Biological Species Concept and Species Boundaries

Gualdrón-Diaz Juliette

Almost all studies in biology, whether at the level of molecules, cells, individuals or populations, are typically referenced to the level of the species. In the field of conservation biology, assessments of biodiversity are made at the level of the species: typical criteria include species richness, numbers of endemic species and the number or presence of endangered species in given areas (Myers et al. 2000). The accurate identification of species is crucial both to research in all areas of biology and to biodiversity conservation. Classical taxonomists classified individuals as members of a species based on a suite of shared morphological characters that were diagnostic and differentiated them from other such morphologically defined groups. In recent times, behavioural and ecological characters have also been increasingly used (Balakrishnan 2005).

One of the species concepts has been almost universally adopted by students of behavior, by most ecologist and those animal taxonomists, as well as by the molecular biologists (Avise and Ball 1990) is the Biological Species Concept (BSC). The BSC is most closely associated with Ernst Mayr (1942), who defines species based on their ability to interbreed: species are considered as “natural” entities distinguishable from other species by the criterion of reproductive isolation and not overall phenotypic similarity. The BSC has been criticised for several reasons: its inapplicability to asexual taxa (Mishler and Theriot 2000; Wheeler and Platnick 2000), lack of a temporal dimension (Willmann and Meier 2000), logical fallacy and the difficulty of applying it to allopatric populations (Mallet 1995).

One of the major problems, not only in BSC, but in any species concept is criterion for differentiating and delimiting species. If one accepts reproductive isolation as a sufficient criterion for delimiting species, then how recognize and delimit species using reproductive isolation?, a solution to this problem is delineate species boundaries using morphology, for this is necessary to examine the concordance between morphological and “biological” species boundaries (Balakrishnan 2005). Nevertheless, in certain groups is relatively easy such as crickets because the calling songs of cricket species are often reliable indicators of reproductively isolated populations (Shaw, 1999). Therefore the concordance in such behavioural and morphological characters would imply that phenetic clusters based on morphology correctly reflect the species boundaries defined by reproductive isolation. But for taxa that do not possess such behavioural associated with morphological is difficult to define species by BSC. In practice, even strong adherents of the BSC use phenetic similarities and discontinuities for delimiting species. If the organisms are phenotypically similar, they are considered conspecific until a reproductive barrier is demonstrated. However a practical difficulty encountered is that some populations may acquire reproductive isolation but minimal morphological difference, whereas other populations may acquire conspicuously different morphologies but no isolating mechanisms (Mayr 2000). Reproductive isolation may thus be looked upon as a sufficient but not necessary condition for delimiting species boundaries.

Although the BSC is the species concept most widely adopted, and is potentially useful in the analysis of speciation from the perspective of population genetics (Templeton 1989), it has some serious difficulties that make it inadequate, besides question unresolvable using the biological definition. As a result, most taxonomists, even those that accept the biological species concept, continue to use morphology and other phenotypic characters in order to delineate species boundaries.

References

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Myers, N., Mittermeier, R. A., Mittermeier, C. G., da Fonseca, G. A. B., and J. Kent. 2000. Biodiversity hotspots for conservation priorities. Nature 403:853-858.

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