Principios de Taxonomia

2.2 Can Species be Defined and Delimited from one Another?j11
Many people devalue the species problem by saying that there are many other
objects in our lives and in the natural world that also cannot be unambiguously
defined. For example, no one can clearly say what a gene is (Paulsen and Nellen,
2008). The modern concept of a gene is no longer exclusively restricted to proteincoding
units in the genome but encompasses a wide range of genetic and epigenetic
variations, without scientists being able to find a dividing line in the genome where
one gene ends and another begins.
With regard to differences in the content and biological functions of genome
segments, which are all included within the common concept of a gene, and the
vagueness of species delimitation, there is actually a parallel between the species
concept and the concept of the gene. This parallel is, however, weak. No one would be
as inflamed by this argument if it concerned deciding whether a particular genome
segment is a single gene or two separate genes. Almost every geneticist would answer
that it is unimportant whether these two genome segments are considered to be two
separate genes.
The situation is different for biological species. For example, a bird watcher would
become excited if someone questioned whether two bird species might be a single
species rather than separate species. That the African elephant is not a single species,
but breaks up into two species is viewed as a matter of such importance that it was
reported in the journal Science (Vogel, 2001). A similar report of the splitting of a
gene, thus far considered to be a single entity, into two separate genes would not be
considered to be of such merit. What is the explanation for this difference?
At first glance, everything seems obvious. The biodiversity of organisms seen in
nature is not a uniform continuum: living organisms are arranged into groups. If a
number of tits are seen in the gardens of a village in Europe, it is not a continuum of
tits that is observed. Instead, some are recognized as Great Tits (Parus major), while
others are Blue Tits (P. caeruleus). Additionally, the diversity of traits among different
organisms is not distributed uniformly; there are peaks and troughs in the distribution
of traits. It can immediately be seen that biodiversity falls within structured
groups that can be referred to as species.
The first hints of trouble that emerge from this apparent unambiguousness are
the smooth transitions between species, that is, the blurry valleys among the peaks.
There are high valleys with rugged slopes and flat mountains with gentle slopes.
There might even be some hint of valleys between two peaks. There are no deep
crevasses, provoking the question of how deep such gashes have to be to separate
one mountain into two mountains. The question arises of whether certain stones at
the bottom of a valley belong to the mountain to the left or to the right, simultaneously
to both mountains, or to neither of them. Mixed zones are problematic with
regard to group formation.
Yet it seems that the phenomenon of gradual transitions between mountains does
not change the fact that there are mountains. Almost no one would infer that
mountains do not exist based on the occurrence of gradual border zones. Nevertheless,
a few taxonomists have drawn just that conclusion: owing to the overlap
between two species and the resultant impossibility of assigning these hybrids
unequivocally to one of the two species, they infer that species may not exist at all