Principios de Taxonomia

46j 3 Is the Biological Species a Class or is it an Individual?
sufficient for the element to belong to the class of this element. This type of logic
applies to chemical elements.
However, is there a trait in an animal species that every organism must necessarily
possess to belong to the species? Would every organism on the planet need to belong
to this animal species if it had the trait? The answer is no (Wilkins, 2010). Biological
organisms have no traits that are necessary and sufficient for taxonomic membership.
Every trait that characterizes the species membership of an organism can be
absent from each individual organism, and if such a trait that characterizes the
species membership of an organism is absent, this organism nevertheless does not
lose its species membership. A non-striped Tiger is still a Tiger.
This is very surprising; the fact that the absence of a species-specific trait does not
preclude membership of the organism to a species clearly shows that traits cannot be
the essence of species membership. Why is the number of protons in a chemical
element a trait that does not tolerate exceptions, in contrast to the traits of living
beings that do tolerate exceptions? The essence of species membership must be
something other than traits (see Section below: The relational properties of the
members of a species are the essence of the species ).
Darwin s theory of evolution explains the fundamental differences between the
essences of chemical elements and living beings. Darwin put an end to the idea that
animals or plants have essential traits. The intrinsic traits of biological organisms
cannot be essential for these organisms affiliations with a species because all of the
biological organisms intrinsic traits are subject to mutative change thus evolution.
Neither morphological, physiological, ethological, chromosomal nor genomic
traits are essential for the members of a species and neither are certain DNA
sequences. This is because all intrinsic traits can change permanently, including
the DNA sequences in the genome. In this respect, there is no difference between a
DNA sequence and a phenotypic trait. Both are diagnostic of the species, but neither
is essential. If a species-specific DNA sequence mutates and thus changes, the
affected organism does not lose its species membership. If one were to assume that
a species is only that which has a very particular set of traits, then a species could, in
principle, only exist for a single moment because some traits may already have
minimally changed by the next moment. Therefore, a species cannot be defined by
its intrinsic traits.
Species are subject to reproduction and continuing mutative changes. In chemistry,
the elements do not multiply but remain constant. No gold atom divides into two
daughter gold atoms and then dies; this means that the objects of chemistry are
present as rigid classes. The chemical elements have always possessed this quality,
and they are likely to continue to express this quality in the far distant future. The
elements have properties on Earth that are identical to their properties on distant
planets. The elements are universals in the philosophical sense (see below), at least in
the frame of our currently existing, spatiotemporally restricted universe.
The objects of biology are an entirely different matter; they change constantly, and
it is impossible for even one of our animal or plant species to exist simultaneously on
a distant planet. If there are animal or plant species on distant planets, they must be
completely different species or they must have migrated there from Earth.