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15

sometimes
present,
in
some
populations
of
wood
mice
(Fig.
1).
This
shows
shared
developmental

potentialities
across
species
however
displaying
marked
differences
in
their
dental
pattern,
having

diverged
more
than
10
million
years
ago.

Both
developmental
mechanisms
change
the
shape
of
the
tooth
without
perturbing
the
longitudinal

alignment
of
the
cusps.
This
matches
a
strong
functional
requirement
during
occlusion
of
the
molars.

Murine
rodents
chew
along
a
longitudinal
(propalinal)
direction.
In
agreement,
the
arrangement
of

the
cusps
evolved
into
longitudinal
rows
that
slide
into
gutters
in
the
occluding
molar
row
(Lazzari
et

al.
2008).
Changes
related
to
Pmax,
being
the
concerted
spacing
of
all
cusps
or
a
local
anterior

elongation,
occur
in
agreement
with
this
constraint.
Intra-­‐population
variants
along
Pmax,
and

evolution
along
this
direction,
are
thus
prone
to
occur
because
such
mice,
at
any
step
of
their

evolution,
display
functional
molar
rows
despite
variation
in
their
overall
shape.

Conclusion

This
study
aimed
at
considering
evolutionary
patterns
in
wood
mouse
molar
shape
to
the
light
of
the

intra-­‐population
variation
as
possible
‘line
of
least
resistance
to
evolution’.
It
showed
the
potential
of

this
approach
to
reconsider
striking
cases
of
parallel,
pronounced
and
fast
evolution,
mostly
on

islands.
Considering
the
intra-­‐population
variance
as
a
hint
for
underlying
genetic/developmental

networks,
this
means
that
evolution
on
islands
can
produce
fast
response
to
selection
and/or
drift
by

modulating
existing
networks.
In
contrast,
only
reduced
differentiation
accumulated
between

ancient
mainland
phylogenetic
lineages,
and
the
shape
changes
involved
were
independent
of
the

direction
of
main
intra-­‐population
variance.
Evolution
among
the
continental
lineages
was
probably

constrained
by
stabilizing
selection
in
this
generalist
species,
and
possibly
involved
small
genetic

changes
not
related
to
the
networks
producing
the
main
variation,
accumulating
together
with

genetic
divergence.
The
questions
opened
by
such
approaches
challenge
further
studies,
integrating

evolutionary,
ecological,
functional
and
developmental
investigations,
that
are
needed
to
develop
a

comprehensive
interpretation
of
the
processes
underlying
morphological
diversification
in
an
‘eco-­‐
evo-­‐devo’
perspective.

Acknowledgements