Principios de Taxonomia

118j 5 Diversity within the Species: Polymorphisms and the Polytypic Species
speciated remains unanswered. This question likely ranks among the crucial questions
in the field of taxonomy.
5.15
The Beak Polymorphism in the Black-Bellied Seedcracker Finch Pyrenestes ostrinus
The beak size of finches is strongly controlled by selection. Given that beaks can be
used for cracking certain seeds for food procurement, fluctuations in beak size as
small as a tenth of a millimeter can lead to the affected birds no longer being
competitive against their conspecifics. The size of the beak, whether it is slightly
smaller or bigger, is not the result of special training by the bird; rather, this trait is
genetically predetermined (Abzhanov et al., 2004).
The finch s beak offers a fine example of a stable polymorphism within a
population, similar to blood groups among humans, as discussed above. The West
African Black-bellied Seedcracker (Pyrenestes ostrinus) provides a textbook example of
adaptive polymorphisms, which refers to the coexistence of different adaptations to
different habitats by representatives of the same species. In Cameroon, two morphs
of the Black-bellied Seedcracker are characterized by different beak sizes, and both
morphs occur simultaneously in a single population (Smith, 1993; Smith, 2008). The
small-beaked individuals primarily feed on soft seeds, whereas the large-beaked birds
are specialized in cracking hard seeds. This is a case of dimorphism.
This dimorphism is comparable to sexual dimorphism. Only thin-beaked and
thick-beaked morphs exist; no animals with intermediate beak sizes are observed,
just as no intermediate sexes are observed. However, there is a significant difference
between these two examples of dimorphism. In the case of sexual dimorphism, the
sole occurrence of two distinct types is because of a genetic mechanism that
determines the either-or result. With respect to the finch s beak, however, selection
is responsible for the fact that no mixed types are observed. Individuals with beaks of
intermediate size admittedly develop due to various allelic combinations, but because
of selection s severe control, these animals have no chance for survival compared to
their pure small- or large-beaked conspecifics that are specialized to eat very
particular seeds. All of the intermediate phenotypes are continuously weeded out.
Selection divides the population into only two morphs. In the case of these finches,
there is neither a genetic mechanism for the development of only two beak sizes nor
any mating barriers between the two morphs with the two beak sizes. Both of the
morphs belong the same species given that they are members of the same gene-flow
community.
The simultaneous coexistence of two phenotypes within the same population
makes biological sense. However, it is not the selective advantage of the thick or thin
beak that is at work here. Instead, it is the clear selective advantage of there being two
variations that together enrich the entire population. The existence of two morphs
with differing beak types broadens the food spectrum of the species. If all of the birds
had the same beak type, then they would all eat the same food. Accordingly, the food
resources in the habitat would become depleted much more rapidly.