Principios de Taxonomia

94j 5 Diversity within the Species: Polymorphisms and the Polytypic Species
One should also not confuse species-specific phenotypic traits with the genome.
Many DNA sequences of the genome are neutral and thus, are largely not controlled by
natural selection. They mutate and diverge between separated populations by chance
and, hence, indicate evolutionary distances, but not species-specificity. Traits that are
controlled by natural selection can be very similar among the individuals of a species,
even if the individuals of the species have evolutionary diverged due to geographical
distance. In turn, selection- controlled traits can be similar between the individuals of
different species, even if these individuals have evolutionary largely diverged.
Although genetic differences are a benchmark for evolutionary closeness or distance,
some populations that are genetically very similar could belong to different species,
whereas, in turn, other species that are evolutionarily old and geographically widely
distributed and whose individuals differ genetically could belong to the same species.
Theforcesofnaturalselectioncanresultinphenotypicdivergenceswithinaspecies,
even in the presence of appreciable gene flow. This phenomenon is called intraspecific
polymorphism. Intraspecific polymorphism is a serious problem for trait-oriented
taxonomy (Ford, 1954). Different traits do not necessarily mean different species.
Because most traits have nothing to do with maintaining species differences, two
individuals of the same species can be significantly more different than two individuals
of two different species (P€a€abo, 2001; Avise, Walker, and Johns, 1998). Intraspecific
divergences, in some cases, can be greater than the differences that are found
between congeneric, but distinct, species (Smith, Schneider, and Holder, 2001).
5.2
Differences in Traits do not Necessarily Mean Species Differences
On page 424 of the first edition of The Origin of Species, Darwin writes (cited in
Ghiselin, 1997) the following: With species in a state of nature, every naturalist has
in fact brought descent into his classification; for he includes in his lowest grade, or
that of a species, the two sexes; and how enormously these sometimes differ in the
most important traits, is known to every naturalist: scarcely a single fact can be
predicated in common of the males and hermaphrodites of certain barnacles
(Cirripedia), when adult, and yet no one dreams of separating them. There is no
more accurate expression of how fraught with problems it is to represent the position,
this organism looks different, so it must belong to a different species.
Darwin s adversary, and friend, Alfred Russel Wallace, titled a publication released
in 1858: On the tendency of varieties to depart indefinitely from the original type.
(Wallace, 1858). With this title, he expressed that species are everything but
homogeneous. Species tend to vary indefinitely in their traits. How can it then
be acceptable to make the phenomenon of trait differences into a species criterion? It
is a difficult problem to compare the nature of the trait differences among the
organisms within a species with the nature of the trait differences between the
organisms of different species (see Chapter 4).
Only the traits that keep populations separate from each other have something to
do with the definition of what a species is. Only these traits are worthy of being called