372.4.1 Many Possible Classifi cation Schemes
While it is unlikely that anyone would ever place oaks and robins in the same taxo-
nomic category, the process of scientifi c classifi cation is not always as straightfor-
ward, and existing groupings are constantly challenged as new data and/or analytical
techniques become available. The “biological species concept” of Ernst Mayr,
which is frequently cited by Berlin , is currently the most infl uential of all species
concepts, but it is by no means the only one. Ereshefsky [ 114 ] describes no less than
seven prominent species concepts,^3 which can be placed into three broad categories
on the basis of the criteria they use to determine what constitutes a species: inter-
breeding, ecology and phylogeny. A description of each of these concepts would be
beyond the scope of this chapter, but it is important to note that the use of different
species concepts can radically change the nature of the classifi cation system: cot-
tonwoods and balsam poplars, for instance, can interbreed, despite their being rec-
ognised as different species on the basis of their genetic, morphological and
ecological distinctiveness. For Ereshefsky, all three types of criteria stem from
legitimate biological phenomena, which is why he believes that a plurality of valid
classifi cation systems is possible:
Because biologists disagree on the correct approach to species, they provide different clas-
sifi cations of the organic world. Moreover, these approaches cross-classify the world’s
organisms by placing the same organisms in different species taxa. (p. 133)
In addition, he takes his argument even further, and provides reasons why he is not
only a “taxonomic pluralist”, but also a “metaphysical pluralist”, i.e. someone who
believes “ that the world is carved in multiple ways, and the most fruitful way to rep-
resent that world is with a plurality of classifi cations ” (p. 43). Species pluralism,
then, is a real feature of the world, and is the result of the interplay of the three evo-
lutionary forces named above. While not everyone shares Ereshefsky’s opinion,
other researchers may still be characterised as taxonomic pluralists, who are simulta-
neously “metaphysical monists” or “metaphysical agnostics”, to use his terminology.
Agapow et al. [ 116 ], for instance, advocate an intuitive idea of a species as “ a popula-
tion on an autonomous historical trajectory ”, but nevertheless admit that
...the very term “species” is deeply ambiguous. While biologists customarily treat species
as tangible entities—at least more so than any other sub- or supraspecifi c group... there is
nonetheless a vast spectrum of meanings attached to the word. The argument over how spe-
cies should be defi ned is endless... [the] concepts encompass many operational and empiri-
cal defi nitions, often resulting in a given group of organisms being viewed in drastically
different ways by different workers.
The phylogenetic species concept, in particular, which currently has equally
vocal and passionate supporters and detractors, has the potential to completely alter
the way in which species are perceived by taxonomists. In a meta-analysis of almost
90 phylogenetic studies on taxa ranging from lichens to whales, Agapow et al. [ 116 ]
(^3) Mayden [ 115 ] lists over 20 species concepts.