Principios de Taxonomia

174j 6 Biological Species as a Gene-Flow Community
1) Self-fertilization: The first important difference between plants and animals is a
plant s ability for self-fertilization, a property that occurs much more rarely in
animals. Species hybrids would remain as evolutionarily insignificant and rare
incidences if the hybrids on their own would not have the ability to rapidly erect a
new population of numerous new individuals. Only a population that is rich in
individuals would be able to compete with both parental species as a distinct new
species in the struggle for life. Only this scenario gives the hybrids the chance to
prevail and survive as a newly evolved species.
Normally, species-hybrid individuals cannot build their own populations
because they have little chance of encountering equal hybrids as sexual partners.
Instead, they only encounter the individuals of the two parental species. If,
however, the hybrids mate again with the parental species, then this represents
a genetic backcrossing. The hybrid genomes blend again with the parental
genomes, and except for a limited introgression of a few genes from the foreign
species, nothing changes in the two parental species. The two parental species
remain preserved, and a third species cannot evolve.
However, many plants are able to self-pollinate. Thus, they do not need any
sexual partners, and therefore, hybrids do not risk mating with the organisms of
the parental species, which would eliminate the chance to propagate as a hybrid.
Due to self-pollination, however, they are able to build a distinct population that is
strong in number, which is reproductively isolated from the parental populations
from the start and can compete with these populations because of their own
reproduction potency. Accordingly, three species have evolved from two parental
species.
Of course, it must be considered that the new group of exclusively selffertilizing
organisms is not a gene-flow community and thus cannot be a species
in this sense. However, many self-fertilizing organisms are not exclusively selffertilizing
all of the time. They occasionally undergo a biparental gene exchange
(see above).
In animals, in contrast to plants, the representatives of few taxa are capable of
self-fertilization, for example, many trematodes (flukes) and cestodes (tapeworms).
The well-known pork tapeworm Taenia solium is almost exclusively
self-fertilizing because the human (the final host in the vast majority of cases)
can only sustain a single worm. Flukes and tapeworms are therefore candidates
for hybridogenic speciation in the animal kingdom. Indications that this has
actually happened, however, are rare (Hirai and Agatsuma, 1991).
2) Tetraploidy: Self-fertilization, a major difference between plants and animals, is
the first important reason why hybridogenic speciation occurs more commonly in
plants than in animals. There is, however, a second important reason. If the
members of two different species interbreed, then an F1 hybrid results, whose
diploid genome consists of the chromosomal sets of two different species. This
can lead to disturbances in meiotic chromosome pairing because the chromosome
partner available for tetrad formation is from another species, causing
misalignment of some chromosomes and deranged chromosome pairing in
meiosis. However, a lack of correct tetrad formation means that correct separation