Principios de Taxonomia

6.27 Is the Italian Sparrow (Passer italiae) a Hybrid Species?j177
In the region of joint occurrence, they populate separate habitats, with the House
Sparrow occupying the traditional habitats, cities and villages and the Spanish
Sparrow progressing into rural habitats. In contrast, in Tunisia and eastern Algeria,
a hybridization of the two species occurs, with variable populations of sparrows being
produced there.
The Italian Sparrow can be found from the Apennine Peninsula to the southern
edge of the Alps. A transitory region exists south of the Alps, roughly 35 to
40 kilometers wide, between the populations of the House and the Italian Sparrow,
in which hybridizations also frequently occur. In the south of Italy, the Italian
Sparrow is connected to the Spanish Sparrow through a broad, smooth transitory
region. The animals on Corsica are similar in appearance to those of northern Italy,
while the sparrows on Sardinia exhibit distinct transitions between the Italian and the
Spanish Sparrow.
This presents a situation that is difficult to interpret. Neither the concept of
isolation by distance nor the concept of a ring species can be applied. There is no
distinct geographical expansion line along which the continually decreasing genetic
compatibility could be traced, as in the case of isolation by distance or in ring species.
Moreover, the House and Spanish Sparrows do not occupy separate geographical
ranges with only a small contact zone, as in the case of the Carrion and Hooded Crow.
Instead, the intermediate regions occur in a ragtag fashion at some locations, and
they are absent at others. Third, the phenomenon of partial genetic introgression in
an otherwise unambiguous side-by-side coexistence of two species is not realized
here, as in the case of the Wolf and the Coyote. Instead, pronounced intermediate
populations exist, living in particular geographical regions.
In this book, I have tried to avoid the term transitional stage for species status.
This term is frequently used for several species with overlapping breeding areas and
occasional hybridizations. This practice ignores, however, the fact that a limited
introgression of genes among otherwise separate species is a normal process and that
in some cases, this even has a biological importance (see below). In several examples,
it is not justified to refer to occasional hybridizations and gene flow across the species
border as a transitional stage, indicating the origin of two new species. Open
species boundaries may simply be a stable continuous situation in the relationship
of two species.
In the rare case of the sparrows, however, we deal with a true case of a transitional
stage, where it is logically impossible to decide whether these are different species.
House and Spanish Sparrows are a real borderline case. There are convincing
arguments against the species status of house and Spanish Sparrows, even though
House and Spanish Sparrows are not unambiguously conspecific.
There is, however, not only the problem of whether House and Spanish Sparrows
are species or races. In addition, the Italian Sparrow is often designated as a hybrid
species. It is supposed to have originated as a third species from house and Spanish
Sparrows. This opinion dates back to Wilhelm Meise half a century ago, who because
of the intermediate appearance of the Italian Sparrow, which bears the traits of both
the House Sparrow and the Spanish Sparrow, arrived at the conclusion that the Italian
Sparrow was a hybrid species (T€opfer, 2007).