Developmental Plasticity and Evolution. In: Mary Jane West-Eberhard, Editor, Oxford University Press (2003) Pp. xi+794. Price $50.00 paper.

Introduction to Experimental Statistics.

Convergent evolution can occur through similar or different evolutionary pathways, which are the sequences of trait changes that led to convergent phenotypic endpoints. These evolutionary pathways may differ, owing to historically contingent events during the evolution of each lineage, or can arise deterministically due to similar histories of selection or evolutionary constraints. Thus, the relative contribution of determinism and contingency to the evolutionary history of convergent clades affects the evolutionary pathway that each has taken. We tested for morphological convergence in body elongation and limb reduction and the evolutionary pathways that gave rise to them in two major clades of Lerista, a species-rich genus of semi-fossorial lizards endemic to Australia. Our analyses showed strong evidence that the two clades evolved deterministically: both clades shared multiple convergent trait optima and similar patterns of integration of the hind limbs. However, the analyses also showed evidence of historical contingency because not all trait optima were realized by both clades, front limbs were not similarly integrated, and the body parts related by linear or threshold relationships differed between clades. Our findings suggest convergence occurs through deterministic pathways that are nevertheless contingent on historical events, and may have functional and ecological implications for convergent organisms.

Convergent body shapes have evolved via deterministic and historically contingent pathways in Lerista lizards

The zygomatic arch is morphologically complex, providing a key interface between the viscerocranium and neurocranium. It also serves as an attachment site for masticatory muscles, thereby linking it to the feeding apparatus. Though morphological variation related to differential loading is well known for many craniomandibular elements, the adaptive osteogenic response of the zygomatic arch remains to be investigated. Here, experimental data are presented that address the naturalistic influence of masticatory loading on the postweaning development of the zygoma and other cranial elements. Given the similarity of bone-strain levels among the zygoma and maxillomandibular elements, a rabbit and pig model were used to test the hypothesis that variation in cortical bone formation and biomineralization along the zygomatic arch and masticatory structures are linked to increased stresses. It was also hypothesized that neurocranial structures would be minimally affected by varying loads. Rabbits and pigs were raised for 48 weeks and 8 weeks, respectively. In both experimental models, CT analyses indicated that elevated masticatory loading did not induce differences in cortical bone thickness of the zygomatic arch, though biomineralization was positively affected. Hypotheses were supported regarding bone formation for maxillomandibular and neurocranial elements. Varying osteogenic responses in the arch suggests that skeletal adaptation, and corresponding variation in performance, may reside differentially at one level of bony architecture. Thus, it is possible that phenotypic diversity in the mammalian zygoma is due more singularly to natural selection (vs. plasticity). These findings underscore the complexity of the zygomatic arch and, more generally, determinants of skull form. Anat Rec, 299:1646–1660, 2016. © 2016 Wiley Periodicals, Inc.

Betwixt and Between: Intracranial Perspective on Zygomatic Arch Plasticity and Function in Mammals.

Within-species skull shape variation of marsupial mammals is widely considered low and strongly size-dependent (allometric), possibly due to developmental constraints arising from the altricial birth of marsupials. However, species whose skulls are impacted by strong muscular stresses – particularly those produced through mastication of tough food items – may not display such intrinsic patterns very clearly because of the known plastic response of bone to muscle activity of the individual. In such cases, allometry may not dominate within-species shape variation, even if it is a driver of evolutionary shape divergence; ordination of shape in a geometric morphometric context through principal component analysis (PCA) should reveal main variation in areas under masticatory stress (incisor region/zygomatic arches/mandibular ramus); but this main variation should emerge from high individual variability and thus have low eigenvalues. We assessed the evidence for high individual variation through 3D geometric morphometric shape analysis of crania and mandibles of three species of grazing-specialized wombats, whose diet of tough grasses puts considerable strain on their masticatory system. As expected, we found little allometry and low Principal Component 1 (PC1) eigenvalues within crania and mandibles of all three species. Also as expected, the main variation was in the muzzle, zygomatic arches, and masticatory muscle attachments of the mandibular ramus. We then implemented a new test to ask if the landmark variation reflected on PC1 was reflected in individuals with opposite PC1 scores and with opposite shapes in Procrustes space. This showed that correspondence between individual and ordinated shape variation was limited, indicating high levels of individual variability in the masticatory apparatus. Our results are inconsistent with hypotheses that skull shape variation within marsupial species reflects a constraint pattern. Rather, they support suggestions that individual plasticity can be an important determinant of within-species shape variation in marsupials (and possibly other mammals) with high masticatory stresses, making it difficult to understand the degree to which intrinsic constraints act on shape variation at the within-species level. We conclude that studies that link micro- and macroevolutionary patterns of shape variation might benefit from a focus on species with low-impact mastication, such as carnivorous or frugivorous species.

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Individual variation of the masticatory system dominates 3D skull shape in the herbivory-adapted marsupial wombats

Intracranial and hierarchical perspective on dietary plasticity in mammals

Department of Applied Forensic Sciences, Mercyhurst University, Erie, PA 16546KEY WORDS directional asymmetry; antisymmetry; fluctuating asymmetry; asymmetryindicesABSTRACT Traditionally, the study of metric skele-tal asymmetry has relied largely on univariate analyses,utilizing ratio transformations when the goal is compar-ing asymmetries in skeletal elements or populations ofdissimilar dimensions. Under this approach, raw asym-metries are divided by a size marker, such as a bilateralaverage, in an attempt to produce size-free asymmetryindices. Henceforth, this will be referred to as“controlling for size” (see Smith: Curr Anthropol 46(2005) 249-273). Ratios obtained in this manner oftenrequire further transformations to interpret the meaningand sources of asymmetry. This model frequently ignoresthe fundamental assumption of ratios: the relationshipbetween the variables entered in the ratio must be iso-metric. Violations of this assumption can obscure exist-ing asymmetries and render spurious results. In thisstudy, we examined the performance of the classic indi-ces in detecting and portraying the asymmetry patternsin four human appendicular bones and explored poten-tial methodological alternatives. Examination of theratio model revealed that it does not fulﬁll its intendedgoals in the bones examined, as the numerator anddenominator are independent in all cases. The ratiosalso introduced strong biases in the comparisonsbetween different elements and variables, generatingspurious asymmetry patterns. Multivariate analysesstrongly suggest that any transformation to control foroverall size or variable range must be conducted before,rather than after, calculating the asymmetries. A combi-nation of exploratory multivariate techniques, such asPrincipal Components Analysis, and conﬁrmatory linearmethods, such as regression and analysis of covariance,appear as a promising and powerful alternative to theuse of ratios. Am J Phys Anthropol 000:000–000,2014.

Quantifying asymmetry: ratios and alternatives.

The effect of dietary properties on craniofacial form has been the focus of numerous functional studies, with increasingly more work dedicated to the importance of phenotypic plasticity. As bone is a dynamic tissue, morphological variation related to differential loading is well established for many masticatory structures. However, the adaptive osteogenic response of several cranial sites across multiple levels of bony organization remains to be investigated. Here, rabbits were obtained at weaning and raised for 48 weeks until adulthood in order to address the naturalistic influence of altered loading on the long-term development of masticatory and non-masticatory elements. Longitudinal data from micro-computed tomography (μCT) scans were used to test the hypothesis that variation in cortical bone formation and biomineralization in masticatory structures is linked to increased stresses during oral processing of mechanically challenging foods. It was also hypothesized that similar parameters for neurocranial structures would be minimally affected by varying loads as this area is characterized by low strains during mastication and reduced hard-tissue mechanosensitivity. Hypotheses were supported regarding bone formation for maxillomandibular and neurocranial elements, though biomineralization trends of masticatory structures did not mirror macroscale findings. Varying osteogenic responses in masticatory elements suggest that physiological adaptation, and corresponding variation in skeletal performance, may reside differentially at one level of bony architecture, potentially affecting the accuracy of behavioral and in silico reconstructions. Together, these findings underscore the complexity of bone adaptation and highlight functional and developmental variation in determinants of skull form.

Erin M. Franks

Papers

Betwixt and Between: Intracranial Perspective on Zygomatic Arch Plasticity and Function in Mammals.

Intracranial and hierarchical perspective on dietary plasticity in mammals

Quantifying asymmetry: ratios and alternatives.

Game of bones: intracranial and hierarchical perspective on dietary plasticity in mammals

Biting off more than you can chew: a regional assessment of diet-induced plasticity