Principios de Taxonomia

6.22 Allopatrically Separated Populations are Always Different Speciesj165
Unlike the Herring Gull, a valid example for a ring species is the Greenish Warbler
(Phylloscopus trochiloides). This species forms a racial circle in Central Asia (Irwin,
Bensch, and Price, 2001; Irwin et al., 2005). Climate data and the comparison of
mitochondrial microsatellite sequences permit us to reconstruct that the animals
spread from the south of the Tibetan Plateau to both the western side of the plateau
and the eastern side around the entire Tibetan Plateau. The Tibetan Plateau is, today,
surrounded by several races of Greenish Warblers, which are all reproductively
connected to each other. The molecular data show that gene flow occurs between the
neighboring races. On the northern side of the Tibetan Plateau, however, the races
from the west and the east encounter each other; here, they are mutually genetically
incompatible. Between the two races in the contact zone, gene exchanges no longer
occur. Thus, the Greenish Warbler, as established, forms a real ring species.
A ring species is often considered to be an exceptional peculiarity. However, this
idea is not correct. A ring species is exceptional only with respect to the fact that
distant individuals of a species encounter each other under natural conditions.
Otherwise, a ring species is nothing other than the normal phenomenon of isolation
by distance, the phenomenon that the individuals from geographically distant
populations of a species in many cases cannot be successfully crossed with each other.
A ring species is often considered to be the start of a speciation, as though a species
would be on the brink of bifurcating into two separate species. However, this
conclusion is not mandatory because there is no necessity of assuming that the
ring species is not stable as a species and would shortly have completely fallen apart
into two or more species (Irwin, Bensch, and Price, 2001; Irwin et al., 2005).
No natural law compels us to consider the coexistence of reproductively compatible
and incompatible populations within the same species as an evolutionarily shortlived
transitional process. This state can very well be stable. Only the purely human
need to set rigid class boundaries converts a ring species into a transitional
phenomenon.
6.22
Allopatrically Separated Populations are Always Different Species
The examples of isolation by distance and the example of the ring species clearly show
that it is not possible to define a species as a community of organisms that are all
cross-fertile. Instead, the mutual cross-fertility of the organisms of a species in some
cases is restricted to adjacent populations only. The cohesion of the individuals within
a species must be understood by stepwise allele exchange through intermediate
populations. With increasing distance, the potency of the organisms of a species
for successfully interbreeding often decreases gradually (Ford, 1954; Baldwin et al.,
2010).
However, although the distant individuals of a species could lose their mutual
ability to be crossed over large geographic distances, each population is connected
with at least an adjacent population. This scenario distinguishes isolation by distance
and distinguishes the ring species from allopatry. In the case of isolation by distance,