Principios de Taxonomia

5.18 Urbanization in Certain Bird Species is based on Genetic Polymorphismj121
allelic differences among the beak-morphs of the finches cannot be observed in the
structural gene Bmp4 itself but in its regulatory elements; this structural gene is
variably regulated but is otherwise unaffected. Experimental interventions that
artificially increase the level of Bmp4 expression have indeed resulted in alterations
of beak morphology.
5.17
Intraspecies Morphs in the Garter Snake Thamnophis ordinoides
In the case of the North American Garter Snake (Thamnophis ordinoides), several
morphs coexist in the same population (Brodie, 1989). These morphs differ significantly
with regard to their color pattern and behavior. The different morphs also vary
in having large or small hunting grounds. In the face of danger, some morphs flee,
while others stay motionless or exhibit aggressive behavior. It is interesting that the
different traits (i.e., pattern, color, and behavior) are correctly tuned with one other.
Snakes that display camouflage-colored dorsal patterns also exhibit motionless
behavior when in danger; in contrast, snakes that display more a striking coloration
behave aggressively or flee.
The genetic foundation for the respective coexistence of these traits is the linkage
of their encoding genes. The genes for the different traits (i.e., pattern, color and
behavior) are located on a single chromosome, where they form a linkage group that
is protected from recombination. This arrangement leads to clearly identifiable
intraspecific morphs and to a situation in which a particular color pattern is combined
with a particular antipredator behavior.
Again, the selective advantage of the existence of morphs is that multiple adaptations
are present within the same species. In the example of the Garter Snake, in
contrast to the Black-bellied Seedcracker finch, however, the multiple adaptations are
genetically controlled, not environmentally by selection.
5.18
Urbanization in Certain Bird Species is based on Genetic Polymorphism
Preadaptive genetic dispositions often appear to be responsible for behavioral
changes; training or learning processes are not always involved. An increasing
number of bird species has moved into the cities over the last centuries, particularly
during the previous few decades. There are examples supporting the view that
urbanization is based on genetic preadaptation.
The timidity of many animals to both humans and certain predators is the result of
experience and genetic dispositions. Imprinting during early life is crucial, as are
later experiences and education by parents and other conspecifics. However, a great
deal of timidity toward potential predators is genetically founded. In the case of birds,
it appears that the timidity toward humans differs widely between geographical
regions and has changed in time.