Principios de Taxonomia

134j 6 Biological Species as a Gene-Flow Community
Therefore, descent communities cannot be natural groups because they form a
continuum, not a structured set of groups. Genealogical connection implies the birth
of children or grandchildren, not the birth of taxa. Nothing in the succession of
consecutive generations constitutes the end of one taxon and the beginning
of another taxon (Simpson, 1961). The descent connection exists in nature, but
there are no defined boundaries. The boundaries in the descent community must be
created by human-imposed criteria. In contrast, gene-flow connections have natural
boundaries and are therefore acceptable for use in forming a natural species concept.
6.5
The Concept of the Gene-Flow Community in Eukaryotes and in Bacteria
Lateral gene transfer is not restricted to eukaryotic sperm-egg fusion. Lateral gene flow
also exists in prokaryotes (bacteria). The peculiarity of bacteria, however, is that the
horizontal gene transfer does not only occur between related organisms, but bridges
wide evolutionary divides. Bacteria that are evolutionary distant from each other can
exchange genes (Dagan, rtzy-Randrup, and Martin, 2008). There is even occasional
transfer between archaebacteria and eubacteria, which are assigned to different
kingdoms because of their evolutionary distance (Woese, Kandler, and Wheelis, 1990).
Recently, an essay was published with the noteworthy title, Species do not really
mean anything in the bacterial world (Hollrichter, 2007). This title signifies that
there are no reliably covarying traits in bacteria that would allow a consistent scheme
of classification. Bacteria can be classified according to one property, but another
property may overlap across the units; classification by this property would result in
other delimitations. By classifying according to one trait, one may, of course, obtain
groups. However, by then classifying according to a second trait, one may obtain
completely different groups.
After bacteria were classified by morphological criteria 150 years ago, the age of
bacterial cultures began. Accordingly, bacteria were arranged by physiological and
biochemical properties or by their specific pathogenicities. The chemotaxonomical
classification was based on fatty acid and sugar components. In the
1960s, however, a completely new classification was conducted according to genomic
similarity. Carl Woese has given preference to the comparison of 16S rRNA gene
sequences for creating taxonomic classifications.
As of today, about 7000 species of bacteria have been designated and classified into
1194 genera, 240 families, 88 orders, 41 classes and 26 phyla (Hollrichter, 2007). This
classification is based on the following definition of species: two organisms belong to
the same species if their genomes are more than 70% identical, their 16S rRNA
sequences are at least 97% identical and their phenotypes are very similar. This
definition, however, cannot be applied to most bacteria, as the three criteria are not
consistent. Due to lateral gene transfer, there are bacteria whose genomes are more
than 70% identical, which thus should belong to the same species, but which
also exhibit large differences in their 16S rRNA sequences, and so should belong
to different species. The reverse is also seen.