Principios de Taxonomia

156j 6 Biological Species as a Gene-Flow Community
Sympatric and allopatric separations are two entirely different evolutionary ways
for speciation, and the question arises as to whether both types of group separation
should receive the same name and whether both should be called species:
1) sympatric speciation is the origin of separated groups that are in competition and
are separated by a control of selection at the same location, while
2) allopatric speciation is the formation of species by chance due to genetic drift at
locations that are isolated from each other.
Sympatric speciation can occur if the sexual partners evolve new preferences for
partner recognition. Females with newly evolved demands for specific traits in their
male partners and the respective males, which conform to these demands, can split
off as a distinct species and can separate from the rest of the members of the original
species. This scenario is sympatric speciation resulting from sexual preference.
An additional possibility for sympatric speciation is the segregation of a few
organisms into a new ecological niche, which opens up new resources and, thus,
signifies an advantageous adaptation, for example, if a group of a monophagous
beetle species switches to a new food plant and becomes adapted to this new food
plant. For this separation to become stable, a corresponding selective partner choice
must coevolve at the same time. Only under this condition, the conquest of the new
ecological niche is not reversed again by backcrossing with the organisms that still
have a preference for the former food plant. If, therefore, a few individuals within
species A conquer a new food plant and rely on this new plant in the future, then they
can only turn into the new species B if, simultaneously, an assortative mating
is guaranteed that makes certain that B-organisms from now on mate only with
B-organisms and not with A-organisms any more (Dres and Mallet, 2002).
Sympatric speciation means that the force of gene flow among the organisms of an
established gene-flow community to again and again homogenize the genomes
among the different individuals is overcome by other forces, such as (1) ecological
adaptation and (2) sexual selection.
1) New niches offer a selective advantage to a new founder population, if the old
habitats are completely occupied.
2) However, if a new resource is populated, only then is there a selective advantage,
whereby the organisms find and select sexual partners with the same type of
adaptation. Because the partner choice depends on specific signals, the conquest
of new food resources together with newly developed signals for mutual partner
recognition can immediately split a gene pool.
Sympatric speciation is exactly the opposite of the allopatric paradigm because
selection is the driving force and not the contingency of an allopatric separation. In
the case of sympatric speciation, adaptation to specific habitats, together with the
erection of crossing barriers are subject to a high selective pressure, to prevent the
remixing of the separating populations. Because of the importance of the role that
selection plays, sympatric speciation is true Darwinism (Tautz, 2009).
Allopatric speciation, in contrast, is pure coincidence; it is simply the result of
genetic drift. The theory of allopatric speciation is based on a twofold coincidence.