A diagrammatic representation of speciation (metapopulation
lineage divergence). Even if every taxonomist using a different species definition based on different properties (SC1 thru SC8) agreed that originally there was one species that subsequently evolved to become two, there would necessarily be disagreement about the number of species present in the gray zones. Drawing from de Queiroz1.
According to Kevin de Queiroz1, during the period of the Modern Evolutionary Synthesis species (including those that are asexual) "were equated with groups of interconnected populations [metapopulations] that form an extended reproductive community and an unevenly distributed but unitary gene pool or field for gene recombination". Moreover, as de Queiroz points out, even in descriptions of species as lineages, the general metapopulation concept of species was implied.
de Queiroz adopts this notion and proposes that the only necessary defining property of species is that they are separately evolving metapopulation lineages. All other properties that have previously been attributed to species (SC1 thru SC8 in the drawing above), for example, reproductive isolation, or the occupancy of a distinct niche, become properties that species as metapopulation lineages may or may not acquire. In de Queiroz’s words:
...metapopulation lineages do not have to be phenetically distinguishable, or diagnosable, or monophyletic, or reproductively isolated, or ecologically divergent, to be species. They only have to be evolving separately from other such lineages.
It is not clear to me, however, how de Queiroz’s proposal, which otherwise makes perfect sense, would pinpoint the moment of appearance of two species from one if we really cared to do so (see the drawing above). This is, of course, not a new issue and was discussed a while ago by Ernst Mayr2:
...the two daughter species are virtually identical at the moment of the split, and if any species differences evolve in the two separated lines, it is by gradual transformation. This makes it impossible to designate a precise point of origin of the new daughter species.
1Kevin de Queiroz. Ernst Mayr and the modern concept of species. PNAS 2005 102:6600-6607. pdf
2 Ernst Mayr. 1989. Toward a New Philosophy of Biology. (p. 325).