Principios de Taxonomia

4.2 What Basic Rule Defines Traits as Being Taxonomically Relevant?j69
traits that distinguish every particular organism from others in a population. Every
organism is unique in its complexity.
Every biologist is aware of this phenomenon. However, less commonly known is
the obvious problem of how one can actually speak of species differences if the
members of a species differ so drastically in regard to their traits. What is the actual
difference in traits between species when all organisms within a species differ from
each other? Only monozygotic twins are identical in traits.
The phenomenon of allelic diversity conjures the difficult question of what are
intraspecific differences in contrast to interspecific differences. What is behind the
fact that different species are distinguished from each other by their traits? Every user
of an animal or plant identification guide uses the option of distinguishing one
species from another species by certain traits. Is it the extent of differences that makes
it possible to distinguish species, such as the principle that few differences equals
belonging to the same species but many differences equals belonging to a different
species? Or is it the quality of differences that makes it possible to distinguish species,
such as the principle that certain differences equals belonging to the same species
and other differences equals belonging to a different species?
Neither is the case. Numeric criterion can be eliminated immediately. It cannot be
the extent of differences. Otherwise, males and females from a great number of
animal species would have to belong to different species. Qualitative criteria can also
be refused. The principle of What only differs in the number of bristles belongs to
one and the same species, but what differs in the number of legs are different
species, cannot be generalized either.
Every multicellular organism has many thousands of traits. Even today in the age of
genome sequencing, in which the number of animal and plant species whose
genomes are completely sequenced continually grows, there is no understanding
of how many phenotypic traits a particular species possesses in contrast to another
species. The logical conclusion is that the characterization of a species by its traits
always has to be based on a restricted selection of traits and that all traits can never be
considered. We presumably will be able to do this one day after the sequencing of all
genes and all gene regulatory and synthesis pathways have been deciphered.
However, this day still lies in the far distant future. Currently, every taxonomical
classification is assigned according to traits that are based on the selection of a few
traits. This selection is subjective.
The scientist, however, should have the desire to establish an underlying system of
rules for general validity, which define the quality of traits suitable for taxon
membership. Such a theoretical foundation for a trait-based taxonomy should define
for all organisms a system of rules for which traits are relevant for taxonomic
classification and which are not. However, this desire cannot be fulfilled.
The traits that distinguish one species from another are not quantifiable. It is not
possible to determine a number of traits that are species specific. From this, it follows
that a statement such as the following is senseless: The organisms of two different
populations match with regard to half of their traits. For example, Wilhelm Meise
claimed 70 years ago that the Italian Sparrow (Passer italiae) was a hybrid species of
the House Sparrow (Passer domesticus) and the Spanish Sparrow (Passer hispaniolensis)