to be reclassified as a single species.

The PSC tends to identify a greater number of species than the BSC. One review of 89 studies concluded that the PSC identified 48.7 percent more species than the BSC (Agapow et al., 2004; see Figure 7.2). If the increasingly popular PSC replaces the BSC as the most widely accepted species concept, the number of endangered species will increase and the geographical range of many will decrease. This in turn would lead to a wide-scale re-evaluation of numerous conservation programmes, for example the location of high-profile biological hotspots, in which large numbers of endemic species can be found, may change depending on which concept is used to determine the number of species in a given region (Peterson and Navarro-Siguenza, 1999). Many biologists therefore advocate a less dramatic

Figure 7.2 Some examples of how the number of species in different taxonomic groups varies, depending on whether or not the phylogenetic species concept (PSC) is used for classification purposes. Adapted from Agapow et al. (2004) and references therein

approach in which multiple species concepts are retained, provided that it is clear which concept is being employed at any given time; some situations will lend themselves to the BSC, others to the PSC, whereas others (e.g. those involving many unicellular or parasitic taxa) may lend themselves to another approach altogether (de Meeus, Durand and Renaud, 2003). This tactic has the advantage of being well balanced but suffers from the uncertainties that surround variable taxonomic criteria.

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