Principios de Taxonomia

180j 6 Biological Species as a Gene-Flow Community
6.29
Gene theft between two Species of Green Frogs (Pelophylax ridibunda
and P. lessonae)
The well-known European Water Frog (Pelophylax esculenta) is a taxonomically
problematic case whose status as a species is disputed. The Water Frog is not a
distinct species; it is not reproductively isolated from the two species to which it is
related, the Marsh Frog (P. ridibunda) and the Pool Frog (P. lessonae) (Color Plate 8).
The Water Frog is diagnostically distinguished from the Marsh Frog and the Pool
Frog, but it does not form a separate gene-flow community that is isolated from the
other two species.
Pelophylax esculenta results from a hybridization of two species, the Marsh
Frog and the Pool Frog. No doubt, the Water Frog is a hybrid. The Water Frog,
however, exists only as an F1 product and not as an F2 or even an F3 product. The
Water Frog, as an F1 hybrid with the genomes of the Marsh Frog and the Pool Frog,
does not continue into a second generation. Instead, the Water Frog must be
repeatedly recreated. The hybrid status of the Water Frog initially resembles the
many known cases of species hybrids, which result from accidents without
evolutionary importance, because they do not reproduce among themselves any
further but die off again at the end of their individual lives (see above). The Water
Frog, however, is not an exceptional transient species. Instead, it is found permanently
in most areas.
Water Frogs reproduce very well among themselves, as can be observed in the
garden pond. They are vital and completely fertile, but if two Water Frogs reproduce
with each other, then only part of the offspring of this reproduction are Water Frogs;
other offspring are again pure Marsh or Pool Frogs. The offspring of the Water Frog
do not continue a distinct line of Water Frogs. The remarkable hybridization between
the Marsh Frog and the Pool Frog does not produce a new line of organisms that
would then exist separate from Marsh and Pool Frogs because there is no autonomous
Water Frog genome that exists separately from the genomes of the Marsh and
Pool Frogs and could pursue a separate evolutionary line. The Water Frog is not a third
species alongside Marsh and Pool Frogs that evolved hybridogenically. Instead, the
Water Frog evolves again and again anew.
How is this explained? It starts with a hybridization between the two species,
Marsh Frog and Pool Frog. The Marsh and Pool Frog mate with each other unrestrictedly;
there is no prezygotic mating barrier. In doing so, they do not forfeit their
identities. The hybrid, the Water Frog, is vital and fertile. The zygote resulting from
the hybridization, as well as all the Water Frog s somatic cells, are allodiploid; they
contain a genome of the Marsh Frog and a genome of the Pool Frog. Its germ line cells
are allodiploid, but only for a certain time in the ontogenetic development of an
individual Water Frog. Before meiosis starts in the testes of the male or in the ovaries
of the female, that is to say, before the preliminary germ cells start to differentiate into
spermatogonia or oogonia, one of the two genomes is completely removed from the
preliminary germ cells. Accordingly, the premeiotic germ cells only contain one of
the two parental genomes. Only the somatic cells of the Water Frog are equipped with