Principios de Taxonomia

106j 5 Diversity within the Species: Polymorphisms and the Polytypic Species
In contrast to natural selection, where only positive alleles have a chance at
survival, in the context of genetic drift, the extinction or fixation of an allele in a
population is a matter of chance. It therefore follows that in every population,
allelic diversity is short-lived and only exists because of the constant addition of
new mutations.
5.10
Stable Polymorphisms – The Selective Advantage is Diversity
The principle that allelic diversity is evolutionarily short-lived is, however, upset by a
remarkable exception: stable polymorphisms. Stable polymorphisms refer to the
long-term persistence of different allelic variants within a population, without the
elimination of single variants by selection or drift.
Selection normally favors only one of several allelic variants because only one
variant is optimally adapted to the specific task of the given gene. The other allelic
variants are most likely less able to perform this task and therefore should have a short
lifetime.
Stable polymorphisms, however, are a remarkable exception. How can several
variants of a single gene survive for a long period of time in a population? Individual
allelic variants cannot have equal fitnesses. In the case of stable polymorphisms, the
selective advantage for allelic survival appears not to be the quality of the individual
allele but rather the coexistence of several parallel allelic variations in the population;
that is, selection is interested in multiple allelic forms coexisting within the
population. Such a population is composed of individuals that carry different allelic
variants of a gene. Therefore, the population always maintains a number of organisms
that are adaptively prepared for potential environmental changes. In this way,
the population can react more flexibly to varying environmental changes. Such allelic
variations can therefore be said to be kept in reserve by the population to be
recruited in a vital moment. Such maintenance of variation is referred to as
preadaptation.
The selective advantages of allelic diversity in a population, as well as preadaptation,
cannot be easily understood from a Darwinian perspective. Ultimately,
however, individual organisms benefit from the populations advantage. This concept
amounts to group selection, which is rejected by many evolutionary biologists. The
relationship between individual and group fitness is a controversial matter in the field
of biophilosophy. Multiple allelisms and polymorphisms are a matter of group
fitness. Theorists, however, disagree whether natural selection acts primarily on
genes, individual organisms or whole populations (Okasha, 2003; Okasha, 2006;
Okasha, 2009).
It is argued that only entities that replicate and reproduce, that is, leaving offspring,
can be units of selection and that a necessary condition of reproduction is that the
offspring outlives its parent (Dawkins, 1976). Does a population reproduce, or do only
the single organisms of a population reproduce? The group selection controversy
concerns whether natural selection can operate at the group level rather than at the