glaciations, four.
(^) Fleminger and Hulsemann (1974) showed some of the possible ramifications of the
continental barrier notion by a study of the copepod genus Pontellina. Prior to 1974,
all of the reports of this tropical group were attributed to one species: P. plumata.
Fleminger and Hulsemann described three new forms: P. morii, P. sobrina, and P.
platychela. They retained P. plumata for a fourth form, the one most widely
distributed. Since the new species were not recognized in a very large body of work
before this study, it is clear that the genus is made up of very similar or “sibling”
species. We cannot represent the sorts of differences that distinguish these species
without going into extreme detail, so in order to learn those distinctions please refer to
the original paper. All of the morphological differences are on anatomical parts that
are important in mating. Fleminger and Hulsemann’s tabulation of differences implies
that the overall relationships are as follows:
Pontellina plumata, which ranges from 40°N to approximately 40°S (Fig. 10.16),
has a nearly constant body form worldwide. The species more restricted latitudinally
are those that also are restricted longitudinally. As an exercise, try to come up with a
sequence of range extensions and restrictions that could have produced the speciation
sequence suggested by the relationships.
Fig. 10.16 Current distributions of four closely related species of the copepod genus
Pontellina: plumata, platychela (dotted in Atlantic), sobrina (horizontal lines in
eastern Pacific) and morii (vertical lines across tropical Indo-Pacific).
(After Fleminger & Hulsemann 1974.)