Principios de Taxonomia

6.21 The Herring Gull (Larus argentatus) and the Greenish Warblerj163
between the Polar Bear and the Brown Bear (Wayne, 1993). Nevertheless, hybridizations
frequently occur between the Grey Wolf and Coyote. While the Coyote was
originally native in the south of the United States, it successfully spread to the north
in the last 100 years and has penetrated into most of the Grey Wolf s living habitats.
Here, successful hybridizations frequently occur; as a result, most of the North
American Grey Wolves already contain Coyote genes. In the province of Quebec in
Canada, almost 100% of the Grey Wolves contain portions of Coyote DNA in their
genome (Wayne, 1993). Apparently, strict speciation genes are missing that could
prevent a hybridization between the Grey Wolf and the Coyote. It would be faulty to
infer a close kinship from the frequency of hybridizations between the Grey Wolf and
the Coyote. Furthermore, disregarding one exception (the so-called Red Wolf; see
Chapter 2), there are no pronounced hybrid populations, but the animals are still
distinctly recognizable as Grey Wolves or as Coyotes in spite of all of the blending.
From these and other examples, it follows that the breaching of species boundaries
is not directly correlated with the degree of phylogenetic distance. Furthermore,
there is no universally valid biological law that would prove the idea that a considerable
percentage of mixed matings between two species must necessarily lead to the
blending of these species (and thus to the disappearance of one of the two species),
even though it is often assumed to be the case. As long as the percentage of hybrids
remains constant (and does not increase in an ongoing fashion) the continuance of
the species is not endangered, and species hybridizations can be tolerated over long
evolutionary time frames (see the example of the black European Flycatchers in
Chapter 4). Apparently, some species can tolerate well that a significant percentage
of their conspecifics enter mixed matings and produce hybrids, while other species
do not tolerate this occurrence, which, in those cases, has the consequence that one
of the two species can be exterminated by hybridization (see the example of the Ruddy
Ducks further below).
6.21
The Herring Gull (Larus argentatus) and the Greenish Warbler (Phylloscopus
trochiloides), a False and a True Model for the Ring Species
As mentioned above, the individuals from geographically distant populations of a
species in many cases cannot be successfully crossed with each other (Figure 6.2a).
This scenario is termed isolation by distance. A special case of this phenomenon is
the ring species. A ring species is a group of populations that show the normal
phenomenon of isolation by distance between their very distant individuals; however,
they exhibit the exceptional phenomenon that those distant individuals encounter
each other under natural conditions (Figure 6.2b).
This scenario can happen in those cases in which the different populations of a
species surround an inhospitable geographic region, in which they cannot live. This
region could be a mountain massif, an ocean or the polar region. The different
populations of the species surround this inhospitable region until the expansion is
closed at a geographic area where the populations encounter each other at both ends