Principios de Taxonomia

5.12 Partially Migratory Birds – an Example of Genetic Polymorphismsj113
into a sedentary bird population. The individuals that exhibit the necessary preadaptive
traits replace other individuals given their sudden colossal selective advantage
(Pulido et al., 2001). Because the genes that control migration theoretically survive in
some individuals in the sedentary population, the entire population of sedentary
birds preserves the opportunity to become migratory in the event of a climate shift.
The analogous situation is true in populations of migratory birds; if the environmental
conditions change, then a population of migratory birds can, via selection,
become sedentary after a small number of generations.
A historic example of such a transition is the Eurasian Blackbird (Turdus merula), a
sedentary bird in Central Europe. However, two hundred years ago in Germany, in
Goethe s times, this blackbird was migratory and was not observed in the winter.
During this period, the winters were much colder, particularly in the cities.
The question arises of whether migratory birds and sedentary birds are races or
morphs. Because Eurasian migratory populations generally breed in the north, and
sedentary populations generally breed in the south, the two populations live in
specific geographical regions. This coexistence of two different populations as a local
adaptation to different geographic environmental conditions suggests, at first glance,
that these populations are geographic races (see above).
However, there are geographical regions in which migratory and sedentary birds of
a species live side by side. In such clinal regions of migratory and sedentary birds, the
two populations interbreed. In these situations, it can be directly tested whether the
two populations are races or morphs. If the populations are races, the mixed
population should consist of intermediary phenotypes, indicating that their genes
recombine. That is, a portion of the migratory alleles should recombine with a
portion of the sedentary alleles. The result of this, however, would be expected to be
lethal given that a bird carrying half of the migratory alleles and half of the sedentary
alleles would not survive. However, contrary to expectations, where migratory and
sedentary birds overlap in the clinal transitory region, the mixed populations
constitute a viable reproductive coherence. These populations do not in fact produce
predominantly lethal hybrids but rather two very distinct phenotypes: viable migratory
individuals and viable sedentary individuals.
Consequently, the birds that live in the regions where migratory and sedentary
populations of birds overlap are referred to as partially migratory birds. The migratory
portion of this population leaves the breeding habitat in the fall and returns in the
spring of the following year. The sedentary portion of the same population in the
same region does not migrate.
How can this be? The numerous hereditary dispositions of migratory and sedentary
birds are certainly mutually incompatible. It should be expected that migratory and
sedentary birds cannot successfully crossbred with each other if their genes recombine.
If the two populations breed, it should be expected that the intermediate hybrid is
partially equipped with migratory allelic constitutions (i.e., alleles that control
migratory restlessness, orientation and migration distance, fat metabolism, wing
dimensions, etc.) and partially with alleles that confer sedentary traits.
That the breeding of these two different phenotypes does not result in intermediate
phenotypes but rather in two distinct phenotypes is precisely what characterizes the