Shape coordinates and centroid size for adults and ontogenetic series analyzed in predictable complexity of evolutionary allometry

Allometry has been a paradigm of constraints, including intrinsic
constraints on the evolvability of allometry, as a source of developmental
and genetic constraints on the evolution of form, and of functional
constraints, maintaining functional equivalence as body size evolves. Yet,
allometry may be the simplest case of varied constraints, and of
morphological integration, even though allometry itself is not simple.
Evolutionary allometry may be especially complex because it depends not
only on the developmental origins of allometry and determinants of
allometric variation but also on the evolutionary dynamics of size and
shape. It should also depend on the ecological opportunity for
size-dependent ecomorphological specialization. We predict that lineages
that converge in those would exhibit similar evolutionary allometries but
otherwise, evolutionary allometries would be heterogeneous. Countering
this expectation are familiar craniofacial evolutionary allometries, often
ascribed to developmental bias. To test both those hypotheses, we compare
evolutionary allometries of mandibles across lineages of squirrels and
evolutionary to growth allometries. As expected, lineages that converge on
size-dependent specializations exhibit similar evolutionary allometries,
but otherwise, their allometries are no more similar than expected by
chance. Growth allometries of squirrels (and a cricetid rodent) slightly
resemble the evolutionary allometry of one lineage, but growth allometries
of species from other lineages are orthogonal to their own lineages’
evolutionary allometry. We would expect that craniofacial allometries that
are not brain-driven would, like mandibular evolutionary allometries, be
predictable only from size-dependent ecological specializations.