MOLECULAR ECOLOGY AND SYSTEMATICS OF SOTALIA DOLPHINS - Biology, Evolution and Conservation of River Dolphins Within South America and Asia - page 273

(Avise, 2004). Phylogeographic analyses focus on the species' past, but provide important
insights on its present-day population structure. Most endangered species are highly
structured, because reductions in abundance contribute to the isolation of populations
(O'Brien, 1994; Frankham, 1996; Avise, 2004) and small population sizes increase genetic
drift, which accelerates population differentiation. As a result, endangered species are often
subdivided in demographically independent units, each with a population size more affected
by local birth and death rates than by migration rates. Hence, the persistence of each unit is
linked to the evolutionary and demographic processes acting upon it (Moritz, 1994; Avise,
2004; Palsbøll et al., 2007). Population units that should be considered independently for
evolutionary biology purposes have been named ―Evolutionarily Significant Units‖ (ESU)
(Ryder, 1986). Later, Moritz (1994) proposed the term ―Management Units‖ (MU) to
designate units for conservation purposes. MUs are different from ESUs because they are less
restrictive and closer to the demographic present of species.
The population structure and phylogeography of S. guianensis along the Brazilian coast
was investigated by Cunha (2007), using mtDNA control region sequences. Analysis of
molecular variance (AMOVA; Excoffier et al. 1992), spatial analysis of molecular variance
(SAMOVA; Dupanloup et al 2002) and Nested Clade Analysis (NCA; Templeton 1998, 2001)
showed evidence for at least six MUs in Brazil: Pará, Ceará, Rio Grande do Norte, Bahia,
Espírito Santo and the South-Southeastern area (from Rio de Janeiro to Santa Catarina states,
Figure 1). Those MUs were highly differentiated (Ф CT = 0,485, P < 10 -5 ), indicating severe
restrictions to gene flow among them. An interesting finding was a lack of variation in the
control region of dolphins from South-Southeastern Brazil (between parallels 22º and 25ºS,
extending 900 km). NCA and genetic diversity patterns suggest that this homogeneity might
have been caused by a recent colonization of the Brazilian coast through a range extension
from north to south, which could be linked to a warming up of the Western Atlantic during the
Holocene. Thus, the observed homogeneity is probably not due to gene flow within the
region, but a consequence of recent foundation (Cunha & Solé-Cava, 2006; Cunha, 2007).
Populations of S. guianensis from the northern part of South America and the Caribbean
were analyzed by Caballero et al. (2006), who proposed two MU for that area: one for Central
America, Colombia and Venezuela, and another for Guyana, Surinamee and French Guiana.
The authors advised that dolphins from the Maracaibo Lake, despite being included in the
first MU, had some unique haplotypes and their genetic distinctiveness should be further
investigated. However, only three individuals from southern Maracaibo were analyzed: the
others were from the northern portion of the lake, where it opens to the Gulf of Venezuela.
Clearly, further analyses of samples from the Maracaibo must be analyzed to verify their
possible genetic distinctiveness.
To date, there is no information on the population structure of S. fluviatilis . The only data
available suggest that the species has moderate to high genetic diversity, since 12 individuals
from the same location in the Central Brazilian Amazon had five different control region
haplotypes (Cunha et al., 2005), and 21 dolphins from the Peruvian, Colombian and Brazilian
Amazon had 13 haplotypes (combining the control region and ND2, Caballero et al., 2007).
Microsatellite variation was also larger in S. fluviatilis (H = 0.531) than in S. guianensis (H =
0.364; Cunha and Watts, 2007). The reason for a higher level of gene variation in S.
fluviatilis , in spite of its probably smaller population size, remains to be determined.