MOLECULAR ECOLOGY AND SYSTEMATICS OF SOTALIA DOLPHINS - Biology, Evolution and Conservation of River Dolphins Within South America and Asia - page 266

single species, without subspecific differentiation (Borobia, 1989). Since then, most authors
adopted the binomial S. fluviatilis , regarding S. guianensis as a synonym, but acknowledging
marine and riverine populations as different ecotypes (Borobia et al., 1991; Jefferson et al.,
1993; da Silva & Best, 1996; Rice, 1998; Flores, 2002). Other researchers preferred to
distinguish the two Sotalia forms using the subspecific denomination S. fluviatilis fluviatilis
and S. fluviatilis guianensis . A summary of the taxonomic changes in the genus Sotalia and
the type localities of the species described for this genus in South America are displayed in
Figures 2 and 3 respectively.
The first indication that the lumping of Sotalia species should be reassessed was given by
Furtado-Neto (1998). A phylogenetic analysis of mitochondrial cytochrome b sequences
showed that marine and riverine Sotalia were different, but that result needed further
confirmation, since only a single riverine sample was analyzed.
The second indication was provided by geometric morphometrics: Monteiro-Filho and
co-workers (2002) found significant differences in shape and size between marine and
riverine Sotalia skulls, suggesting that they belonged to different species. The main difference
was in the alignment of the rostrum and occipital condyle: in marine animals, the location of
the foramen magnum is posterior, indicating that the cranium would be in line with the
vertebral column. In freshwater specimens, the foramen magnum is located more ventrally, so
the cranium would point downwards (Monteiro-Filho et al., 2002).
Molecular Systematics
Taxonomy
As morphological analyses revealed significant differences between marine and riverine
Sotalia (Monteiro-Filho et al., 2002), genetic analyses were essential to settle the issue of
specific differentiation. This is because morphological differences might arise in response to
different selection regimes and might not reflect reproductive isolation. Additionally,
Monteiro-Filho et al. (2002) did not examine any skull from the Amazon Estuary, so the
possibility that marine and riverine Sotalia formed extremes of a cline could not be ruled out.
Significant differences in the skull had been previously reported by Borobia (1989), but a
conservative conclusion supporting a single species was reached, among other reasons, due to
the lack of samples from the Amazon Estuary, which could represent a transitional zone.
The use of molecular data in taxonomy and phylogeny has intensified over the last
decades. Molecular systematics has benefits and disadvantages over traditional, morphology-
based systematics (Hillis, 1987). Molecular markers are useful because they reveal a larger
amount of variation, due to the large number of characters available in comparison with
morphological analyses. Besides, genetic differences usually accumulate faster than
phenotypic differences and, when genotypes are analyzed, environmental effects such as
plasticity or convergence do not confound the analyses (Mayr, 1963; Avise, 2004). Those are
invaluable features, especially in the delimitation of species. The detection of reproductive
isolation and of monophyletism, which are pre-requisites of many species concepts, is also
straightforward when genotypes are analyzed (Mayr, 1963; Hillis, 1987; Knowlton, 2000;
Avise, 2004). On the other hand, molecular analyses demand expensive equipment and
samples preserved in a way not to destroy DNA. Hence, integrating molecular and