Principios de Taxonomia

142j 6 Biological Species as a Gene-Flow Community
environments. Races, therefore, always emerge from geographical differences. It has
to be considered, however, that races are typologically defined. Only those trait
differences that are clearly diagnosable for the human eye, define races. Populations
that differ by cryptic traits are not considered to be races.
Species that only live in a small geographical area of distribution usually do not
produce races, apparently because gene flow allows alleles to reach all members of the
species with high frequency and because the constant backcrossing prevents groups of
individuals with separate sets of traits from pursuing their own evolutionary paths.
Species with an extended geographical spread, however, almost always produce races.
There are still exceptions. Among birds, the Northern Lapwing (Vanellus vanellus),
despite its distribution from Spain throughout the entire Palearctic to the East-Asian
Pacific coast, does not produce races (del Hoyo, Elliott, and Sargatal, 1996). In most bird
species, however, many races exist (Rensch, 1951). The widespread Tits (Paridae)have
an especially large number of races; the Great Tit (Parus major) for example, has 34
subspecies from Western Europe to East Asia (del Hoyo, Elliott, and Christie, 2007).
Races can look distinctly different from each other, as in the case of the Australian
Magpie (Cracticus tibicen), which occurs at different locations in Australia with more
than ten significantly different plumages. Races always must be recognizably
different; otherwise, there is no logical justification for a race designation. When
geographically separated populations are adapted to the local environments but
the traits of adaptation are not clearly visible to the human eye, the local populations
do not constitute races. The fact that races are only defined by trait differences which
are recognized by humans, documents that races are human constructs, created only
for the convenience of classification (Chapter 5).
Races cannot coexist at the same location because then crossbreeding would reverse
the distinguishing trait differences. The homogenizing force of gene flow reverses
divergence. If, however, this homogenizing force wanes with geographic distance,
then the force of local adaptation prevails and the homogenizing force can no
longer prevent the formation of races. For this reason, races always must be geographical
phenomena (Paulus and Gack, 1983). If races nevertheless were to coexist in
the same place, as is currently the case with human races, then this coexistence results
from secondary immigration due to colonization and globalization. Today, many
members of distinct human races no longer live in their ancestral homelands. The
sympatric occurrence of different races is unstable and thus always of short duration.
Blending immediately commences, and the human races presumably will disappear.
Currently, blonde- and black-haired humans live alongside each other in many
parts of the world. They have, however, originated from separate regions and were
only reunited relatively recently. If the coexistence of different races remains
established in the long run because of inherent aversions or traditional prohibitions
against marriage, and if these separation prevents blending, then one must consistently
speak of different species because the gene flow has become disrupted. The
disruption of gene flow, which leads to speciation, does not necessarily need to have
genetic causes. Traditions and different educations are sufficient to create species
differences. Certain traditions can also cause a long-lasting disruption of gene flow, as
is the case with the commandments of certain religions (Hackstein, 1997).