Principios de Taxonomia

100j 5 Diversity within the Species: Polymorphisms and the Polytypic Species
similar coexistence of the Carrion and Hooded Crow has already existed for centuries
and will continue to exist in the same fashion in the coming centuries, also (Haas and
Brodin, 2005; Saino and Villa, 1992). None of the arguments support the view that
both crows would become completely separated in the future. In biology, it is more
appropriate to look for reasons that allow a partially cross-breeding species pair to
continue to remain distinct in spite of mutual hybridization, than evading the
problem and speaking of a temporary situation.
Another borderline case between a race or species is provided by Guppies (Poecilia
reticulate) on Trinidad (Magurran, 1999). The Guppies are another example of the fact
that a large degree of diversification can successfully coexist with ongoing gene flow
of long duration between different populations. Diversity among populations should
not be considered as necessarily leading to a coming speciation. Speciation is not the
inevitable consequence of a large degree of diversification in a population.
The Guppies that are widespread over the inshore waters of Trinidad split up into a
multitude of small populations that have different traits. These trait differences
surpass the diversity of well-established species. Molecular analyses have revealed
that the different individual populations have existed for half a million years, which is
more than a million generations. In spite of this extent, the different populations are
reproductively connected to each other in broad transitions. There is neither a
homogenizing merger of the population differences nor is there an erection of
reproductive barriers such as in the case of another family of fish, the African
Cichlides, which are subject to rapid speciation (Chapter 6). It is an unresolved
mystery why certain groups of animals form many new species in evolutionarily short
times whereas others, sometimes closely related groups of animals, do not form any
new species over long periods of time (Seehausen et al., 2008; Verheyen et al., 2003).
Why is there no speciation in the case of the Guppies? Possibly, the rigorous
pursuits, in which the Guppy males conquer the females for reproduction, play a
role. This behavior, to a great extent, suspends the usual biological mechanism of
female choice (Magurran, 1999). Normally, the selective choice of males by females
is the main reason why speciation occurs. In most higher animal species, males that
deviate from the norm are not allowed to reproduce by the females, which can lead to
the formation of species barriers that then remain in existence. In most species, this
choice and decision behavior is initiated by the female. It is called female choice
(Chapter 6) and is one of the decisive forces that leads to species formation. The males
act against this force. In the case of the Guppies, the males breach these barriers and,
thus, maintain the gene flow even between rather different populations. The main
information that can be learned from the Guppy example is that population diversity
within a species can be evolutionary stable, despite ongoing gene flow.
5.6
What are Morphs?
Another example of the fact that diversity within a species can benefit the species
itself and should definitively not be thought of as leading to a speciation is provided by