Principios de Taxonomia

128j 6 Biological Species as a Gene-Flow Community
uniparental organisms, then each birth would be a speciation because, due to the
lack of biparental reproduction, no individual will ever recombine its genome
with the genome of another. Of course, it is not reasonable to apply the term
species to an individual.
A particular problem is posed by prokaryotic organisms, as they also have lateral
gene transfer, because in its general sense each transfer of genes between two
organisms other than a transfer from a mother into a daughter is a lateral gene
transfer (see below). Although the biological mechanisms of prokaryotic lateral
gene transfer are different from the mechanism of sperm-egg fusion in eukaryotes,
the final population-genetic and evolutionary results of both forms of gene transfer
are similar. In both cases, organisms that may have a long evolutionary history of
separation merge again and exchange genes. In both cases, the result of such contacts
is the recombination of genomes. In both cases, lateral gene transfer makes it difficult
to generate a dichotomously branching phylogenetic tree. Even distant branches
may merge, converting the tree into a connected network (Figure 6.1).
Figure 6.1 In a cladistic tree of biparental
organisms, the branches refer to taxa, not to
organisms, whereas the reticular cross
connections within the shrouded branches refer
to individual organisms. A cladistic tree of
biparental organisms cannot be displayed at the
organismic level; it only can be displayed
at the taxon level. This difference between
the cladistic trees of bi- and uniparental
organisms reflects the reason why
uniparental organisms cannot be considered
to be species.