The idea that there is no single, right species definition, is known as pluralism, this being one response to the multiple species definitions (Maclaurin & Sterelny.2008). This pluralism is reflected in the diversity of species concepts. (Wilkins 2002) has listed 26 distinct species concepts along with synonyms. Here, I include one concepts broadly used and debated, in order to consider how to recognize species.
The BSC (biological species concept), is considered as an interbreeding concept or reproductive isolation concept (Lee 2003; Wilkins 2009 ) . The basis for this consideration lie in the concept´s specifications: Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups (Mayr 1942),
So the reproductive isolation in nature, was the key factor in identifying and maintaining species as discrete entities (Claridge 2009). But this key factor, was the principal trouble at the time to recognize the species. Thus, in practice the BSC would recognize that different species are characterized by distinct barriers, isolating mechanism or by the SMRS (the Specific Mate Recognition System of Paterson). However the species taxa are only rarely recognized by direct studies of the SMRS (Claridge 2009). Further, neither asexual nor Parthenogenetic organism can be considered under the BSC because they don’t have a functional system of mate that leads to the fusion of gametes and the reproductive isolation. The geographical variation of the populations vary from almost nothing to large differences but the reproductive isolation in the field can be determined only for sympatric populations , then the allopatric forms began to be recognized with a pragmatic approach as subspecies (Mallet 2005).
Perhaps one of the principal problems with the BSC is the practical impossibility of establish the reproductive isolation between natural populations (Balakrishnan 2005). Another point of criticism, including lack of temporal dimensión, the BSC defines the species only in some point in time (Lee 2003; Balakrishnan 2005). Under the BSC, as the other concepts related to the interbreeding concepts, the species at a given point in time are groups of organisms which interbreed with each other and are reproductively isolated from other species. Likely employing gene flow would tend to delimit the same taxa as species, and thus to depict the boundary of the species rank in the same place (Lee 2003).
Claridge, M. 2009. Species Are Real Biological Entities. In: Contemporary debates in Philosophy of Biology. (Ed. F. Ayala and R. Arp). Wiley-Blackwell. p 91-109
Lee, M. S. 2003 Species concepts and species reality: salvaging a Linnaean rank. J. Evol. Biol. 16. Blackwell Publishing ltda.
Maclaurin, J. & Sterelny, K. 2008. What is biodiversity?. The University of Chicago Press, Ltd., London. p 31.
Mallet, J. 2005. Species Concept. p367-373.
Mayr, E. 1942. Systematics and the Origin of Species from the Viewpoint of a Zoologist. Columbia University Press: New York.
Wilkins, J. 2002. Summary of 26 species concepts. Copyright © 2002 John S. Wilkins
Wilkins, J. 2009. Species: A History of the Idea. University of California Press, Ltd. London.p 197- 201.