Principios de Taxonomia

84j 4 What are Traits in Taxonomy?
In contrast, the same or similar developmental processes in different taxa can be
controlled by markedly different genes that are non-homologous to each other. For
example, gastrulation in the early embryonic development is referred to as homologous
across all vertebrates, although this developmental process is controlled by
significantly different genetic mechanisms. Thus, it is not consistent to label the
gastrulae of different animal phyla as homologous. This constraint is at best present
for some of the partial components that are involved in gastrulation. Only these
components can be considered to be homologous. Another example are the supposedly
homologous body segments of Drosophila and of the Migratory Locust
(Locusta), which are, however, genetically controlled by different transcription
factors. Thus, these body segments, as a whole, cannot be homologous to each
other, because only some components are homologous. Similarly, one would not
label two things as being colored red when they are colored both red and blue, so that
the commonness of both is not only the color red.
Furthermore, the concept of homology becomes less and less useful in its
significance and meaning when going further back into phylogeny. If two structures
descend from a common ancestor that traces far back into phylogeny, then the
assessment of the common descent and, thus, the prevailing homology relation does
not produce any specific gain in scientific knowledge, although it is logically
unassailable to call all of these structures homologous. Because of the common
descent of all of life, every structure is, in the end, homologous to every other
structure.
4.9
The Vertebrate Eye and the Squid Eye: They Cannot be Homologous Nor can they
be Non-Homologous
Most traits are complex. This observation means that what is considered by the
taxonomist to be a trait in reality is a complex structure that is composed of several
components, because evolution has structured the traits of a phenotype according
to a modular principle. The individual components of a single trait sometimes
share very different evolutionary histories. This scenario clearly means that many
traits cannot be homologous and also not non-homologous, because they are
composed of components with different phylogenetic descents. Even several
proteins can be partially homologous to each other and partially not homologous,
because different proteins can be modularly assembled from different domains. If
two proteins in two different organisms are compared, some of the domains of a
specific protein might be homologous, whereas other domains of the same protein
might be convergent parts that have independent evolutionary origins. Examples of
this phenomenon are some trans-membrane domains and specific receptor
domains (Ast, 2005).
The concept of homology is often misunderstood. The disagreement of scientists
on the concept s formation has led to the homology problem (Bock, 1989).
Researchers with differing interests, experiences and objectives have defined the