Principios de Taxonomia

4.8 What are Homologous Traits?j83
The four-footedness of a newt and a horse traces back to a common ancestor. This
may be considered an example of homology because it is unrelated to environmental
adaptation. The newt, with its clumsiness, would perhaps move better on six feet. On
the other hand, the fins of a seal, a whale and a fish look similar; however, this type of
construct obviously does not constitute homology, because it is a result of environmental
adaptation. The striking similarity of the fins has nothing to do with common
descent but is a result of the same selection pressure. This phenomenon is called
convergence (parallel evolution). When similar traits are observed in different
organisms, it is often difficult to substantiate whether the traits are similar because
of homology or because of convergence.
For a long time, it has been clear that the many similarities between Swifts (family
Apodidae) and Swallows (family Hirundinidae) trace back to convergence. Both
families belong to different orders; they are, therefore, only slightly related to each
other. The streamlined body and the sickle-shaped wings are not the same between
Swifts and Swallows because they share the same ancestor but because they share the
same behavior. Only a few years ago, it has been detected that the great similarities in
morphology and behavior between New World Vultures (Cathartidae) and Old World
Vultures (Aegypiinae) also do not result from common descent but instead result from
convergence (del Hoyo, Elliott, and Sargatal, 1994). The seven species of New World
Vultures, to which the Andean Condor (Vultur gryphus) belongs, are more closely
related to Storks (order Ciconiiformes) than to the Raptors (order Falconiformes). The
short, raptor-like hooked beak and the bald facial skin are adaptations to the gathering
of food from the interior of carcasses; these are not traits that trace back to a common
descent.
At first view, one would say that New World Vultures and Old World Vultures are
not homologous to one another. But the question arises whether the term homologues
can be applied to an entire organism, such as a Vulture. The concept of
homology appears to refer to replicating structures, such as cells or DNA molecules.
Can a trait, in principle, be homologous to another trait? Can phenotypic traits such as
body parts, construction plans, behavioral patterns, and developmental paths in
principle all be treated as homologous to each other? Two phenotypic traits cannot
have a common ancestor because they do not replicate. All morphological structures
are complex, meaning that they are assembled from individual parts. It is not possible
to trace these structures back to homologous genes. If one tries to trace complex
structures back to common ancestors, this succeeds only if the complexity of the
morphological structure is dismantled into its parts. Then some of the parts are
homologous to each other and others are not.
Homologous genes in no way need to encode similar phenotypic traits. The evodevo
research of recent years has shown that many single genes can change their
role in the developmental paths of different species, so that they then influence
markedly different morphological structures. Examples of this phenomenon are the
transcription factors distal-less, engrailed and orthodenticle, which, as orthologs in
various metazoic taxa, influence the pattern formation of distinctly different structures
(Mindell and Meyer, 2001). Thus, markedly different developmental processes
can be governed by homologous genes.