Principios de Taxonomia

6.31 The Problem of Smooth Boundaries between two Gene-Flow Communitiesj183
In addition, there is also the phenomenon of maternal inheritance. The homozygotically
recessive l/l combination does not affect the individual itself that possesses
this genotype, but only the structure of the cytoplasm of the eggs that are
produced by this individual, if it is a female. The structural organization of the
cytoplasm determines the direction of coiling of the daughter snails that develop
from these eggs. Right- or left-handedness consequently does not affect the generation
that has the phenotype, but only the subsequent generation.
What is important for the snail species problem is the fact that right- and lefthandedness
each result in mating barriers. A right-handed snail cannot copulate with
a left-handed one. The copulation position is such that the penis and vagina do not fit
if two mirror-inverted, differently handed snails want to mate. This phenomenon
alone offers the possibility of a rapid reproductive isolation and thus the formation of
a new species, which would require the left-handed snails pooling together.
That these are favorable conditions for species formation is further supported by
the fact that because of the maternal inheritance, the l allele can already preadaptively
spread in a population, without the left-handed snails phenotypically appearing in
this population. This can statistically lead to a sudden appearance of many lefthanded
snails, which can then build up a population of many individuals that is
strong enough to compete with the right-handed snails.
The statistical chance that an autonomous left-handed population will build up in
an existing right-handed population is still a matter of controversial discussion. Allen
Orr (Orr, 1991), however, has posited via a computer simulation that an entire snail
population can turn from a right-handed population into a left-handed population.
Such a population would immediately become isolated from the other populations.
This would then be a one-gene speciation. This model is not purely speculative, for
among fossil snails, examples can be found of snail species that became left-handed
in a relatively short evolutionary period.
The capability of an immediate speciation in only one generation has already been
mentioned for tetra- and polyploid organisms that can emerge in plants. If a plant
doubles its chromosomal set and thus becomes tetraploid, gene flow is instantly
disrupted because the tetraploid plant can no longer backcross with the parental
plants. Tetraploidy is a remarkable form of speciation that happens without the
alteration of even a single gene because no gene in the newly evolved species is
different from those in the original species (Schilthuizen, 2001).
These examples again make clear that the species that is defined as a delimited
gene-flow community is a fundamentally different concept from the species as a
phylogenetically distinct group of organisms, which is the kind of species that is
aimed at by the barcoding approach (Chapter 4).
6.31
The Problem of Smooth Boundaries between two Gene-Flow Communities
A frequently observed misunderstanding of the species concept of the gene-flow
community lies in the problem of delimitation. Gene flow barriers are such