Species with-out evolutionary assumptions

Erika Jazmín parada Vargas.

The Species concept and its delimitation are old problems in biology and multiple debates have been done since multiple disciplines (Cracraft, 2000). Here is presented The Phylogenetic Species Concept (PSC) (sensu Wheeler and Platnick), and are showed some of its advantages.

“Species delimitation, like character-state definition, is a preliminary activity of phylogenetic analysis, and both of these elements, characters and terminals, are the fundamental components of which hypotheses of phylogenetic relationship are constructed” (Davis and Nixon, 1992). This is an important premise because there are concepts (i.e., Biological Species Concept) that rooted the identification of species in presumptions of speciation. The presumption of speciation anchors the concept in the inverted view that understanding speciation is a prerequisite to defining species rather than a product of that delimitation (Goldstein & DeSalle, 2001).

By definition (for PSC) a species is “the smallest aggregation of populations or lineages diagnosable by a unique combination of character states in comparable individuals (Semaphoronts) (Wheeler & Platnick, 2000). It is important to make the distinction between the PSC and the monophyletic species concept (sensu Mishler and Theriot), because the second one is based on the concept of shared derived characters, called synapomorphies (in this case autapomorphies, because they belongs to a single group, and the monophyletic groups are defined by the autapomorphies (Pleijel, 1999)). “If two or more individuals or populations share a derived character then they are assumed to be more closely related than individuals or populations lacking that character” (Goldstein & DeSallle, 2001). But whether or not some individuals within a particular species are more or less closely related to one another than to members of another phylogenetic species is irrelevant to the reconstruction of relationships among species (Goldstein & DeSallle, 2001).

So, returning to the PSC “diagnosability could be described in terms of character constancy in one species versus absence of the character in another, this condition is not necessary for two species to be distinct. One phylogenetic species may carry alleles A and C at a particular locus, whereas another carries B and D” (Davis & Nixon, 1992). Fig1

One of the most interesting points of the PSC is that can predict that a population is a separate species and that it will retain character distintictness; this prediction is potentially falsified by additional observations of character distributions (any character genetically derived) (Wheeler & Platnick, 2000). This is an advantageous methodological tool because it does not need a lot of abstract evolutionary mechanisms to view the rise of a new character. It also allows testing the evolutionary hypothesis because the experimental units are independent from the experiment.

By minimizing unnecessary assumptions, a program does not become antievolutionary. To the contrary, it allows the recognition of other elements of cladistic analysis (i.e., species) so that a pattern may be established that is in need of evolutionary explanation. This and related transformed cladistic analytical methods are not only consistent with evolutionary theory, they are necessary if any particular evolutionary process is to be studied in a testable framework (Wheeler & Platnick, 2000).

Lastly with the redefinition of the species and the definition of groups by the phylogenetic analysis a nomenclatural problem comes, since maybe the new groups do not match with the classical Linnean groups. Based on the Linnean system we could redefine the groups with a tree-thinking and not just abandon widely used nomenclature, causing a communicational mess (Potter & Freudestein, 2005).