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random
processes
such
as
founder
effects
and
drift.
Hence,
directions
of
main
variance
have
been

proposed
to
constitute
‘lines
of
least
resistance
to
evolution’
(Schluter
1996).

The
concept
was
first
developed
for
the
direction
of
main
genetic
variance
(Gmax)
which
is
the

principal
vector
of
the
genetic
variance-­‐covariance
(VCV)
matrix
(G
matrix).
However,
considering
the

phenotypic
variance
(P
matrix
and
its
main
direction
Pmax)
presents
several
advantages.
Phenotypic

variance
can
be
assessed
from
any
set
of
morphological
measurements
on
a
sample
of
specimens,

whereas
estimating
the
G
matrix
requires
known
genealogies
(Steppan
et
al.
2002).
Pmax
can
thus
be

studied
in
wild
populations
(e.g.
Ackermann
and
Cheverud
2000;
Marroig
and
Cheverud
2005)
and

even
in
fossils
(e.g.
Renaud
et
al.
2006;
Hunt
2007).
G
matrix
and
P
matrix
share
most
of
their

directions
of
variance
(Siahsarvie
2012),
suggesting
that
the
latter
can
be
confidently
used
as
a

surrogate
for
the
former.
Furthermore,
compared
to
the
G
matrix,
considering
the
P
matrix

incorporates
developmental
and
environmental
component
of
the
phenotypic
variation.

The
direction
of
main
phenotypic
variance,
Pmax,
corresponds
to
a
vector
pointing
in
the
direction
of

the
most
widespread
variation.
A
first
question
is
to
know
if
this
direction
is
conserved
across

populations
and
even
species,
a
hint
that
the
underlying
genetic
/
developmental
networks
would
be

conserved
along
evolutionary
time
scales
(Ackermann
and
Cheverud
2000;
Bégin
and
Roff
2003;

Renaud
et
al.
2006;
Renaud
and
Auffray
2013).
However,
the
structure
of
the
variance
itself
can

evolve
due
to
selection
or
drift
(Roff
and
Mousseau
2005).
A
second
question
is
to
know
if
the
pre-­‐
existing
variance
actually
tends
to
favor
evolution
into
this
direction.
This
can
be
addressed
by

comparing
the
direction
of
Pmax
to
evolutionary
directions
among
populations
or
species
(Marroig

and
Cheverud
2005,
2010;
Renaud
et
al.
2006;
Hunt
2007).

The
aim
of
the
present
contribution
is
to
illustrate
the
study
of
Pmax
and
its
use
for
deciphering

processes
driving
morphological
diversification
on
the
model
of
the
European
wood
mouse

(Apodemus
sylvaticus).
The
first
upper
molar
(Fig.
1)
was
selected
as
trait
of
interest
because
its

evolution
among
modern
populations
and
along
fossil
lineages
of
murine
rodents
has
been

evidenced
to
match
such
a
line
of
least
resistance
to
evolution,
matching
an
overall
conserved

direction
of
main
intra-­‐population
variation
(Renaud
et
al.
2006;
Renaud
and
Auffray
2013).
This

direction
of
shape
change
tends
to
oppose
broad
vs.
elongated
molars
(e.g.
Fig.
1).

Intricate
patterns
of
differentiation
have
been
previously
evidenced
in
the
wood
mouse.
Molar
shape

differences
have
been
found
among
phylogenetic
lineages
(Renaud
and
Michaux
2007)
inherited

from
repeated
isolations
in
separate
refuges
during
glacial-­‐interglacial
cycles
(Michaux
et
al.
2003).

Interfering
with
this
differentiation
between
isolated
populations,
a
morphological
cline
was