Principios de Taxonomia

6.17 Sympatric and Allopatric Speciationj157
First, it is based on the assumption of a coincidental separation of two populations
caused by external forces, such as, for example, geological forces or climatic events,
and second, it is based on the assumption that, in the course of the allopatric
lifetimes, mutations that lead to genetic incompatibility accumulate coincidentally.
Neither of these two coincidences is founded on a quantifiable scientific theory.
The allopatric paradigm is nothing but the putting-into-words of an ad hoc concept
(Tautz, 2009).
If allopatrically separated populations become different, then selection has no
interest in erecting prezygotic crossing barriers because the populations, which
have become different, do not encounter each other anyway. For this reason,
allopatric speciation usually needs a long time (Orr, 2009), while sympatric speciation
can be a matter of only a few generations because it is stimulated by positive selection.
If the allopatrically separated populations were to meet again later, then the
differences that have originated by chance in allopatry can be an obstacle to remating.
However, these differences do not necessarily have to be an obstacle.
Mayr has defended allopatric speciation as the only way in which species can
originate (Mayr, 1963). The theory that speciation must almost always be allopatric
has consequences that cannot readily be brought into agreement with our current
knowledge of speciation. For example, according to the allopatric paradigm of
speciation, those populations that live sympatrically without an external separation
cannot split and, hence, would have to remain in existence eternally. As long as
speciation must almost always be allopatric, a population can never undergo
speciation under sympatric conditions because, without geographical separation,
there can be no split. (In this consideration, anagenetic alterations of traits are, of
course, not counted as speciation; see Chapter 7).
Furthermore, the theory of allopatric speciation does not explain why there is rapid
speciation in some groups of animals but not in other groups of animals, even though
the groups should, in principle, have the same chance of being torn apart into isolated
groups by external factors. For example, over approximately ten to a hundred
thousand years, the Cichlides of the African lakes have produced, in every lake in
which they occur, ten to several hundred species, while the members of other families
of fish in the same lakes did not (Verheyen et al., 2003) (see below). From this
scenario, it clearly follows that the ability of a population to speciate also depends on
the internal genetic factors of the organism itself, and these factors are specific to the
respective groups of animals. In these cases, speciation is definitively not based on a
separation by external factors but instead is based on the genetic constitution that is
present, to a varying extent, in the different groups of animals.
Additionally, the paradigm of allopatric speciation does not explain why the
number of species is so large in beetles even though most beetles are able to fly
and, therefore, could easily overcome many external barriers. The (apparently) very
large number of beetle species, however, must be acknowledged with some care.
Currently, the order of Coleoptera (beetles) is divided into approximately 400 000
species. This order would be the order with the highest number of species in the
animal kingdom except that the majority of these species are delimited by typological
trait differences, which have often been determined in only a small number of