Principios de Taxonomia

164j 6 Biological Species as a Gene-Flow Community
of the expansion. In the end, they form a ring-shaped distribution zone. Whereas all
of the adjacent populations are reproductively connected to each other, the most
distant populations lose their reproductive compatibility when they encounter each
other at both ends of the circle (Figure 6.2b). They are no longer able to successfully
crossbreed under the natural conditions present there. Isolation by distance has
more and more estranged the populations over time, during the expansion. With
the example of such a geographic ring-shaped chain of populations, surrounding
the polar region or other inhospitable regions such as mountains or deserts, the term
Rassenkreis (racial circle) was originally introduced (Rensch, 1947). This name is a
correct name; however, it has not become established terminology. It has been
replaced by the less appropriate term ring species.
For a long time, the Herring Gull (Larus argentatus) was considered to be the classic
example of a ring species. The following model was stated: The Herring Gull
originated in southwestern Asia, and from there, it spread through northern Asia
and continued through North America, until it finally migrated across the North
Atlantic Ocean to penetrate Europe. Because of this scenario, the Herring Gull was
said to be the prime example for a ring species that has spread around the North Pole.
According to Mayr (1942), the Herring Gull originated in the Aral-Caspian region
and spread through Mongolia to northeastern Siberia. The local race of northeastern
Siberia received the distinct species name Larus vegae. The expansion continued to
North America, where the Herring Gull was designated as L. smithsonianus. In North
America, probably caused by the ice age, a new race L. argentatus was separated
from L. smithsonianus, which then immigrated into Europe after the ice age.
In Europe, L. argentatus encountered the Lesser Black-backed Gull (L. fuscus), which
was originally native in the Baltic region. Here, at the hypothetical end point of the
circumpolar expansion ring, the two gull species do not mate with each other. From
west to east in Eurasia and North America, all of the adjacent races are reproductively
compatible with each other, while the two end populations of the expansion ring
(the Herring Gull and the Lesser Black-backed Gull) have reached full reproductive
isolation and coexist in Europe similar to distinct species alongside each other.
The Herring-Gull example has been the standard model for a ring species for
more than half a century and, as such, has entered every text book. However, recent
investigations have not been able to confirm this orderly model (Liebers,
de Knijff, and Helbig, 2004). A comparison of mitochondrial DNA sequences could
not support a key element of the ring species hypothesis, namely the postulated close
relation of the North-American L. smithsonianus to the European L. argentatus.
The haplotype sequences that are distinctive for the Nearctic L. smithsonianus were
not rediscovered in European L. argentatus gulls, not even in Iceland. There are
no indications for an evolutionarily young bifurcation between L. smithsonianus and
L. argentatus.
Thus, the Herring-Gull complex is not a ring species. The available data do not
support the conception of a ring-shaped expansion. Instead, this case is a case of
diverging expansion waves that emerged from two separate glacial refuges.
The mitochondrial data speak for an expansion from a continental Eurasian retreat
area, on the one hand, and a second retreat from a North Atlantic refuge.