Principios de Taxonomia

176j 6 Biological Species as a Gene-Flow Community
organisms. Consequently, the hybrid organisms can only continue to exist if they
mate again with equal hybrid individuals. Backcrossing with their parental species
would create a dead line of sterile F1 offspring, and they would not be able to
continue reproduction.
Now the first peculiarity of plants comes into effect: the ability to self-fertilize.
The tetraploid hybrid organisms do not need a sexual partner who may be diploid,
a requirement that (in animals) would bring further reproduction to an end. They
can reproduce with themselves. The mature allodiploid sperm fertilizes a mature
egg of its own mother individual, and therefore this egg is also allodiploid. Thus,
the resulting zygote is tetraploid like the mother organism.
Due to a combination of self-fertilization with allotetraploidy, hybridogenic
speciation is possible. The capability for allotetraploidy is the second important
reason, after self-fertilization, why plants are much more frequently able to
generate hybridogenic species than animals. Allopolyploid hybrids are often
especially resistant because they combine in themselves the optimal properties
of both of their parental species.
Allotetraploid hybrid formation is a remarkable example of a speciation
within only a single generation because a post-zygotic barrier immediately
comes into being in the first generation of hybridogenesis. This barrier immediately
stops gene flow between the hybrid and its parental organisms. In only a
single generation, the genome duplication produces plants that can no
longer reproduce with their parents, but only with themselves. Furthermore,
this is a speciation without any changes in genes. Not a single DNA sequence
distinguishes the individuals of the new species from the individuals if its
stem species.
6.27
Is the Italian Sparrow (Passer italiae) a Hybrid Species?
In Europe, three different forms of house sparrows exist, whose species status is even
today still debated: the well-known House Sparrow (Passer domesticus), the Spanish
Sparrow (Passer hispaniolensis) and the Italian Sparrow (Passer italiae). The House
Sparrow populates all of Europe except for the polar region and, interestingly, Italy
and its neighboring islands. The Spanish Sparrow breeds in Spain on the Balkans and
on most Mediterranean islands. The Italian Sparrow mainly inhabits Italy, Sicily,
Corsica and Sardinia, but remarkably, it is also found on Crete. In most of Europe, the
three forms generally exist separately, but there are also broad overlapping regions,
such as in north and south Italy, in Spain, in the Balkans and in North Africa. In these
overlapping regions, the species coexist side by side without blending in some
regions, whereas in other regions, hybridizations occur. In the hybridization
regions, there are extended populations of phenotypically intermediate organisms
(T€opfer, 2007).
On the Iberian Peninsula, on the Balkans and in parts of North Africa, the Spanish
and the house sparrow coexist sympatrically, without any hybridization occurring.