Principios de Taxonomia

112j 5 Diversity within the Species: Polymorphisms and the Polytypic Species
or weeks to maintain its migratory restlessness before ceasing to migrate. Otherwise,
the bird may cease its migration too early or too late and would stop in the wrong
winter habitat.
All of these behavioral properties of migratory birds have a genetic foundation.
Sedentary birds do not require the instincts needed to migrate; the genes responsible
for migratory behavior are inactive in the individuals of the sedentary population of
the same species. Furthermore, migratory birds require a different metabolism than
sedentary birds. Such birds must build up enormous subcutaneous fat deposits as
fuel as well as water deposits before the fall migration. These different metabolic
requirements necessitate a multitude of enzymatic activities, which are all repressed
or differently controlled in sedentary birds. Before beginning their migration,
therefore, migratory birds may increase their weight by a factor of two in the late
summer. During these weeks, migratory birds may weigh twice as much as members
of the sedentary birds of the same species (Berthold and Querner, 1981). In addition,
wing length and wing pointedness are greater in migratory birds than in sedentary
birds (Rensch, 1938; Baldwin et al., 2010). Blackcaps (Sylvia atricapilla) on the Cape
Verde Islands are sedentary birds and have much shorter wings than do blackcaps on
the European mainland. The short wings of the Cape Verde Islands population would
not allow them to fly across the ocean (Pulido et al., 2001).
The enormous number of different traits between the individuals of migratory and
sedentary population of the same bird species raises the taxonomical question of
whether migratory and sedentary birds are different races or morphs.
According to the position of Berthold, there are no bird populations that are either
completely migratory or completely sedentary; the difference in traits between the
migratory and sedentary birds is simply a matter of allelic frequency distribution.
Therefore, a bird population that lives in the north and normally leaves its breeding
habitat in the fall must contain a few individuals that are genetically determined to be
sedentary. In cold winters, these individuals die off.
However, a few alleles survive in the entire population, presumably hidden under
the cloak of recessivity. Recessive alleles are not phenotypically expressed and,
therefore, can survive without being eradicated by natural selection. The analogous
situation can be supposed for sedentary bird populations. The members of this
population remain in their breeding areas over the entire year. They do not migrate
because they find enough food even during the winter. In this population, however, a
few individuals may possess the allelic constitution that encourages them to migrate
to distant winter habitats. This behavior in a normally sedentary population may be
detrimental given that when these birds arrive in spring, their potential breeding
territories may already be occupied. However, migratory alleles will survive in a
sedentary bird population.
Berthold s position is also supported by the fact that, if climate changes demand it,
a population of migratory birds can turn into a population of sedentary birds, or vice
versa, over only a few generations. The allelic polymorphisms in alleles that control
migratory and sedentary behavior provide the individual populations with a remarkable
flexibility over a longer evolutionary timeframe. A change of the local climate
can, within a small number of generations, transform a migratory bird population