Principios de Taxonomia

5
Diversity within the Species: Polymorphisms
and the Polytypic Species
5.1
Preliminary Note
Even today, it is still a common view that organisms that are clearly different with
regard to traits are different species and that organisms that are less different with
regard to traits are subspecies; this belief system does not withstand a persistent
examination. Even on the DNA level, the extent of the differences and the certainty
that they indicate the status of species cannot be equated. There are groups of
individuals that differ genetically but belong to the same species, and there are groups
of individuals that are similar in their DNA sequences but belong to different species.
The first are evolutionarily old species, and the second are young species.
Only a tiny fraction of the genome is involved with species differences (Orr, 1991).
Species arise and are maintained by the action of very few genes, which have earned
the moniker speciation genes. Speciation genes are genes for which the phenotypic
traits cause reproductive isolation (see Section Speciation genes, pre- and postzygotic
barriers in Chapter 6). Speciation genes are often responsible for the speciesspecific
choice of food plants or for the occupation of a specific ecological niche.
Speciation genes are also responsible for the choice of the correct species-specific
sexual partner and hence are responsible for the perpetuation of species barriers.
Speciation genes constitute only a small fraction of the genome (Orr, 2001). The vast
majority of the genome is not involved in differentiating species and keeping them
separate to prevent the merging of diverged populations (Chapter 4).
One should not confuse the species-specific traits that cause reproductive isolation,
and thus are responsible for maintaining species barriers, with those species-specific
traits that became different by chance, that is, as a consequence of long-term
separation leading to the cessation of sexual reproduction and gene exchange. The
latter traits characterize the different species only arbitrarily, because they play no
necessary role in blocking the gene flow. Evolutionarily young species differ in only a
few traits, and these are mainly the traits for the maintenance of the species barriers
(Schliewen et al., 2001). Evolutionary old species also differ in those traits that are not
involved with keeping species separate from each other but which became different
just by chance (Ferguson, 2002).
Do Species Exist? Principles of Taxonomic Classification, First Edition. Werner Kunz.
Ó 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.
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