de Queiroz 2007

de Queiroz, K. 2007. Species concepts and species delimitation. Syst. Biol. 56(6): 879-886

I've always felt that the competing species concepts had an underlying commonality to them. In this paper de Queiroz has clearly confirmed this for me - there IS a unifying species concept.

The unifying species concept is:

Species are separately evolving metapopulation lineages.

I love it! So clear, so easy. Undergrads can understand it. And it is a clearly separate issue from species delineation, as de Queiroz points out.
Here are some more clarifying definitions that de Queiroz gives:

lineage = refers to an ancestor-descendant series (Simpson 1961; Hull 1980) [not to be confused with a clade or monophyletic group which is sometimes also called a lineage!]

metapopulation = an inclusive population made up of connected subpopulations (Levins 1970, Hanski and Gaggiotti 2004)

-most of the old alternative species concepts adopt different properties of lineages as secondary defining properties
-these secondary properties (secondary species criteria) arise at different times during the process of speciation, ergo their incompatibility

-one of the great things about a unified concept is that it "clarifies the issue of species delimitation by clearly separating the conceptual problem of defining the species catagory (species conceptualization) from the methodological problem of inferring the boundaries and numbers of species (species delimitation)

-under a unified species concept, the 'old species concepts' (now appropriately called species properties), are now more appropriately viewed as lines of evidence relevant to the fundamentally different methodologies. This seems to be the trend with new systematics papers, where authors go through the old concepts and see if their species holds up to them. I like it because, as de Queiroz states:
"disagreements about species delimitation should result from disagreements or differences concerning one or more of the following issues:

1) the reliability of particular methods (i.e., for inferring lineage separation)
2) the relevance of particular data
3) temporal scale (years versus decades versus centuries, etc.)
4) prospective versus retrospective perspectives [huh?]
5) cases of incomplete lineage separation"

*the main point being that a highly corroborated hypothesis of lineage separation (separate species) requires multiple lines of evidence.

Good paper!