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S. Renaud and J. R. Michaux

Thesleff, 2000) and significant changes in the final shape of Conversely, molar differentiation is minimal on islands of
the tooth can be caused by slight changes early in intermediate size. This effect is due to the differentiation of
development (Jernvall, 2000; Salazar-Ciudad & Jernvall, the Sicilian molars. Despite the large size of this island, it is
2002; Kangas et al., 2004). This mode of development makes occupied by a specific genetic clade (Michaux et al., 1998b),
a single developmental module with interdependence of and the molar difference is linked to a genetic divergence.
cusps. Furthermore, once the molars are erupted they should Additionally, molar differentiation is important on small
2
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only vary with wear (Renaud, 2005). The mandible, however, islands such as Porquerolles (12.5 km ), Port-Cros (6.4 km ),
2
2
2
seems to consist of at least two modules corresponding to the Ole ´ron (17.5 km ), Noirmoutier (4.9 km ) and Re ´ (9.3 km ),
alveolar region and the ascending ramus (Atchley et al., 1992; where a reduced population size seems to trigger the process.
Mezey et al., 2000) and the shape of the mandible is Notably, the existence of an artificial bridge linking an island
influenced by bone remodelling occurring due to an inter- to the mainland does not seem to increase gene flow enough
action between muscular functioning and mandible growth to homogenize island and mainland populations, as shown by
(Lightfoot & German, 1998; Bresin et al., 1999). These factors the results from the islands of Ole ´ron and Noirmoutier. This
may make mandible shape more prone to change in response can be explained partly by research showing that resident
to environmental factors than molar shape. populations of rodents on small islands limit introgression
These results suggest that molars, which are significantly from mainland genes by an increased aggressiveness towards
more abundant in the fossil record than fragile mandibles, can immigrants (Granjon & Cheylan, 1989, 1990). Accordingly,
provide valuable information on the phylogenetic background genetic data suggest that the proximity to the mainland does
of populations even at an intraspecific scale. However, the not impede significant genetic divergence on smaller islands
assumption of tracking the phylogenetic background of (Michaux et al., 2002b). An adaptive component for such
populations using morphometric analyses of teeth may be molar divergence on small islands is suggested by a common
challenged. Firstly, given recent developmental data, slight trend of insular molars to be wider, interpreted on the
changes early in development can induce major change in the mainland as allowing the consumption of more resistant
final shape of the tooth. Second, different genes or changes in food. This response might be enhanced by a long history of
gene expression can produce similar morphologies (Cheverud isolation of a reduced population favouring genetic diver-
et al., 1997; Klingenberg et al., 2001; Klingenberg & Leamy, gence.
2001) that increase the likelihood of convergent evolution. A differential response of mandibles and molars to (1)
Third, dental characters may experience rapid changes during ecological factors and (2) genetic differentiation is supported
periods of environmental changes (Kangas et al., 2004; Renaud by the pattern of mosaic evolution observed on the islands we
et al., 2005). However, our results, together with recent data studied. These data corroborate the results based on mainland
on the intraspecific variation of the mouse tooth (Cucchi, patterns of differentiation.
2005), support the molar as a marker of genetic divergence, at
least for relatively recent events.
Insular effect on size: mosaic variation, gigantism and
macrodonty
Mosaic evolution of mandible and molar shape on
Fewer genes are apparently required to control the size of
islands
mandibles and molars than the shape (Workman et al., 2002).
The pattern observed on the mainland suggests that molars Size, however, is known to vary with numerous factors (Nevo,
should differentiate according to patterns of gene flow. The 1989; Ganem et al., 1995; Dayan & Simberloff, 1998;
response time for molars should be slower than for mandibles, Kingsolver & Pfennig, 2004) and the large influence of the
which could vary rapidly due to various environmental factors. endocrine system on size may explain the lability of this
Insular populations are prone both to breakdown of gene flow character (Bu ¨nger & Hill, 1999; Dupont & Holzenberger,
and to important changes in ecological factors compared with 2003). The discrepancies in the response of molar and
the mainland populations. Island populations, therefore, mandible shape to various factors may therefore not be valid
should display more intricate patterns than populations on for size. Yet, our results point to distinct factors influencing
the mainland, which have an integrated mandible–molar molar and mandible size (Table 6).
latitudinal signal. Size is, however, a commonly studied feature, and under-
Mandibles tend to diverge more on islands of intermediate standing the factors underlying its differentiation is of general
remoteness (Yeu, Corsica, Ibiza) and intermediate competition interest. A well-documented effect of insularity on small
pressure (Yeu, Re ´, Ibiza). Divergence on Ibiza may be mammals is a trend towards larger size (Foster, 1964;
influenced by allometric divergence due to the very large size Lomolino, 1985, 2005). This effect has been described for
of the animals. The island’s remoteness and middle level of body size, and it is usually interpreted as due to a weakening of
competition pressure may cause environmental conditions interspecific competition (Angerbjo ¨rn, 1986; Dayan &
distinct from the mainland, such as a widening of the wood Simberloff, 1998) or predation (Michaux et al., 2002b),
mouse niche permitting a more varied diet (Orsini & Cheylan, allowing the animals to converge towards an optimal energetic
1988), therefore causing mandible shape to change. size (Damuth, 1993). The consequent change in the niche

350 Journal of Biogeography 34, 339–355
ª 2006 The Authors. Journal compilation ª 2006 Blackwell Publishing Ltd