Principios de Taxonomia

200j 7 The Cohesion of Organisms Through Genealogical Lineage (Cladistics)
reasons why paraphyly cannot be eradicated. What justification is there for a
paraphyletic taxonomy? There are only pragmatic reasons for this; there is no
consistent underlying theory.
Only the human desire to combine organisms into a common group that look
similar to the human eye can motivate the use of paraphyletic groups (Figure 2.4:
B þ C is a paraphyletic group). This is not at all a scientific line of thinking. The
formation of paraphyletic groups means that two different classification principles
are intermixed: grouping according to the descent relation and classification according
to trait resemblance.
The best-known example of paraphyly in biology is the aforementioned class of
reptiles along with the class of birds within the phylum of vertebrates (Figure 7.6). The
reptiles are an incomplete, truncated taxon because the birds that branch off from
the rest of the reptiles at an evolutionarily later time have been withdrawn from the
reptile class. However, the birds are the sisters of the dinosaurs and are closely related to
these, while both together (the birds and the dinosaurs) are less closely related to the
crocodiles, snakes and turtles than birds and dinosaurs are to each other.
Thus, the formation of the class of reptiles without the birds contradicts the logic of
a kinship-oriented taxonomy. This is not taxonomy according to kinship, but
taxonomy according to the assessment of traits. However, the assessment of traits
is based on subjective standards that are set by humans ourselves. This means that
paraphyla (in contrast to monophyla) are artificial groups, whose coherence and
boundaries as separate groups are not demonstrated in nature (Figure 2.4: B þ C is a
paraphyletic group). For the empirically driven scientist who wants to restrict himself
to phenomena that can be objectively observed in nature, paraphyly is a group
formation that is highly questionable, if not to be rejected altogether.
We humans think that birds simply look very different from the reptiles, but we are
unable to quantify this distinction. Birds are warm-blooded, have feathers, are usually
multicolored and often sing in a manner pleasant to the human ear. Accordingly, in
the eyes of humans, they distinguish themselves drastically from crocodiles, snakes
and turtles. These are cold-blooded, have scales or horny scutes, are usually colored
inconspicuously, and make utterances that are almost without exception unpleasant
to humans. The qualities of these criteria cannot be reason to group birds and reptiles
into separate equal-ranked taxa.
For the modern, factually driven scientist, it is disconcerting that these kinds of
criteria serve as a reason to separate the birds from the reptiles and award them an
equal rank along with the rest of the reptiles. Why do the possession of a plumage and
warm-bloodedness rank so high that the birds are ascribed such dissimilarity to the
remaining reptiles? By reverting less to subjective human perception and instead
perhaps valuing certain metabolic enzymes that are not visible to the human eye, the
birds would not be such a unique case compared to the rest of the reptiles. Then, the
birds autonomy would be ended, and the birds would unambiguously belong to
the reptiles, with which they are closely related and from which they should not be
segregated.
Let the contrariness of a trait-based and a kinship-based grouping once again be
demonstrated by the following example (Figure 7.7). Of the four grandchildren