0.5 Species diversity  (Page 2/14)

To count the number of species, we must define what constitutes a species. There are several competing theories, or"species concepts" ( Mayden, 1997 ). The most widely accepted are the morphological species concept, thebiological species concept, and the phylogenetic species concept.

Although the morphological species concept (MSC) is largely outdated as a theoretical definition, it is still widely used. According to this concept:

species are the smallest groups that are consistently and persistently distinct, and distinguishable byordinary means. ( Cronquist, 1978 ).

The biological species concept (BSC), as described by Mayr and Ashlock (1991) , states that

"a species is a group of interbreeding natural populations that is reproductively isolated from other such groups".

According to the phylogenetic species concept (PSC), as defined by Cracraft (1983) , a species :

"is the smallest diagnosable cluster of individual organism [that is, the cluster of organisms are identifiably distinctfrom other clusters] within which there is a parental patternof ancestry and descent".

In practice, systematists usually group specimens together according to shared features (genetic, morphological,physiological). When two or more groups show different sets of shared characters, and the shared characters for each groupallow all the members of that group to be distinguished relatively easily and consistently from the members of anothergroup, then the groups are considered different species. This approach relies on the objectivity of the phylogenetic speciesconcept ( i.e. , the use of intrinsic, shared, characters to define or diagnose a species) and applies it tothe practicality of the morphological species concept, in terms of sorting specimens into groups ( Kottelat, 1995 , 1997 ).

Despite their differences, all species concepts are based on the understanding that there are parameters that make a species adiscrete and identifiable evolutionary entity. If populations of a species become isolated, either through differences in theirdistribution ( i.e. , geographic isolation) or through differences in their reproductive biology( i.e. , reproductive isolation), they can diverge, ultimately resulting in speciation. During thisprocess, we expect to see distinct populations representing incipient species - species in the process of formation. Some researchers may describe these as subspecies orsome other sub-category, according to the species concept used by these researchers. However, it is very difficult to decidewhen a population is sufficiently different from other populations to merit its ranking as a subspecies. For thesereasons, subspecific and infrasubspecific ranks may become extremely subjective decisions of the degree of distinctivenessbetween groups of organisms ( Kottelat, 1997 ).