Principios de Taxonomia

5.10 Stable Polymorphisms – The Selective Advantage is Diversityj107
level of individual organisms. Most contemporary evolutionary biologists are highly
skeptical of the hypothesis of group selection, which they regard as biologically
implausible (Okasha, 2001).
How can an allele survive the long term in a population if it is not advantageous, or
even disadvantageous? Such an allele would certainly be selected against relatively
quickly and thus disappear in the long term. As far as allelic polymorphisms are
understood today, many are not immediately advantageous for the individual.
Instead, the advantage for survival lies in the population itself surviving in the case
of an environmental change. In the case of such an environmental change, the
survival of a population can depend on the few surviving individuals that carry alleles
that were previously disadvantageous (see the example of Darwin s Finches below).
However, how do organisms (and their alleles) that are disadvantaged survive in an
unchanging environment?
This difficulty is not simple to resolve. Which genetic mechanisms allow for the
survival of occasionally disadvantageous alleles? One mechanism is immediately
obvious: recessivity. Many alleles, even disadvantageous ones, can survive for a long
period of time if they are expressed in the heterozygous state. This allelic survival is
possible because the homologous dominant allele determines the phenotype and is
therefore the only allele that is exposed to selection. Such recessive alleles are under a
invisibility cloak. Certain authors even believe that the biological meaning of
diploidy is founded in this concept.
A second genetic mechanism that may preserve disadvantageous alleles over the
long term is the fixed coupling of these alleles with neighboring genes. These groups
of genes would be linked on the chromosome and only rarely, if ever, separated by
recombination. If the neighboring genes have a selective advantage, they could carry
disadvantageous coupled genes with them from one generation to the next, provided
that the disadvantage of the linked allele is not too great.
The following simple example further illustrates this concept. Sexual dimorphism
can be thought of as a stable polymorphism. There are several examples of a particular
sex exhibiting selective disadvantages. For example, there is a clear selective disadvantage
for the female sex in some human cultures. Despite this, females are not
displaced through selection because the selective advantage of sexual dimorphism is
not the advantage of being a male or a female. Rather, the advantage comes from there
being two sexes in the population. The sex that is disadvantaged by inherited
properties or unfavorable ethical or social constraints does not have a selective
disadvantage; its survival is due to the advantage of the two sexes coexisting, as
disadvantageous as it may be to be of a given sex.
Stable polymorphisms are not restricted to sexual dimorphism. These polymorphisms
occur in various forms and are the primary foundation of intraspecific trait
variabilities (Aguilar et al., 2004). Known examples of stable polymorphisms include
the major histocompatibility gene complex (MHC) and the different blood groups. In
the case of the MHC, allelic diversity is evolutionarily older than the separation of
humans and chimpanzees. With respect to particular MHC alleles, humans and
chimpanzees are more similar than particular humans are among themselves
(Figueroa, G€unther, and Klein, 1988). Careful thought should be given to the fact