Principios de Taxonomia

162j 6 Biological Species as a Gene-Flow Community
However, in a geographically widely spread species, gene flow can strongly decrease
with distance. It may be possible that many de novo mutated alleles never bridge the
distance between populations of a species that are geographically far apart (isolation
by distance, see above).
6.20
Phylogenetic Distance and Reproductive Incompatibility in Two Species Pairs, Polar
Bear (Ursus maritimus) and Brown Bear (U. arctos), in Comparison to Grey Wolf
(Canis lupus) and Coyote (C. latrans)
The Polar Bear (Ursus maritimus) is a relatively young species, which split off of a
population of Brown Bears (Ursus arctos) living in north-east Siberia approximately
200 000 years ago (Breiter, 2008). The usual classification into two species, Polar Bear
and Brown Bear, has been conducted according to phenotypic trait differences and
does not correspond to descent relations. Brown Bears are widely spread across the
northern Holarctic and occur southwards as far as Spain, Italy, Persia and India. They
are distributed across several separate territories.
Some populations of Brown Bears have long since been isolated and are phylogenetically
further apart from each other than the Polar Bear is from the northeastern
Siberian Brown Bear (Talbot and Shields, 1996). The term Brown Bear does not
correspond to a species because it is neither a natural gene-flow community nor is it a
monophylum. Instead, Brown Bears is a collective term for several separate gene-flow
communities that have a different evolutionary relation towards each other. Retaining
the traditional term Brown Bear as an artificial species for pragmatic reasons
would mean that this group would be a paraphylum (Chapter 7) because the Polar
Bear would have to belong to it also.
Brown Bears are only classified into a common species for subjective reasons
because of their brown fur, their relatively long ears and their terrestrial way of living.
As is easily apparent, the Polar Bear looks different, which is only an evolutionarily
young adaptation to its life in ice and water; however, these are a small number of
traits that are outwardly visible to humans and do not determine any information
about kinship.
Despite the close kinship of the Polar Bear and the northeastern Siberian Brown
Bear, hybrids are very rare, although they are fertile if produced in captivity (Breiter,
2008). At least in the case of the female partner being the Polar Bear, it is evident why
hybrids are so rare. The significantly smaller Brown Bears are apparently unable to
compete with the male Polar Bears for mating rights because the female Polar Bears
exhibit a distinctive preference for large partners, and the larger partners are solely
the Polar Bears. Thus, it is not the phylogenetic distance between the Brown Bear and
the Polar Bear that makes hybrids among them so rare; instead, it is an ethological
barrier that prevents mating.
In contrast to the Bears is the relation between the Grey Wolf (Canis lupus) and the
Coyote (C. latrans) in North America. The Grey Wolf and Coyote separated roughly
two million years ago, which is a ten times greater phylogenetic distance than