Principios de Taxonomia

4.9 The Vertebrate Eye and the Squid Eye: They Cannot be Homologousj85
concept of homology differently, and the definition has also changed over time
(Mindell and Meyer, 2001).
Here is an example of the difficulty of concept formation: The relationship between
the vertebrate eye and the squid eye is a popular textbook example of convergence.
Both of these visual organs have evolved independently from each other over the
course of evolution; the vertebrate eye evolved from the brain, and the squid eye
evolved from the skin. In spite of these different origins, both eyes show amazing
similarities in the entire organization of the lens, vitreous body and retina. These
similarities are non-related secondary similarities that are caused by equivalent
selection pressures. For this reason, the vertebrate eye and the squid eye are
considered to be convergent.
However, in spite of this scenario, there are also reasons to consider them to be
homologous. This proposal is not contradictory; instead, it is simply based on the fact
that both organs are assembled from parts that have similarities that are based on
homology, which is a real kinship, as well as from parts that have similarities that are
based on parallel evolution. Some components of the vertebrate eye and the squid eye
are homologous, whereas other parts are not homologous and, instead, are only
convergent.
For example, the gene pax 6 (paired-box gene) is an important key gene for the eye s
development and function. Pax 6 is a tissue-specific transcription factor that
regulates the eye s development in an early development stage and plays a role with
regard to the differentiation of the lens and retina (Gehring, 2002). The pax gene is
strongly conserved and, in vertebrates and squids, originates from a common
ancestral stem gene. This gene is, therefore, a distinctly homologous component
of both eyes – the squid eye and the eye of the vertebrate (Piatigorsky and Kozmik,
2004). Other genes, however, which also shape the traits of the eye, are not related
with respect to kinship. Thus, the eye is composed of traits that are partly the products
of homologous genes but are also partly the products of non-homologous genes.
What, then, is a homologous organ? Is homology based on the construction plan
(in which case the vertebrate and squid eye would not be homologous to each other),
or is homology based on specific traits (in which case the vertebrate and squid eye
would be homologous as well as non-homologous to each other)? Clearly, it is not
possible to apply the concept of homology to complex structures in biology. Organs
are almost never structures of common descent. Their components and tissues are
not descendants of a common ancestor. Most organs are assembled from individual
parts, each with a different evolutionary history. Some of these components could be
homologous but others of the same trait could well be non-homologous. If a complex
trait is compared between two organisms, it often can be neither homologous nor
non-homologous. Only the individual parts can be homologous to one other. One
would not consider two objects as being colored red when they are colored both red
and blue.
It appears that the basic question of whether two organs are homologous or not was
a meaningless question from the start. The application of the concept of homology to
entire systems is an incorrect way of reasoning, because this approach ignores the
modular principle of evolution. It may be that an aversion of conservative biologists