Donald L. Swiderski

Trade‐offs are inherent features of many biomechanical systems and are often seen as evolutionary constraints. Structural decoupling may provide a way to escape those limits in some systems but not for structures that transmit large forces, such as mammalian mandibles. For such structures to evolve in multiple directions on a complex adaptive landscape, different regions must change shape while maintaining structural integrity. We evaluated the complexity of the adaptive landscape for mandibular shape in Marmotini, a lineage of ground squirrels that varies in the proportions of seeds and foliage in their diets, by comparing the fit of models based on traits that predict changes in mandibular loading. The adaptive landscape was more complex than predicted by a two‐peak model with a single dietary shift. The large number of adaptive peaks reflects a high diversity of directions of shape evolution. The number of adaptive peaks also reflects a multiplicity of functional trade‐offs posed by the conflicting demands of processing foods with various combinations of material properties. The ability to balance trade‐offs for diets with different proportions of the same foods may account for diversification and disparity of lineages in heterogeneous environments. Rather than constraints, trade‐offs may be the impetus of evolutionary change.

The mouse mandible is a popular model system that continues to be the focus of studies in evo-devo and other fields. Yet, little attention has been given to the role of postnatal growth in producing the adult form. Using cleared and stained specimens, we describe the timing of tooth and jaw development and changes in jaw size and shape from postnatal day 1 (p1) through weaning to adulthood. We found that tooth development is relatively advanced at birth, and that the functional adult dentition is in place by p15 (just before the start of weaning). Shape analysis showed that the trajectory of mandible shape changes direction at least twice between birth and adulthood, at p7 and p15. At each stage there are changes in shape to all tooth- and muscle-bearing regions and, at each change of direction, all of these regions change their pattern of growth. The timing of the changes in direction in Mus suggests there are signals that redirect growth patterns independently of changes in function and loading associated with weaning and jaw muscle growth. A better understanding of these signals and how they produce a functionally integrated mandible may help explain the mechanisms guiding evolutionary trends and patterns of plasticity and may also provide valuable clues to therapeutic manipulation of growth to alleviate the consequences of trauma or disease.

The fundamentals of mechanics provide a strong basis for the hypothesis that allometry could be an important constraint on evolutionary changes in shape. Empirical evidence that allometry accounts for sufficient shape variation to be an effective constraint is much weaker. Numerous studies claim to show that most shape variation is correlated with size, but nearly all these studies use morphometric methods that confound size and shape. Consequently, the proportion of the shape variation correlated with size remains unknown. To address this issue, I examined ontogenetic change and adult variation in lower jaws of the fox squirrel, Sciurus niger. Jaw morphologies were quantified using traditional distance measurements and geometric shape variables computed from landmark coordinates. The same analyses were performed on each data set. As expected, analyses of the distance measurements found that allometry accounted for nearly all ontogenetic variation. The same analysis on the coordinate data found that allometry accounts for 50% of the variation in distance measurements but <25% of the variation in shape. These results confirm that analyses of distance measurements confound size and shape and that this can lead to erroneous claims about the importance of allometry.

H. David Sheets, Miriam L. Zelditch, Donald L. Swiderski, Gavin Naylor; Growth and Shape: Measurements and Metrics, Systematic Biology, Volume 51, Issue 5, 1 S

Reconstruction of phylogenetic relationships can be an arduous and prolonged task. Large numbers of specimens must be examined to produce even larger numbers of potentially informative traits. These descriptive data are scored and fed into phylogenetic analysis routines to determine which of the myriad possible phylogenies are supported by the data. Often, these analyses are repeated several times, exploring various options, in the hope that one of them will produce a reasonably well resolved and strongly supported tree.