Principios de Taxonomia

6.19 Reproductive Incompatibility is Different than Phylogenetic Distancej161
6.19
Reproductive Incompatibility is Different than Phylogenetic Distance
Reproductive incompatibility is not the same as phylogenetic distance. Therefore,
both criteria cannot be used for a common species concept. A species that, as a group,
has an evolutionary distance to another species is not the same as a species that, as a
group, is reproductively isolated from another species. In other words, the phylogenetic
species is not the gene-flow community, at least not as long as the terms are
applied to evolutionarily young species.
The phylogenetic distance of two organisms and their mutual genetic compatibility
must not be lumped together, although a long-lasting phylogenetic separation in
several cases has resulted in genetic incompatibility. However, there are evolutionarily
and genetically far distant organisms that are still reproductively compatible
with each other (Lande, 1980; Coyne and Orr, 1997). Vice versa, there are organisms
that separated from each other only a short time ago, which could differ less
genetically than humans of different populations but which are reproductively
well-separated from each other. Among these organisms are the Cichlids in many
African lakes (Schliewen et al., 2001; Meyer, 1993). Kinship and sexual compatibility
are not the same thing. The action of defining a species to be a group that has
exceeded a minimum of phylogenetic distance does not represent the species concept
of the gene-flow community (see the criticism on barcoding in Chapter 4).
In contrast to expectation, the occurrence of hybrids between different species is
not always a consequence of a close kinship of the parental species. The concept that
two species would not be able to combine with each other, and if they were able, then
they simply would not be real species yet because they were still too closely related to
each other, is not sustainable in this simple form. There are evolutionarily young
species (meaning closely related species) that are cleanly separated and that hardly
hybridize with each other. Indeed, only a single or a few gene mutations are sufficient
to raise reproductive species barriers (see below) (Phadnis and Orr, 2008;
Prud homme et al., 2006).
That the equation of phylogenetic distance and reproductive isolation is not
justified follows already from the difference between plants and animals. Among
other reasons, plants differ from animals in that they break species barriers much
more often than animals, and they hybridize with each other. However, the ability of
plant species to form species hybrids much more often than animals is not related to
the fact that plant species are more closely related to each other than animal species.
The same consideration applies to bacteria (see above). It is not the phylogenetic
proximity of plants to each other that causes them to be able to hybridize; instead, it is
the plants different biology that makes them have the tendency to have more species
hybridization than animals.
The commonly found equation of phylogenetic distance and reproductive isolation
results from the outdated assumption that the gene flow within a species is always
strong and extensive, so that de novo evolution of allelic mutants either disappears
rapidly or quickly spreads across all organisms of the gene-flow community
(Dobzhansky, 1937; Mayr, 1942). This phenomenon is called fixation of the alleles.