Principios de Taxonomia

4.10 The DNA Barcoding Approach – is Taxonomy Nothing more than Phylogenetic Distance?j89
(Stiassny and Meyer, 1999; Verheyen et al., 2003) (Chapter 6). Over presumably less
than ten thousand years, there has been an adaptive radiation of enormous extent.
Lake Victoria was completely dried out approximately ten thousand years ago. By
some mechanism, a few Cichlids entered the lake afterwards. In the short time until
the present, 400–800 species originated from this very small founder population.
These species drastically differ from each other with regard to their traits, which
are responsible for ecological adaptation and partner recognition. They have varying
body sizes and color patterns, varying fin forms and also widely varying mouth and
jaw shapes, which reflects an adaptation to varying types of food consumption and
mutual partner recognition. The Cichlid species of Lake Victoria and other African
lakes differ genetically even less than humans of different populations (Schliewen
et al., 2001; Verheyen et al., 2003). This example clearly documents that the
phylogenetic distance between two species is not a measure of the extent of their
mutual reproductive incompatibility, and it is clearly not a measure for what is
thought to define a species in a general sense.
The equation of phylogenetic distance and reproductive isolation results from a
misconception of the extent of gene flow within a species (Chapter 6). The exchange
of alleles among distant populations of a species could become very weak across long
geographic distances. In many cases, newly arisen allelic mutants do not reach all
distant populations within a species and, thus, they promote genetic divergence
within a species. Alleles are not fixed anymore along the entire distribution area of a
gene-flow community, but only within limited ranges of the entire distribution area.
As a consequence, geographically distant populations could differ genetically,
although they still belong to a connected gene-flow community (Ehrlich and Raven,
1969). A scenario in which a gene-flow community attains considerable adaptive
intraspecific genome variability is conceivable (Andolfatto, 2001), and a gene-flow
community could be composed of several local races that are genetically distinct
(Chapter 5).
The extent of internal genetic cohesion of the individuals of a biparental species
has been overestimated in the past (Mishler and Donoghue, 1994). This belief can be
traced back to the advocates of the species concept of the gene-flow community,
Theodosius Dobzhansky and Ernst Mayr, who were convinced that gene flow and
genetic recombination would be the main forces that make the individuals of a
species look so similar to one another (Dobzhansky, 1937; Mayr, 1942). However,
Ehrlich and Raven (1969) countered that the extent of gene flow is very limited
between geographically distant organisms of many species and suggested that the
effective population size in plants is to be measured in meters and not in kilometers
(Chapter 6). The individuals of populations that are separated by several kilometers
may rarely, if ever, exchange genes and, as such, could evolve independently. Their
phenotypic similarity could be conserved mainly by selection pressure, not so much
by genetic recombination, as Dobzhansky and Mayr assumed (Bradshaw, 1972).
Lande (1980) has stressed that there has been an overemphasis on the genetic
cohesion of widespread species and argued that of the major forces conserving
phenotypic uniformity in time and space, stabilizing selection is by far the most
powerful.