Principios de Taxonomia

190j 7 The Cohesion of Organisms Through Genealogical Lineage (Cladistics)
Figure 7.3 Reproduction of biparental organisms cannot be displayed in a simple cladistic tree
because the fusion of gametes (fertilization) reverses bifurcations into a network. The branches do
not bifurcate in an upward direction along the time scale but, rather, fuse upwards.
Because the phylogenetic tree displays only reduplication and bifurcation, in the
first instance, only uniparental reproduction processes can be displayed in a
phylogenetic tree. Uniparental reproduction is the reproduction of a single individual
without the sexual merging of two parents. It is vegetative reproduction, parthenogenesis
or self-fertilization (Chapter 6). Only this kind of simple cleavage of a mother
organism into two daughter organisms represents the branching phylogenetic tree as
it is displayed in Figure 7.2. Biparental sexuality is not a kind of replication and
branching, and thus the reproduction of biparental organisms cannot be displayed in
a simple phylogenetic tree (Figure 7.3).
Many single-celled protists, for example, amoebae or flagellates, reproduce uniparentally.
Also, the vegetative reproductive stages of sponges and Coelenterata, that is,
multicellular organisms, as well as the parthenogenetic life cycles of rotifers and water
fleas are also uniparental. These replication processes can easily be displayed in a
bifurcating phylogenetic tree. Bisexual processes, however, cannot be displayed in a
simple phylogenetic tree because sexual reproduction is the opposite of bifurcation.
Biparental sexuality is the lateral fusion of separated branches. The phylogenetic tree
does not apply to sexually reproducing organisms. The fusion of separated branches
turnsthephylogenetictreeintoareticularnetwork,whichisnotaphylogenetictree,for
aphylogenetictreeisexpectedtobeatreewithdivergingbranchesthatonlydivergebut
do not reconvene. The diagram in Figure 7.2 cannot be used for the representation of a
family s phylogenetic tree. A family s tree (Figure 7.3) includes biparental sexuality,
and therefore it cannot be a phylogenetic tree, but rather, it is a reticular network.
Every fusion of gametes (fertilization) reverses the basic element of a phylogenetic
tree: bifurcation. The bifurcating phylogenetic tree is tarnished by sexual processes.
Biparental sexuality means that the offspring of an individual becomes
contaminated by its sexual partner. The children of a spouse are only one half
its offspring, its grandchildren only one quarter. As shown in Figure 7.3, biparental
reproduction reverses the bifurcating phylogenetic tree. The branches do not
bifurcate in an upward direction, but to the contrary, they fuse upward along the
time scale.
However, there certainly are phylogenetic trees of biparental organisms. How is
this possible? This apparent incongruity results from a confusion of the hierarchical