Principios de Taxonomia

5.5 Are Carrion Crow and Hooded Crow (Corvus corone and C. cornix) in Eurasiaj99
The House Sparrow was originally a Palearctic bird species. In the middle of the
nineteenth century, the first specimens were introduced into North America and
have then, at the beginning of the twentieth century, spread across the entire northern
continent. In the course of only about a hundred generations, considerable local
adaptations evolved with respect to coloration, beak and wing size, distinguishing
individual races from each other. It would be of special interest to know whether the
distant races can actually still cross-breed successfully for a long-ranging number of
generations. It is possible that the expected intermediate phenotype of the F1descendants
of a cross-breeding of the distantly living organisms would have lost
its ability to successfully compete with pure-bred individuals.
5.5
Are Carrion Crow and Hooded Crow (Corvus corone and C. cornix) in Eurasia and the
Guppy Populations on Trinidad Species or Races?
Often, the question arises of whether two populations are races or species. An
example is the Eurasian species pair the Carrion Crow and the Hooded Crow (Corvus
corone and C. cornix) (see Section The origin of reproductive isolation through
reinforcement in Chapter 6). The breeding areas of the Carrion Crow are geographically
separated from those of the Hooded Crow, but both encounter each other
in a narrow region of contact, where they successfully hybridize with each other.
Geographical separation and clinal transition are criteria for classifying both crows as
races. However, the region of contact on the border of the entire geographical range of
the Carrion and Hooded Crow is only approximately 50 km wide (Haas and Brodin,
2005). This hybrid zone stretches from Northern Europe across the Elbe region and
Austria to Italy. Furthermore, the mating preferences between the two Crows are not
arbitrary, but they are clearly assortative (Saino and Villa, 1992). This scenario means
that, as a first choice, a Carrion Crow mates with a Carrion Crow, and a Hooded Crow
mates with a Hooded Crow. Mixed pairings occur only as a second choice, if the
conspecific partner is not found, for example, because, at a slightly later time in the
breeding period, most partners are already paired up. The offspring rate of mixed
matings is slightly lower than in purebred couples, indicating a postzygotic restriction.
Furthermore, the two different Crows have a different choice of habitat, by the
Hooded Crow rather than the Carrion Crow preferring stubble fields and corn fields
after harvest (Randler, 2008). Therefore, the criteria for classifying both Crows as
species exceed the criteria for considering them as races.
At this point, it should be added that there is no scientific gain to waving away the
Carrion Crow/Hooded Crow problem with the remark that this case is still an
ongoing speciation process that simply has not finished yet. This notion originated in
the purely human need for stability, which is not, however, constituted in nature.
Everything flows, sometimes more slowly and sometimes faster, and every species is
part of a still ongoing speciation process that has not finished yet.
Old historical data speak in favor of considering the current situation of the
coexistence of the Carrion and Hooded Crow to be rather stable. It is probable that a