Principios de Taxonomia

172j 6 Biological Species as a Gene-Flow Community
produced, which then also successfully inherit the prezygotic speciation genes of the
parents. Then, if the offspring of a purebred mating have advantages, then selection
also benefits those parental properties that guarantee a correct partner choice.
The underlying selective pressure ensures that those traits, then, quickly prevail in
most cases.
Reinforcement has been verified for many examples, as follows:
When, in the 1920s, the Lesser Black-backed Gull (Larus fuscus) penetrated from
its native breeding area in the Baltic Sea into the geographical range of the
Herring Gull (Larus argentatus) in the southern North-Sea region in Germany
and Holland, both species at first mated relatively often. Hybrids of both sexes are
fertile. Today, after only a few decades, the two species have almost completely
stopped interbreeding with each other (Haffer, 1982). Stuffed hybrids can still be
found in Dutch museums and are today considered a valuable rarity. Apparently,
the hybrids had postzygotic disadvantages, although these are not known.
Accordingly, because in the first contact a prezygotic isolation came into being
(or strengthened), in the course of only a few generations and as a result of natural
selection, there is a bias against hybrids.
A second known example is of Drosophila: At Mount Carmel in Israel, there is a
gorge that ironically is called Evolution Canyon (Korol et al., 2000). The northern
and the southern slope of this canyon are only a few hundred meters apart from
each other, but they differ drastically from each other microclimatically because of
having a different solar radiation and humidity. On both canyon slopes, certain
populations of Drosophila melanogaster have adapted to the strongly different
microclimates, for example, with regard to the ground temperature that triggers
egg deposition. Immediately after the evolution of these adaptations, a sexual
blending of the populations of the opposing mountain slopes would be fatal for the
preservation of the linkage of the newly evolved traits. The linkage of the new
alleles would have been destroyed by genetic recombination. Thus, a strong
selective pressure has ensured a quick coevolution of an assortative mating
behavior. A strong assortative partner choice with a preference for only the flies
of the same mountain slope evolved, while the organisms of the opposing slope
were avoided. In doing so, the newly evolved gene pools were kept apart. This
scenario implies the de novo origin of two species of Drosophila under sympatric
conditions because the distance of only a few hundred meters between the two
mountain slopes allowed for a daily encounter of different flies. During the
process of species formation, there were no external, allopatric barriers.
Between the two extremes, there is no reinforcement, but there is apparently
unrestricted blending (in the example of the Ruddy Ducks) and there is strong
reinforcement with fast development of mating barriers in the example of the
European North-Sea Gulls; there are stable intermediate solutions. There are species
that hybridize frequently with each other. However, despite frequent hybridizations,
the species do not lose their identity. This outcome results from the fact that the
percentage of species hybrids does not increase in time in the long run. If this
condition is satisfied, then the occurrence of frequent hybridization apparently does