Principios de Taxonomia

182j 6 Biological Species as a Gene-Flow Community
D. silvestris and D. heteroneura, for example, differ in fewer than ten gene locations.
These genes distinguish the two species by slightly different head forms. As the head
form is important for partner choice, a blending of the two species no longer occurs
(Ahearn and Templeton, 1989). Accordingly, few mutations were necessary to
generate two new, separate Drosophila species.
An even more drastic example is given within the species complex Drosophila
pseudoobscura. The Drosophila pseudoobscura individuals living in Columbia ( Bogota
population ) and in North America ( USA population ) cannot be crossed because
the offspring of such crosses are sterile (Phadnis and Orr, 2008). In this example, only
a single gene separates the two Drosophila populations into two species, without clinal
transition regions. The gene is called Overdrive, and it causes both male sterility
and segregation distortion in the F1 hybrids. Because this gene causes hybrid sterility,
it is a speciation gene (see above).
In North America, there are two closely related species of the monkey flower
Mimulus: M. lewisii and M. cardinalis. Mimulus are plants that belong to the order
Laminales. Both species evolved from a common stem species an evolutionarily short
time ago. The bifurcation into two species was accompanied by a change of flower
coloration, which is essentially controlled by a single gene. M. lewisii has pinkish
flowers, while M. cardinalis has lost the pinkish color through a mutation and now
exhibits scarlet flowers.
The pinkish flower coloration is clearly visible to bumblebees, and indeed,
M. lewisii is pollinated by bumblebees. However, the scarlet flowers of M. cardinalis
are barely visible to bumblebees and accordingly can no longer be effectively
pollinated by bumblebees. Hummingbirds can clearly see the flowers, and indeed,
M. cardinalis is pollinated by hummingbirds. Here we face a case of sympatric
speciation, which has become possible because two alterations have prevailed in a
parallel way: the de novo origin of the red flower coloration and, coevolutionarily, the
pollination by hummingbirds.
In gene-technological experiments, the respective flower coloration genes of the
one Mimulus species have been transferred crosswise into the genome of the other
species. By doing so, nothing was changed in the individual species except for
flower coloration. In field experiments, the pollinators exclusively followed the flower
coloration. Thus, they each visited the wrong plants, but those with the right
flower coloration (Orr, 2009). In this way, a single mutation can induce the pollinators
to visit a new plant and subsequently exclude them from the gene-flow community of
the former species. From this example, it follows that a simple genetic alteration can
affect the origin of a new species.
An additional example of speciation through one or only a few mutations is
provided by snails. Most species of snails are right-handed, that is, viewed from
above, the shell is coiled clockwise around an imaginary axis. In rarer cases, however,
left-handed specimens appear. This has a simple genetic basis. Only a single allelic
pair is responsible for the direction of coiling. One allele (R) causes right-handedness,
the second allele of the same gene (l) causes left-handedness. As R dominates l, most
of the phenotypes are right-handed; only the homozygotic l/l combination results in
left-handedness.