Biology Reference
In-Depth Information
Figure 5.3. Body length of two rhinoceros beetles, Chalcosoma caucasus and C. atlas in
allopatric and sympatric locations. Boxes show 5% reliable range of mean of each
population, vertical lines show maximum and minimum values. Note the significant
differences between the two species in all sympatric locations. From Kawano (
2002
).
Reprinted by permission of the author and the University of Chicago Press.
locations) that reinforcement of reproductive barriers is an important
factor, and perhaps the only factor involved. The same explanation may
apply to the observation that one species occurs at higher and the other at
lower altitudes in sympatric than in allopatric locations. It is unclear how
the two species have evolved. Kawano discusses three possibilities:
(1) species have evolved by allopatric speciation and have come into
secondary contact in some locations; (2) species have evolved sympatri-
cally and in some locations remained sympatric, whereas in others became
allopatric; (3) the two species in the sympatric locations may be phylo-
genetically closer to each other than to the same species in other locations,
i.e., C. caucasus and C. atlas have evolved repeatedly in each sympatric
location. However, the third explanation seems unlikely in view of the
existence of not only one but several sympatric ''species'' pairs.
Our conclusion for this chapter is that alternative explanations to
both niche restriction and segregation by interspecific competition are
possible, and are indeed in many cases more likely than competition. For
