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biological speciesgroups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups. Recently the biological species concept (BSC) has been criticized by the proponents of the PHYLOGENETIC SPECIES concept (PSC) particularly on the basis that:
- It does not rely on rigorous analysis of taxonomically valid characters but on an inferred speciation process.
- That the process of reproductive isolation is usually untestable, giving rise to necessary assumptions concerning reproductive isolation.
- POLYTYPIC species are not single evolutionary units.
However, the PSC presents even more problems and again, in the long run, depends on the opinions of the taxonomist. Disagreements amongst taxonomists concerning subspecies under the BSC would translate to a much more difficult situation of species under PSC, and its supporters include those apparently intent on increasing the overall number of species - more than doubling the number of bird species for example. However the number of species recognized is not a factor in the argument. Speciation is an ongoing process and inevitably the point at which a population ceases to breed freely with other populations (thus making the species the only natural taxonomic unit) is difficult to recognize and this gives rise to controversy. At the present time a good case has not been made out to dispense with the biological species.