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Chapter One: General Introduction
population (Cassens et al. 2005). On a larger scale, Hoelzel et al. (2002b) suggested
a historical bottleneck as a potential cause for the low worldwide genetic diversity of
killer whales. Other processes might also be involved in shaping the genetic diversity
in this species (see below in this section).
Hayano et al. (2004) suggested that the low genetic diversity found in the population
of Pacific white-sided dolphins along the coast of Japan was a result of a population
reduction. This could have occurred when the individuals in the Sea of Japan
became isolated from the rest of the North Pacific population during a glacial period
in the Late Pleistocene. This could also be considered a founder event rather than a
demographic bottleneck, since it does not necessarily represent a real decrease in
abundance. Although they represent different demographic processes, bottleneck
and founder events can have the same effect on the level of genetic diversity in a
population. Cases of founder events have been suggested for several nearshore
populations of bottlenose dolphins (notably in the western North Atlantic and South
Africa) in order to explain the lower level of genetic diversity than that in the offshore
populations from which they could have originated (Hoelzel et al. 1998b, Natoli et al.
2004).
Social behaviour can also reduce genetic diversity within local populations, for
instance as a result of high philopatry. This could be the case for the population of
about 130 bottlenose dolphins at Moray Firth, Scotland, that show no evidence of
contemporary exchange with neighbour populations (Wilson et al. 1999). Indeed,
Parsons et al. (2002) found a very low level of mtDNA variability among these
dolphins, which, in such a small population, could be due to the relative importance
of genetic drift. A similar case of low mtDNA diversity is observed in Doubtful Sound,
New Zealand (de Tezanos Pinto et al., unpublished data), within a small and isolated
community of bottlenose dolphins living in a fiord.
Unusually low levels of mitochondrial diversity were also found in several species of
dolphins thought to live in matrilineal societies (Whitehead 1998). These include killer
whales, long-finned pilot whales and short-finned pilot whales (with sperm whales
following a similar pattern). To explain this trend, Whitehead (1998, 2005)
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