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Chapter One: General Introduction
information may help reveal and clarify important features of cetacean taxonomy
(e.g., Milinkovitch 1997), it also has some inherent limitations as a tool for
systematics. The alleles of sister species reach monophyly after all ancestral
polymorphism is lost through genetic drift or directional selection. Although drift is
expected to be much quicker for mitochondrial genes than nuclear genes (as a
consequence of a smaller effective population size for the former), incomplete
lineage sorting can still affect mitochondrial gene trees, especially in the case of
rapidly radiating taxa in which successive speciation events occur before sorting is
completed (Funk & Omland 2003). Such an effect is suspected to obscure the
phylogeny of delphinids, in particular within the group commonly referred to as STDL,
which encompass the species of Stenella, Sousa, Tursiops, Delphinus and
Lagenodelphis (Reeves et al. 2004). As a result, several molecular markers,
including mitochondrial and nuclear markers, are now commonly combined to resolve
phylogenetic relationships among cetacean species (e.g., Harlin-Cognato &
Honeycutt 2006, Caballero et al. 2007).
Other sources of uncertainty may exist. Natoli et al. (2006) recently suggested that
the populations of long-beaked common dolphin (Delphinus capensis) in the Pacific
and South Africa could have each radiated independently from their sister-species,
the short-beaked common dolphin (Delphinus delphis). This would represent a case
of morphological convergence, rather than being the sole consequence of incomplete
lineage sorting. Similarly, it is believed that the morphotypic similarities between the
Asian and South African populations of Tursiops aduncus are a result of
convergence, the two being highly genetically differentiated (Natoli et al. 2004).
Ultimately, in order to resolve the taxonomic status of dolphin species it has been
recommended that congeneric analyses be conducted, including, for each of the
series of putative taxa, a large number of specimens from across its range (Funk &
Omland 2003, Reeves et al. 2004).
1.2.3. Convergence with terrestrial mammals
Perhaps the most striking and interesting characteristic of cetacean evolution (and of
odontocetes in particular), is the several lines of convergence found with terrestrial
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