412 Marc Ereshefsky
3.1 The Case for Pluralism
Let us start by introducing three prominent species concepts in biology. There
are many more prominent species concepts, but introducing three is sufficient for
providing the argument for pluralism. The most common species concept in the
biological literature is Mayr’s [1970] Biological Species Concept. The Biological
Species Concept defines a species taxon as a group of organisms that can success-
fully interbreed and produce fertile offspring. According to that concept, a species’
integrity is maintained by interbreeding within a species as well as by reproductive
barriers between organisms in different species. The Ecological Species Concept
defines a species taxon as a lineage of organisms maintained and segmented by eco-
logical forces [Van Valen, 1976]. Stabilizing selection maintains a species’ integrity,
while disruptive selection can lead to new species. The Phylogenetic Species Con-
cept (which has multiple versions) defines a species taxon as a basal monophyletic
lineage [Mishler and Brandon, 1987]. A monophyletic lineage contains all and
only the descendants of a common ancestor. Because monophyletic lineages occur
up and down the Linnaean hierarchy, species are defined asbasal monophyletic
lineages — the smallest lineages represented in Linnaean classifications.
These species concepts, the biological, ecological, and phylogenetic, not only
provide different definitions of ‘species,’ their use gives rise to different classifica-
tions of the organic world. This is confirmed by numerous empirical investigations.
The most glaring discrepancy is between the Biological Species Concept (BSC) and
the other two concepts. BSC requires that the organisms of a species exchange ge-
netic information through interbreeding. That requires sexual reproduction. BSC
does not require that every member of a species successfully interbreed, but it does
require that a sufficient number of the organisms sexuality reproduce to maintain
a species’ integrity. The problem is that most of life on this planet does not re-
produce sexually but asexually, through cloning or vegetative means. Asexual
organisms do not form species according to the BSC. Nevertheless, asexual organ-
isms do form species according to the Phylogenetic Species Concept (PSC) and
Ecological Species Concept (ESC). For the PSC, species are defined genealogically,
independent of mode of reproduction. For the ESC, species are defined as lineages
of organisms maintained by selection forces. PSC’s and ESC’s classifications of
the organic world include asexuals, while BSC’s classifications exclude asexuals.
These species concepts carve up the world in different ways.
Other cases of species pluralism are more complicated. For example, the BSC
and the PSC sort the very same organisms into different species. Consider the case
of ancestral species. Many supporters of the BSC believe that a standard form of
speciation occurs when a population of a species becomes isolated from the main
body of a species and undergoes a ‘genetic revolution.’ The parental species, or
‘ancestral species,’ remains intact. For proponents of the BSC, two species are
present in such cases: the ancestral species consisting of A and B, and the new
species C, see Figure 3.1.
However, the PSC cannot allow the existence of two species in this case. Recall