Saturday, March 23, 2013

Classifying the species concepts

This post continues a series on species that began with this introduction.

So looking over the literature and the aforementioned list of 26 species concepts, let's try to classify the concepts into different groups:

Synchronous concepts
    Reproductive community concepts
        Biological species
        Cohesion species
        Genetic species
        Genic species (?)
        Recognition species
        Reproductive competition species
    Phenetic and cluster concepts
        Autapomorphic species
        Genotypic cluster
        Morphospecies or typological species
    Phylogenetic concepts
        Phylogenetic species
        Genealogical concordance species
Asynchronous concepts
    Internodal species
    Composite species
        *Successional species
Incertae sedis
    Evolutionary species

The first division, indicated by bold font, is whether the concepts deal only with species in the here and now or, more generally speaking, with those existing together in one time-slice (synchronous) or whether they have a historical dimension (asynchronous). Note that the latter are rarely of interest for anything beyond theoretical discussion unless we are talking about a group with an excellent continuous fossil record. I am not sure where to place the Evolutionary Species Concept because I may not quite understand it. It sounds a bit like a reproductive community seen from an asynchronous perspective, so I have placed it apart for the moment. (Similarly, I previously understood the genic species to mean something different from how it is described in the list I linked to, so there is a question mark behind it.)

Within the synchronous concepts that deal with delimiting and circumscribing species in the present we have a second division, indicated in italics, where the concepts are grouped by their main grouping and, if defined, ranking criteria. The first group is what we might call the family of the biological species concept, different concepts that attempt to delimit species by the ability or propensity of individual organisms to interbreed. The second group defines species via some kind of character combination or overall similarity. In all these cases, species are groups of individuals that are ecologically, morphologically or genetically more similar to each other than they are to individuals belonging to any other species. In contrast to this essentially phenetic approach, the small third group employs some kind of phylogenetic approach to species circumscription.

I have left out several concepts that are silly or too similar to others. The ones that are further indented and marked with asterisks are also a bit dubious as they appear to represent merely special cases of the concepts directly above them. Even having reduced the number already, it will still be useful to lump them further and discuss, for example, all reproductive community concepts together. On the other hand, it appears useful to take a more granular approach to the phenetic concepts - but that may just reflect my personal biases.

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