Biology Reference
In-Depth Information
Timing of Speciation
The divergence between
S. fluviatilis
and
S. guianensis
observed by Cunha et al. (2005)
was 2.5%, for both the control region and the cytochrome b. The evolutionary rates of those
markers have been estimated at between 0,5% and 1% per million years (My) for the control
region of cetaceans (Hoelzel et al., 1991) and 1%/My for the cytochrome b (Irwin et al.,
1991). Hence, the speciation event that separated both lineages probably happened between 5
and 2.5 mya, during the Pliocene. At that time, the Amazon River was already flowing along
its present course, with its outlet to the Atlantic (Hoorn et al., 1995; Lundberg et al., 1998).
For the last 4 my, several sea level oscillations occurred, as a consequence of glacial and
interglacial periods. During the periods of sea level rise, river discharge was prevented, and
freshwater inflow into the Amazon basin increased, causing the inundation of the Amazon
craton (Lundberg et al., 1998). The highest marine transgression happened around 2.5 mya
(Klammer, 1984). It is possible that
Sotalia
colonized the Amazon basin during one of those
transgression/inundation events. Regardless of the putative timings of speciation, dolphins
that colonized the Amazon River system probably had an Atlantic origin, because the
alternative explanation (entrance from the Caribbean via present day Maracaibo Lake and
Paleo-Orinoco system) would require a much older divergence (>10 mya).
Caballero et al. (2007) calibrated a molecular clock for the control region using the
estimated divergence between
Sotalia
and
Phocoena phocoena
based on the fossil record (10-
11 my). Therefore, they arrived at a faster substitution rate, and dated the divergence between
S. fluviatilis
and
S. guianensis
at 1 to 1.2 mya, during the Pleistocene. This dating is also
compatible with environmental oscillations in the Amazon basin (Caballero et al.,
2007). Due
to the lack of
Sotalia
fossils, it is not possible yet to decide which of the two scenarios is
more likely.
Evolutionary Relationships
Sotalia
is one of the several Delphinidae genera. The Delphinidae family is regarded as a
taxonomic ―trash basket‖, because its members are very diverse in shape and size, and share
no exclusive characteristics. Some of the characteristics of delphinids are a marine
distribution, presence of beak, presence of a falcate dorsal fin and presence of conical teeth.
However, there are exceptions to each of those features (Jefferson et al.,
1993).
The evolutionary relationships among delphinids are far from understood, so at present it
is difficult to ascertain the phylogenetic position of
Sotalia
. Traditionally,
Sotalia
has been
grouped with
Sousa
and
Steno
based on morphology. In fact,
Sousa
dolphins were originally
assigned to
Sotalia
.
The grouping with
Steno
might have resulted from the use of primitive
morphological features in pre-cladistic analyses, but has endured to the latest classifications
(reviewed in LeDuc et al.,
1999). The most accepted morphological classification was
proposed by Perrin (1989). This classification maintains
Sotalia
,
Sousa
and
Steno
as closely
related (Subfamily Stenoninae).
Sousa
is a genus with two recognized species:
S. teuszii
from
the Eastern Atlantic, and
S. chinensis
from the Indo-Pacific. A third species,
S. plumbea
,
occurring in the Western Indian Ocean, is regarded by most authors as a synonym of
S.
chinensis
.
Sousa
dolphins are morphologically similar to
Sotalia
.
Steno
is a monotypic genus
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