Species, Taxonomy, and Systematics 411a number of similarities, including having large, fuzzy antlers. What explains that
similarity? One cause — the proximal cause — is the individual development of
each male elk. A deeper explanation — the distal cause — turns on relationships
between male and female elks. Male antlers are the result of sexual selection. Such
selection requires the participation of both male and female elk. Looked at in this
way, we see that the existence of similarities within lower level groups, here within
the genders, depend on higher level groups (here species) and the diversity within
them. That is, polymorphism at the higher level, and the population structure
that binds polymorphism, is essential in explaining lower level similarities within
the genders and other sub-groups of a species.
Population structure explanations are common, and arguably essential, for un-
derstanding diversity and similarity within species. Such explanations are also
essential for understanding the identity conditions of species. As we have seen,
species are first and foremost genealogical entities. Genealogy is an inter-population
structure — species are lineages of populations. So according to biological sys-
tematics, species identity is defined in terms of population and inter-populational
structures, not organismic similarity. PST theory, unlike HPC theory, properly
captures the identity conditions of species. Stepping back, we see that PST has
three virtues. First, it accounts for both similarity and polymorphism within
species. Second, by citing population and inter-population structures, PST pro-
vides a fundamental explanatory schema for explaining trait distributions in species.
Third, that explanatory schema provides the proper basis for understanding species
identity.
3 TAXONOMIC PLURALISMSo far, we have discussed the nature of species taxa. Let us now move up one level
and ask about the nature of the species category. Typically, biologists and philoso-
phers believe that there is one correct definition of that category; they believe that
there is one correct ‘species concept,’ as biologists call it. However, the biological
literature contains over a dozen species concepts [Claridge, Dawah, and Wilson,
1997]. These concepts are not fringe or crank concepts, but concepts proposed
and investigated by prominent biologists. Biologists and philosophers have taken
two stances to this plethora of species concepts. Monists believe that one concept
is the correct species concept, or one concept is to be preferred over all others.
Monists believe that biologists need to sift through the various proposed species
concepts and determine which concept gives the proper definition of ‘species.’ Plu-
ralists take a different stance. They believe that more than one species concept is
worthy of acceptance. This section explores the arguments for and against species
pluralism.