Principios de Taxonomia

5.14 The Color Pattern Polymorphism of the Shells of the Brown-Lipped Snail Cepaea nemoralisj117
from one to five bands. There are 20–30 different morphs within a single population.
What are the reasons for this diversity? Why is the best-adapted type not dominant? It
cannot be the case that all of the morphs, with their differing colorations and patterns,
have precisely the same chances for survival and reproduction. Why does one
genotype not replace the other? Visual selection by predators or climatic selection
should certainly repeatedly prefer one or another phenotype. And if this represents a
case of a variable adaptation to different ecological niches, why then does the differing
niching not lead to assortative mating and thus to sympatric speciation (Chapter 6).
Whether C. nemoralis exhibits bands may have something to do with the likely
primary predators of the Brown-lipped Snail in several areas: song thrushes (Turdus
philomelos) (Jones, Leith, and Rawlings, 1977). In this regard, many investigations
have been performed to determine whether the extent of banding or the coloration of
the snail s shell has beneficial camouflage effects. Indeed, it was found that song
thrushes have more difficulty spotting banded snails in the vegetation than they do
spotting non-banded snails. This result does not, however, explain the polymorphisms
observed in this species of snails. On the contrary, the polymorphism should
eventually disappear, at least in those locations where the song thrush is the primary
predator of the snail.
Climate factors may exert a selective influence on the presence and color of bands
in C. nemoralis. Dark shells absorb more thermal radiation, whereas yellow shells
reflect light more strongly. The color of the shell therefore influences the temperature
of the snails. Exposure to the sun is crucial. Too much sun leads to heat death, but too
little sun leads to the cold-blooded snails being too stiff and having limited mobility,
which in turn puts the animals at risk for starvation. However, this explanation is also
dissatisfying. If exposure to sunlight is the primary factor in the persistence of the
multiple morphs, why are so many different morphs found at the same location?
C. nemoralis lives in a large geographical range in Europe and is found from
Norway to Spain and from the coast up to an elevation of 1200 meters. One can
observe C. nemoralis in habitats as diverse as dunes, meadows and forests. Admittedly,
the morphs are not uniformly distributed. Instead, some regions are dominated
by certain shell variations. However, it is not the case that every geographical
location, every mountain peak or every habitat hosts its own specific morphs.
Furthermore, it has been observed that different morphs remain in the sun for
differing time periods (Jones, Leith, and Rawlings, 1977). Some morphs prefer the
cool morning hours for their activities, others are active around noon, and still others
are predominantly active in shadows. However, no conclusive connection between
the ambient temperature and the snails coloration or degree of banding has been
observed. In most cases, scientists cannot assign the specific color patterns to the
individual climatic factors of the morphs respective habitats; the situation is
too complex.
The driving force for the intraspecific diversity of Cepaea snails appears to be that
increased species diversity leads to an increase in the number of potentially habitable
environments. By increasing their own diversity, the snails expand their resources
and thus increase their population size. This would without doubt be a selective
advantage. However, the question of why snails living in different niches have not