Principios de Taxonomia

74j 4 What are Traits in Taxonomy?
protecting bone plates (instead of scales). The absence or additional presence of
particular fins, as well as significant differences in jaw shape, body shape, tooth
structure, the defensive spines and in body coloration is also striking.
Most striking are the two forms of Sticklebacks in many North American lakes.
One lives in the open deeper water, and the other lives on the bottom in shallow
areas. The deep-water form exhibits a pronounced, spined ventral fin, similar to the
original sea dweller. The shallow water form lacks both the fin and spine
(Prud homme et al., 2006).
All of these forms are evolutionarily very young. They evolved after the last ice age,
and some forms evolved several times independently from each other. In captivity,
these phenotypically very different fish can be crossed with each other artificially. Of
course, this does not say anything about their species status, but it has an important
practical consequence for the scientist. The genetic cross-compatibility allows for the
identification of genes involved in the reduction of the pelvic fin. Surprisingly, the
remarkable anatomic alterations apparently trace back to only a very small number of
genes that function as the developmental control genes (Kingsley, 2009).
The presence or absence of the ventral spine in the Stickleback essentially traces
back to the expression of only a single gene. This gene is named Pitxl and is (as
usual) controlled by various enhancers. One particular enhancer is responsible for
the expression of the Pitxl gene in the ventral fin, the ventral fin enhancer. In other
body parts, Pitxl codes for the same protein, but it affects the development of other
body structures. Failure of the ventral fin enhancer only causes the failure to express
the gene in the ventral fin area and thus specifically affects only development of the
one spiny ventral fin. The other enhancers remain intact and continue function.
The Stickleback is an exceedingly instructive example for typologically oriented
taxonomists. It shows that the reshaping of entire body structures can trace back to a
single mutation.
4.5
What is the Relevance of Differences in Traits between Two Species?
Two traits can appear different or perform different functions but nevertheless be
encoded by the same gene. What then are two different traits if they are based on the
same gene? If the structural genes hardly differ between two species, what is it that
differs if two species have different traits? If one species possesses a certain trait and
another does not, we assume intuitively that the first species has acquired this trait at
a later time. However, traits can also be lost. The control of the structural genes via
enhancer switches makes apparent that anatomical and morphological alterations
during evolution can be based on the repeated switching on and off of structural
genes that otherwise do not change during the speciation processes.
What is a newly evolved trait if its coding gene is not new? This consideration
questions the concept of Willi Hennig s apomorphy (Hennig, 1966) (Chapter 7).
Hennig s cladistic taxonomy defines the origin of two daughter species from an
ancestor species (stem species) by the fact the two new daughter species must possess