Principios de Taxonomia

4.10 The DNA Barcoding Approach – is Taxonomy Nothing more than Phylogenetic Distance?j87
time; (ii) reproductive isolation is based on the slow accumulation of many
genetic differences throughout the genome; (iii) genetic divergence automatically
leads to reproductive isolation between species; and (iv) premating and
postmating reproductive isolation have a similar genetic basis. I argue that so
many exceptions to these assumptions have been demonstrated that they
cannot be used with any reliability to distinguish different species.
All of these four assumptions are not generally valid and are arguments against the
biological foundation of the barcoding approach. If you believe in the truth of all of
these four assumptions, then you may believe that barcoding is the real alternative to
all of the conservative species concepts. However, the four assumptions do not hold
true for several examples of species formation.
What is the basic principle of the barcoding approach? All DNA sequences are
subject to mutation. Many mutations disappear soon after their appearance because
they are eliminated by selection or genetic drift (Chapter 5). However, a few
mutations survive over the course of time. If the branches of the phylogenetic tree
become separated from each other, then different mutations accumulate in the
different lineages in the course of longer evolutionary timeframes. The lineages
diverge, and the DNA of the genomes becomes more and more different among the
organisms. If it is viewed very roughly, this mechanism proceeds proportionately
with time. For this reason, it is also referred to as a molecular clock. It is possible to
tell, from the number of base differences between two DNA sequences that are
homologous to each other, how long ago the time was at which two lineages separated
from each other. DNA sequence differences are a measure of the kinship of two
lineages. Many base exchanges means that there is a distant kinship and few base
exchanges means that there is a close kinship.
This principle is used by the method of barcoding. It was in 2003 that Paul Hebert
from the University of Guelph, Canada, declared DNA barcoding to be the
trendsetting method for identifying species (Hebert, Ratnasingham and deWaard,
2003). In this way, the pursued objective is easily understood. Barcoding means the
sequencing of a 648-bp region of the cytochrome c oxidase I gene (COI) from the
mitochondria and its comparison among different organisms. Because this sequence
can be read routinely by automatic scanners in the same way as a supermarket
scanner distinguishes products using the black stripes on the packaged goods, this
technique has been called barcoding.
The 648-bp region is being used as the standard barcode for almost all animal
groups. It has been shown that 95% of the tested species of various animal groups
possess distinctive COI sequences that allow species identification. Mitochondrial
DNA sequences do not recombine their genomes after sexual merging of the sperm
and egg, and therefore, they have some advantages compared to nuclear DNA
(Ballard and Dean, 2001). In plants, however, the COI gene of the mitochondria
is not an effective barcode region, but two gene regions in the chloroplast have been
found to be suitable for species identification in plants. It basically remains
unexplained why a specific mitochondrial gene region should work for almost all
animal species but not for plant species. Mitochondria play the same biological role in