Philosophy and Species 279The Ranking Problem
Is species a fixed—that is to say, absolute—rank in taxonomy (in contrast to the
so-called “artificial” higher taxa)? Arguments about whether some or all taxonomic
ranks are natural or not go back a very long way. Linnaeus clearly thought that spe-
cies and genera were natural ranks, while Buffon felt that they were not (at one time),
and even today some present alternative views that species themselves are not natu-
ral, but only individuals, or presently terminal taxa, are. Much of the species debate
has been over what it is that species are that makes them a natural (that is, real) rank.
Discussions of reproductive isolating mechanisms, for example, or ecological niches,
geographic replaceability, and so forth take it for granted that there does exist a real
rank of taxa, that it is appropriate to call that rank the species-level, and that what we
are all disagreeing about is what to use to define that level, or rather what it is that
makes some taxon that level. Other problematic cases, such as ring species, well-
defined geographic races, facultatively interfertile species, superspecies, sympatric
species swarms, and of course asexuals, both quasispecies and secondarily asexual
taxa, lead some to think that the notion of species is a homonym for many distinct
concepts. Moreover, species seem to be constituted in different ways in different
groups of organisms. Many plant species are formed, for example, by hybridization,
a process that is often ignored or glossed over when discussing animal species. Some
fungi have multiple sexual morphs instead of the regulation two, and so reproductive
isolation becomes a much harder concept to qualify in their case than in animals.
Consequently, if we are trying to compose a general concept of species that applies
to all living things, or to compare and contrast the concepts that are relevant only in
particular cases, two questions arise: one is whether there is warrant for thinking that
there is indeed a rank of species in all taxonomic hierarchies or whether it is relative
to the discriminating criteria used in a given group of taxa; the other is whether a
species taxonomic level is required at all.The Commensurability Problem
The most common and popular species concept is, of course, the biological species con-
cept championed by Mayr since 1940. It is typically the definition taught to undergradu-
ate zoology students and often the one taught to botany students. It is not, however, taught
to bacteriology students, nor is it the preferred definition of lichenologists, as the taxa
that are formed by most bacteria do not rely on constant sexual reproduction if any, and
lichens are obligate symbionts formed by a mutualistic association between blue-green
algae and fungi.^11 Immediately the commensurability problem rears its head. Species
among animals are not commensurate with species among many single-celled organ-
isms or species among symbionts. Some, such as Dobzhansky,^12 simply denied, as we
saw, that asexual organisms can form species (since by definition reproductive isolation
is what makes species, and every asexual individual is reproductively isolated). This is
surely putting the definitional cart before the evolutionary horse. If species are outcomes
(^11) Purvis 1997, Leavitt et al. 2016. A recent paper suggests that lichens are the result of three symbionts,
with an ascomycete yeast completing the triad [Spribille et al. 2016].
(^12) Dobzhansky 1937, 320ff.