Principios de Taxonomia

4.7 A Species cannot be Defined by Traitsj81
certain limit, homogenization of traits by genetic recombination declines, and local
races arise (Chapter 5).
Trait similarity and trait differences are certainly phenomena that are causally
connected to biological rules. Similarity of traits could be the result of both common
descent (the species concept of kinship) and sexual gene exchange (the species
concept of the reproductive community). If species are grouped by kinship or as gene
flow communities, similarity in traits has a certain probability. However, the trait
similarities among the members of a descent community, as well as the trait
similarities among the members of a gene-flow community, can be substantially
weakened. Furthermore, the similarities, or rather the dissimilarities, of traits can
also have other causes (see several of the Color Plates).
Thus, it should not be a principle to treat the members of a species as equivalent
with regard to their traits because often, they are not equivalent in this manner. The
inference of status as a species from trait equivalence has the merit only of an initial
suspicion (Mahner and Bunge, 1997). In discovering two groups of organisms with
different traits, one can only claim a lower or higher probability for the assumption
that these groups are two different species. Trait similarity in general is not a criterion
of species membership. It is not possible to establish a species definition based on
criteria that apply only by the majority of examples, but not by all.
Since the organisms of different species are distinguished from each other by their
traits, but the organisms within a species are also distinguished by their traits, the
question arises of whether intraspecific differences (that is, differences between
organisms of the same species) are based on trait qualities that are different in type
from interspecific differences (that is, differences between the organisms of different
species). Basically, there are three lines of thought on this issue:
1) A species has fewer intraspecific differences than there are interspecific differences
between related species.
This relationship could be true, but it does not have to be true in all cases. With
respect to DNA sequence traits, which have the advantage, compared to morphological
traits, that they can be counted, individuals of the same species could
differ more from each other than the members of different species (Avise, Walker,
and Johns, 1998; Verheyen et al., 2003). Thus, it is not admissible, at least in the
case of evolutionarily young species, to elevate the extent of trait differences to a
species criterion, in the sense that small differences indicate the same species,
whereas more pronounced differences must indicate a different species (see the
criticism of barcoding below).
2) The differences between species rely on special trait qualities, whereas intraspecific
differences have different trait qualities.
Is there any type of trait that marks a species as a species, as in chemistry, where
the number of protons determines the identity of a specific element? Can we
simply choose a morphological trait (e.g., the number of limbs) and divide the
animal kingdom accordingly into groups in a law-like manner, similar to a
chemist compiling a table of elements? This approach has certainly been
attempted in the past in taxonomy research (Chen, 2002). The modern response,