Principios de Taxonomia

126j 5 Diversity within the Species: Polymorphisms and the Polytypic Species
females locate suitable nests by observing the host bird preparing its nest and
beginning incubation, waiting for hours in a concealed place if necessary. Moreover,
it is probable that the host birds are identified acoustically by their song. The ability to
locate a proper nest must also be determined to a large extent hereditarily given that
no parental instruction is possible and that the only available memories are from their
own hatching and raising.
Furthermore, brood parasitism can only be successful when the cuckoo s eggs
mimic the eggs of the host birds with respect to size, color and pattern. In the
Eurasian Cuckoo Cuculus canorus, there are more than a hundred genetic variations
that determine the eggs sizes and colorations. The individual female, including its
female descendants, only lays eggs of one particular type. This egg mimicry can only
be explained by the presence of polymorphic alleles.
With respect to understanding the species concept, the primary challenge is not
simply explaining perfect adaptation but how this adaptation can be explained by a
number of linked genes, all of which must work together as a linkage group in a
cuckoo female. The time of egg deposition, the identification of the host and the
coloration of the eggs are supported by genetic dispositions that must not be
separated by genetic crossing-over events. This is because a particular cuckoo female
cannot have half of its traits adapted to one host bird and the other half to another host
bird. None of the adaptation traits that are attuned to a particular host bird can
recombine. The sum of these traits must be inseparably and genetically linked to each
other. Accordingly, there are genetically distinguishable Robin cuckoos, Black
Redstart cuckoos, Thrush cuckoos, Reed Warbler cuckoos, and so on. Why, then,
are not there 200–300 species of cuckoos in Eurasia?
The answer to this question is as follows. The adaptation to a particular species of
host bird is only maintained by the female cuckoos. In males, there is no host
specificity. Accordingly, just as with the mimicry polymorphism of female Swallowtail
butterflies (see above), the cuckoo morphs are sex-related.
Why is it the female sex that exhibits polymorphism? It does not appear to be a
coincidence that both butterfly and bird morphs are determined exclusively by the
female sex. This is because in birds, just as in butterflies, females, as an exception, are
the heterogametic sex. In butterflies and birds, the females are of the XYconstitution
(occasionally referred to as ZW), whereas the males are of the homogametic XX
constitution. The Y (or W) chromosome contains a large percentage of genes that are
embedded in heterochromatin and thus are protected from recombination. These
protected genes are referred to, in this instance, as a linkage group. It is a safe
assumption that all of the genes that determine the distinct phenotype of a particular
morph are located on the Y chromosome, the genes of which are generally protected
from genetic crossing-over. This situation would explain (1) how an entire battery of
genes is linked and inherited as a whole and (2) why these genes are inherited along
the maternal line. If the cuckoo was homogametic in the female sex, as in mammals,
brood parasitism likely could not exist.