Page 12 - Asylv_molars_Pmax_revised_2014_01

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Limited
role
of
allometry
in
molar
shape
evolution

Size
-­‐related
shape
changes
have
been
suggested,
and
indeed
demonstrated
as
a
line
of
least

resistance
to
evolution
in
bony
traits
such
as
skull
and
mandible
(Marroig
and
Cheverud
2005;
Cardini

and
Thorington
2006).
Such
traits
are
prone
to
change
along
growth
due
to
bone
remodeling,
making

of
intra-­‐population
allometry
a
significant
component
to
Pmax
(Renaud
and
Auffray
2013).
In

contrast,
murine
molar
teeth
are
determined
early
during
the
embryonic
development
and
are
only

affected
by
wear
after
eruption.
This
may
make
them
less
prone
to
allometric
variations.
Indeed,

allometric
directions
of
molar
shape
changes
were
not
congruent
between
intra-­‐
and
inter-­‐
population
levels,
were
not
parallel
to
Pmax,
and
did
not
contribute
significantly
to
any
of
the

evolutionary
shape
changes
documented
here.
This
undermines
the
role
of
allometry
as
a
significant

evolutionary
force
in
traits
such
as
murine
molars,
although
allometric
constraints
may
be
of
prime

importance
for
other
traits
such
as
skull
(e.g.,
Cardini
and
Polly
2013).

Divergence
between
phylogenetic
lineages
does
not
follow
Pmax

Evolution
on
the
continent
and
on
islands
may
follow
different
regimes,
due
to
very
different

conditions
regarding
population
size,
demography
and
degree
of
isolation.
On
the
continent,
large

effective
population
size
and
possibility
of
gene
flow
between
neighboring
populations
should
tend

to
buffer
variations
and
smooth
morphological
divergence.
Nevertheless,
morphological
differences

may
slowly
accumulate
proportional
to
genetic
divergence
between
isolated
lineages
(e.g.
Renaud
et

al.
2007).
In
agreement,
a
significant
molar
shape
differentiation
was
observed
between
the
Western

European
and
italo-­‐balkanic
lineages,
separated
since
more
than
1
million
year
(Michaux
et
al.
1998a;

2003).
A
further
differentiation
may
occur
within
the
Western
European
lineage,
with
clustering
of

the
Westernmost
French
populations
suggesting
a
phylogenetic
relatedness
that
challenges
further

studies
in
this
so
far
undersampled
area.
In
both
cases,
the
differentiation
between
these
groups
did

not
occur
parallel
to
Pmax.
Possibly,
stabilizing
selection
may
be
an
important
component
in
the

evolution
of
this
generalist
and
opportunist
rodent
(Renaud
et
al.
2005),
thus
constraining
evolution

along
Pmax.
Mutually
not
exclusive,
minor
changes
may
still
accumulate
along
other
directions

(Renaud
et
al.
2006).

Sicily
represents
in
this
context
a
peculiar
case,
since
this
island
houses
an
endemic
lineage
isolated

for
more
than
800
000
years
(Michaux
et
al.
1998b).
Ancient
and
intense
human
travels
might
have