Principios de Taxonomia

34j 2 Why is there a Species Problem?
that exist in nature. It is highly improbable that more than twenty different concepts
all correctly describe the species that exist in nature.
It is questionable whether the current pluralism of many different species
concepts is desirable or sustainable. The various types of entities that are termed
species differ in their basic regularities are therefore not comparable (Ghiselin,
2002). The impression arises that there is not likely to be a definition of species that
is able to simultaneously combine the characteristics of gene flow, descent cohesion
and trait equivalence in an equally justified manner (Atran, 1999). For this reason, it is
amazing that there are consistent (i.e., monistic) field guides available for the
identification of particular animal and plant groups.
Mayden has itemized the 22 species concepts that are currently in use (Mayden,
1997) and established that many of these concepts are mutually incompatible. No
chemist would accept the existence of different concepts of a chemical element that
depended on the author, especially if these concepts were irreconcilable.
If there are different species concepts, how does one know when to use each
concept? Not every organism with a different set of traits is a different species, and not
all species have markedly different traits. This means that a species cannot be defined
by traits. For example, female Mallards (Anas platyrynchos) are phenotypically very
different from male Mallards, so why are male and female Mallards not different
species? A female Mallard is closer in appearance to a female Pintail (Anas acuta) than
to a male Mallard. Therefore, appearance alone cannot be a criterion for differentiating
species.
An alternative definition of a species that is not based on trait similarity would be as
follows: a species consists of organisms that mate with each other. Organisms that do
not mate with each other belong to different species. Male and female Mallards mate,
but mallards do not regularly mate with Pintails. This example shows that a markedly
different definition of species, based on mating, has to be applied to account for the
fact that male and female Mallards belong to the same species. The species concept of
trait similarity and the species concept of reproductive compatibility are two different
species concepts that cannot be licentiously combined. How does one know when to
apply each species concept?
Mayden presents the 22 species concepts in great detail and with commentary, so
the species concepts will not be repeated here. Instead, it is noted that most species
concepts can be subsumed under three main concepts (Mahner and Bunge, 1997):
1) The phenetic species concept: This concept is understood as classification based on
common traits. Therefore, the species is an entity of maximum covariation between
existing and missing traits. The phenetic species concept is also termed the
numerical species concept because it quantifies the differences in traits between
the taxa and then performs the taxonomical classifications and species delimitations,
a process that is now assisted by computers (Sneath and Sokal, 1973).
The objections for a phenetic species concept are twofold:
(1) First, the phenetic concept is a purely formal, operational concept that treats
biological species as a group of organisms with similar traits. The phenetic
species concept is not concerned with the biological processes that have led to