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subset of LGT that leads to homologous recombination is largely limited to closely related bacteria. This finding supports a view of bacterial species that resembles, in some respects, the biological species concept.

On the matter of the multiplicity of species concepts, Kevin de Queiroz, in “Ernst Mayr and the Modern Concept of Species” (Chapter 13), has contributed an article that directly targets one of the main sources of confusion that arises in species concept debate. That confusion lies between species criteria, as articulated in various species concepts, which are actually contingent properties of species, and the necessary properties of species as they are understood in the general sense of being evolutionary lineages. Biologists who disagree about which contingent properties of species are the most useful for identification and classification should be able to find common ground by recognizing the contingent, as opposed to necessary, aspect of the features they prefer to study.


Mayr, E., ed. (1957) The Species Problem (Am. Assoc. Adv. Sci., Washington, DC).

Mayr, E. (1987) The ontological status of species: Scientific progress and philosophical terminology. Biol. Philos. 2, 145–166.

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