Principios de Taxonomia

perform functions there. All of these genes, however, are distinguished by the fact
that they have diverged extremely quickly in evolution (Orr, 2009). In Drosophila, a
gene of the nuclear pore complex has been identified as the cause of hybrid sterility.
The nuclear pore complex is a structure in the membrane of the cell nucleus and
serves as a checkpoint in the process of the channeling in and out of macromolecules.
The nuclear pore complex is composed of many proteins that coevolve at high speed.
Thus, there is fast incompatibility if the parents stem from different gene pools
(Tautz, 2009).
6.17
Sympatric and Allopatric Speciation
6.17 Sympatric and Allopatric Speciationj155
There are two different basic possibilities for why two organisms are not able to
successfully crossbreed with each other. Either they live together and there exists a
prezygotic barrier between them (see above) that prevents a successful zygote
formation, or they live at different locations and, thus, cannot meet at all. The first
scenario is called a sympatric distribution and the second scenario is called an
allopatric distribution of the populations.
Differences between the two forms of separation are enormous and cannot be
overvalued. The origin of two new species under sympatric conditions is different
from the origin of two new species under allopatric conditions, although both
processes are designated with the same term: speciation.
If two organisms live sympatrically and, therefore, meet each other regularly, they
must evolve intrinsic properties that prevent mating. Completely different is the
situation with two organisms that live allopatrically. Those organisms cannot meet
each other for external reasons. Therefore, any prezygotic crossing barriers do not, in
fact, have to exist. Allopatrically distributed organisms do not need traits that prevent
zygote formation, which would not make sense biologically. Their mutual mating is
prevented only by external limits, and these limits are not properties that the
organisms possess themselves. Such external barriers are often, but not always, of
a geographical nature. They can be oceans, mountain ranges, rivers or only highways
that prevent an encounter of the separately living organisms.
In the case of certain Weevil beetles or nematode worms, however, allopatric
conditions can also be produced by a lifelong confinement in the interior of a host
plant that prevents a mutual encounter with each other, even in the same geographic
position (McCoy, 2003). These animals spend their entire life cycles on or even in the
interior of a single food plant without ever leaving the host plant. These examples
have an external cause for separation that has nothing to do with a geographic
separation. This form of separation is also an allopatry, although not through a
geographical barrier. In every case of allopatry, however, the absence of crossbreeding
is not based on the properties of the organisms themselves but, instead, is based on
external barriers. Thus, the organisms themselves then carry no properties of
separation at all within them. Their separation is not based on speciation genes.
Those genes can, at best, evolve by pure chance.