Zootaxa recently published a brief paper on the philosophical question: how many species concepts are there? The author is John Wilkins, who has written also a book on the subject (if you look it up you may be able to find a free download on the web), and who has a blog (http://evolvingthoughts.net) with philosophical contributions to biology.
Within the context of systematics and taxonomy (Wilkins has an extensive post on his blog), he mentions seven “basic” species concepts (with up to 27 variations or mixtures of them): agamospecies (asexuals), biospecies (reproductively isolated sexual species), ecospecies (ecological niche occupiers), evolutionary species (evolving lineages), genetic species (common gene pool), morphospecies (species defined by their form, or phenotypes), and taxonomic species (whatever a taxonomist calls a species). It may be clear that these are a mixture of concepts of what species are and how we identify species. Wilkins goes back to the simple, biological definition of species by John Ray in 1686: species are those groups of organisms that resemble their parents. He then argues that based on this species concept, all others are conceptions; some think species only exist in the mind of biologists. There are two explanations why species are species: ecological adaptation and reproduction reach.
Thus his final conclusion is 27-7-2-1, and n+1 definitions of “species” in a room of n biologists.
It is good that philosophers from time time to time clear up the mess that we biologists make of our scientific world. Although, it may appear that Wilkins has an opponent with a contrasting view… We’ll see.
Wilkins, J.S., 2011. Philosophically speaking, how many species concepts are there? – Zootaxa 2765: 58-60.