Principios de Taxonomia

6.6 Uniparental Propagation in Eukaryotesj137
(Ghiselin, 1997). Longer-term uniparentality is hard to substantiate, as an occasional
biparental fertilization can only be recognized if the organisms are observed
uninterruptedly for years. As such surveillance is generally impossible, bisexual
stages may be overlooked, especially if they do not occur for years.
The bdelloid rotifers are famous for having persisted without biparental sex for
millions of years (Welch and Meselson). There are no males. Only females are found,
and these individuals are all diploid. As the homologous alleles of the diploid genome
have diverged to an unusually high extent in DNA sequence, it is clear that genetic
recombination between the homologous alleles has not occurred for many generations
of evolutionary time. Under biparentality, the homologous alleles would have
aligned their sequences through occasional recombination. As no exchange has
occurred, it appears that there has been no genetic recombination for millions of
years; however, even among the Bdelloidea, the rare occurrence of mictic stages
cannot be ruled out with complete certainty (Ghiselin, 1997).
The deep skepticism regarding permanent uniparentality in an animal species
results from the awareness that the loss of bisexuality in particular phylogenetic lines
should apparently come to a dead end in the evolutionary long run. Apparently,
sexuality cannot be reinvented repeatedly in nature. There are no indications that a de
novo origin of sexuality has ever occurred in phylogenetic lines that have lost sexual
reproduction (Ghiselin, 1997).
Biparental sexuality apparently is an ancient biological process that originated in
the common ancestors of all currently living eukaryotes and has since been
maintained.
The conclusion that organisms with permanent uniparental reproduction are
species-less because there is no interorganismic connection is unsatisfying.
A taxonomist wants to classify the entire diversity of life and not only a subset of
organisms, but nature is not made to be classified by humans (Sterelny and Griffiths,
1999). Only the need for order forces us to find a place for all and everything.
One interesting example in this context is the species status of workers in
many social Hymenoptera (bees, wasps, ants). These workers are sterile and so do not
have any reproductive connection to each other; however, it makes little sense to
consider the sterility of Hymenoptera workers, which are dead ends of gene flow, as a
species problem. This conclusion results from the following reasoning: one can
imagine a beehive as a super organism, in which the reproducing queen creates
workers that are doomed to die off, like somatic cells split off from the germ line
(Tautz, 2008). Like worker bees, somatic cells are end lines of development no longer
capable of reproduction. Only the germ cells continue living. Similarly, the sterile
Hymenoptera workers die, but the queens are inseminated and continue to
reproduce.
The distinction between an ontogenetic genealogy of cells and a genealogy of
organisms should not be as strong as it currently is. In animals and plants, there are
many transitions from multicellular organisms to colonies of organisms that are
connected like the cells in an individual organism. Examples for these super
organisms are the animal colonies that are formed by many hydrozoans and corals.
The next-higher level would then be the beehive.