Patterns Of Morphological Integration Research Articles

Functional morphological integration related to feeding biomechanics in the hominine skull

Quantifying and characterizing the pattern of trait covariances is crucial for understanding how population-level patterns of integration might constrain or facilitate craniofacial evolution related to the feeding system. This study addresses an important gap in our knowledge by investigating magnitudes and patterns of morphological integration of biomechanically informative traits in the skulls of Homo sapiens, Pan troglodytes, and Gorilla gorilla. We predicted a lower magnitude of integration among human biomechanical traits since humans eat a softer, less biomechanically challenging diet than apes. Indeed, compared to African apes, the magnitudes of integration were lower in H. sapiens skulls for form data (raw dimensions) but were similar or higher for shape data (raw dimensions scaled by geometric mean). Patterns of morphological integration were generally similar, but not identical, across the three species, particularly for the form data compared to the shape data. Traits that load heavily on the primary axis of variation in morphospace are generally associated with size and/or shape of the temporalis and masseter muscles and with dimensions related to the constrained lever model of jaw biomechanics. Given the conserved nature of morphological integration, skull adaptations for food processing in African apes and humans may have been constrained to occur along certain paths of high evolvability. The conserved pattern of functional integration also indicates that extant hominine species can operate as reasonable analogues for extinct hominins in studies that require population-level patterns of trait variance/covariance.

Digest: A new tool will illuminate patterns of morphological integration.

Morphological integration is the result of natural selection influencing the evolution of multiple morphological traits simultaneously. Understanding how and why morphological traits covary is fundamental to uncovering the processes that drive phenotypic diversity. In a new study, Conaway and Adams (2022) review the utility of several methods for quantifying morphological integration and provide a new metric that will facilitate broad, novel comparisons of morphological integration among taxa and between different morphological features.

Allometry and morphological integration shape the chemical detection system in Liolaemus lizards (Squamata, Iguania)

The chemosensory system in tetrapods consists of glands that emit chemical signals and specialized organs that capture these signals. Lizards mostly depend on this chemosensory modality and different relationships have been suggested among components of their chemical detection systems; the vomeronasal organ (VNO), palatal openings (PO) and tongue morphology (TM), as well as epidermal glands and their secretions. In this study, we used phylogenetic geometric morphometrics to examine shape variation in the VNO, PO and TM of 20 species of Liolaemus lizards. First, we tested whether phylogeny, allometry or diet type influenced their shape. Second, we tested an integration hypothesis between the shapes of the VNO, palatal opening and tongue, as well as, their association with the number of precloacal epidermal glands (PEG). Third, we tested the relationship between the evolutionary change rates of trait shape. Results showed a weak phylogenetic effect on shapes and an allometry effect on the shape of PO. The diet type did not affect the shape of these traits as well as, the number of precloacal epidermal glands. The tongue shape showed significant integration patterns with both the shape of VNO and PO. A faster evolutionary rate change was observed for PEG; VNO and PO showed similar medium rates and TM showed the slowest evolutionary rate. Our results suggest that allometry and different morphological integration patterns may affect the chemosensory system in studied species. The integration patterns among the shapes of the tongue, the VNO and the PO support the assumed association of the chemical detection system components in Liolaemus lizards.

Morphological integration and cranial modularity in six genera of echimyid rodents (Rodentia: Echimyidae)

Abstract Rodents of the family Echimyidae present a wide variety of life histories and ecomorphological adaptations. This study evaluated morphological integration patterns, modularity, and evolutionary flexibility in six Echimyid genera representing ecomorphological extremes within the family. The relationships between traits were evaluated by comparing estimated covariance and correlations matrices of populations. The presence of modules was investigated by comparing the patterns of integration between traits and using hypothetical matrices based on shared development/function and masticatory stress. The results point to a common covariance and correlation pattern among the six echimyid genera, suggesting a conserved pattern of covariation (associations among traits) throughout the evolution of this group. The overall magnitude of integration, however, varied greatly. We also found a high degree of modularity in all six echimyid genera. Finally, we observed a clear association between flexibility, i.e., the ability of a species to respond to the direction of selection, with the overall magnitude of integration and degree of modularization. The results of this study provide hypotheses concerning the underlying effects of the association among traits, which may have facilitated or constrained the evolution of morphological variation in the diverse family Echimyidae.

Morphological integration of the canine region within the hominine alveolar arch

The early hominin record is characterized by numerous shifts in dental proportions (e.g., canine reduction and megadontia) linked to changes in diet and social behavior. Recent studies suggest that hominins exhibit a reduction in the magnitude of covariation between the anterior and posterior dental components compared with other extant great apes. They point toward, but do not directly test, the relative independence of canine morphology within the hominin alveolar arch. This study focuses specifically on the how the canine region covaries with other regions of the dental arch because the canine region has drastically reduced in size and changed in shape across human evolution. We examine extant primate species most commonly used as a comparative framework for fossil hominin morphology: Gorilla gorilla (n = 27), Pan troglodytes (n = 27), and Homo sapiens (n = 30). We used geometric morphometric methods to test for size and shape covariation between the canine region with other dental regions. We also examined the influence of sexual dimorphism and allometry on intraspecific and interspecific patterns of covariation. The analysis of size and shape covariation between the mandibular canine and other individual tooth regions elucidated complex, species-specific, and sex-specific morphological relationships in the mandibular alveolar arch. There was little evidence to support different patterns of morphological integration between humans on the one hand and nonhuman apes on the other. Canine region morphology was relatively independent from other dental regions across species based on shape and did not significantly covary more with either the incisor or postcanine region in any species. The size correlations between the canine and other dental regions were moderate to high. The species-specific results of this study question the ability to make a priori assumptions about morphological integration in the extant hominin mandibular alveolar arch and its application to the fossil record.

Evolutionary Potential and Morphological Integration of the Shoulder Girdle Among Primates

The morphology of the shoulder girdle is highly varied among primates, possibly reflecting an array of functional demands placed through the range of locomotor and behavioral movements exercised in the upper limb. Variation in the shoulder girdle, and especially the scapula, is hypothesized to be the result of directional selection for different functional demands. Evolutionary analyses of the shoulder girdle often assess this anatomical region and its traits individually, which implicitly models the evolution of the girdle as independent of other traits. However, the primate shoulder girdle has developmental and functional covariances with the basicranium, pelvis, humerus, and vertebral column that we have recently demonstrated have potential to influence its evolution through correlated responses to directional selection. Whether these relationships are similar across primate taxa and how these relationships may affect morphological variation among primates is unknown.Here we evaluate the strength of the covariance and evolutionary potential among four anatomical regions: shoulder girdle, basicranium, pelvis, and humerus. We obtained cranial and postcranial linear dimensions from Alouatta, Colobus, Homo, Hylobates, and Papio. Measurements were mean standardized prior to analysis. We estimated morphological integration using Wagner's variance of the eigenvalues (VE), and estimated evolutionary potential using Hansen and Houle's evolutionary flexibility (EF) with 1000 simulated random selection gradients. EF captures how closely the evolutionary response in a group of traits follows the direction of the selection. We evaluated differences in VE and EF among taxa by performing a pairwise comparison of the bootstrapped values for each. Analyses were performed using the R statistical environment.Our results show that patterns of morphological integration and evolutionary flexibility among anatomical regions differ across primate taxa, with humans having the greatest evolutionary potential and Papio the lowest. Broadly, the patterns revealed among taxa do not strictly correspond to the functional demands of the upper limb experienced by each; taxa with different locomotor demands on the upper limb have the same kinds of covariance or evolutionary flexibility. Thus, evolutionary hypotheses of primate shoulder girdle morphological variation that evaluate functional demands alone may not account for the effect of these relationships. We conclude that different trait covariances among anatomical regions may manifest as morphological differences in the primate shoulder girdle, which has important implications for models of shoulder evolution.Support or Funding InformationThis research is supported by a National Science Foundation Doctoral Dissertation Improvement Grant (NSF BCS‐1825995).

Patterns of Morphological Integration of Slavic Loan Nouns in Petkevičius’ Catechism (1598) as an Indication of Their Origin and Chronology

Patterns of Morphological Integration of Slavic Loan Nouns in Petkevičius’ Catechism (1598) as an Indication of Their Origin and ChronologyThis article focuses on the morphological integration of Slavic loan nouns featuring in Merkelis Petkevičius’ Catechism (1598) into the Lithuanian language. It attempts to establish whether the pattern of adaptation of a Slavic loanword to a particular Lithuanian stem can suggest its more precise origin. In order to achieve this objective, I extracted all Slavic loan nouns from Petkevičius’ Catechism, identified their stems and meanings, and established their equivalents in Slavic languages of the relevant period (Old Russian, Ruthenian, Old and Middle Polish). Comparing this data made it possible to establish some common patterns of integration of Slavic loanwords into the morphological system of the Lithuanian language. A direct relationship was identified between the endings and gender of the Slavic words and the Lithuanian stems into which they were integrated. Therefore, in some cases the pattern of adaptation of a Slavic loanword can suggest its path into the Lithuanian language. Adaptacja morfologiczna słowiańskich zapożyczeń rzeczownikowych w katechizmie Pietkiewicza (1598) jako wskazówka na temat ich źródła i chronologiiNiniejszy artykuł omawia adaptację morfologiczną słowiańskich zapożyczeń rzeczownikowych w języku litewskim na podstawie materiału leksykalnego katechizmu Melchiora Pietkiewicza (1598). Podjęta analiza stanowi próbę ustalenia, czy ich dostosowanie do określonego rdzenia litewskiego może dokładniej wskazywać na źródło pochodzenia. W toku przeprowadzonych badań z katechizmu wyekscerpowano wszystkie rzeczowniki zapożyczone z języków słowiańskich, zidentyfikowano ich rdzenie i znaczenia oraz ustalono ich ekwiwalenty w językach słowiańskich w odpowiednim okresie (staroruskim, ruskim, staro- i średniopolskim). Porównanie tych danych pozwoliło uchwycić pewne prawidłowości adaptacji zapożyczeń słowiańskich do systemu morfologicznego języka litewskiego. Ustalono bezpośredni związek pomiędzy końcówkami i rodzajem gramatycznym leksemów słowiańskich a ich adaptacją do określonych rdzeni litewskich. W pewnych przypadkach wzorzec adaptacji może zatem wskazywać na źródło zapożyczenia słowiańskiego w języku litewskim.

Unravelling the hybrid vigor in domestic equids: the effect of hybridization on bone shape variation and covariation

BackgroundHybridization has been widely practiced in plant and animal breeding as a means to enhance the quality and fitness of the organisms. In domestic equids, this hybrid vigor takes the form of improved physical and physiological characteristics, notably for strength or endurance. Because the offspring of horse and donkey is generally sterile, this widely recognized vigor is expressed in the first generation (F1). However, in the absence of recombination between the two parental genomes, F1 hybrids can be expected to be phenotypically intermediate between their parents which could potentially restrict the possibilities of an increase in overall fitness. In this study, we examine the morphology of the main limb bones of domestic horses, donkeys and their hybrids to investigate the phenotypic impact of hybridization on the locomotor system. We explore bone shape variation and covariation to gain insights into the morphological and functional expressions of the hybrid vigor commonly described in domestic equids.ResultsOur data reveal the occurrence of transgressive effects on several bones in the F1 generation. The patterns of morphological integration further demonstrate that the developmental processes producing covariation are not disrupted by hybridization, contrary to functional ones.ConclusionsThese results suggest that an increase in overall fitness could be related to more flexibility in shape change in hybrids, except for the main forelimb long bones of which the morphology is strongly driven by muscle interactions. More broadly, this study illustrates the interest of investigating not only bone shape variation but also underlying processes, in order to contribute to better understanding how developmental and functional mechanisms are affected by hybridization.

Pharyngeal Jaws Converge by Similar Means, Not to Similar Ends, When Minnows (Cypriniformes: Leuciscidae) Adapt to New Dietary Niches.

Convergent evolution is at the forefront of many form-function studies. There are many examples of multiple independent lineages evolving a similar morphology in response to similar functional demands, providing a framework for testing hypotheses of form-function evolution. However, there are numerous clades with underappreciated convergence, in which there is a perceived homogeneity in morphology. In these groups, it can be difficult to investigate causal relationships of form and function (e.g., diet influencing the evolution of jaw morphology) without the ability to disentangle phylogenetic signal from convergence. Leuciscids (Cypriniformes: Leuciscidae; formerly nested within Cyprinidae) are a species-rich clade of fishes that have diversified to occupy nearly every freshwater trophic niche, yet are considered to have relatively low morphological diversity relative to other large freshwater clades. Within the North American leuciscids, many genera contain at least one herbivore, insectivore, and larvaphage. We created 3D models from micro-computed tomography scans of 165 leuciscid species to measure functionally relevant traits within the pharyngeal jaws of these fishes. Using a published phylogeny, we tested these metrics for evolutionary integration, phylogenetic signal, and correlation with diet. Measurements of the pharyngeal jaws, muscle attachment areas, and teeth showed strong positive evolutionary correlation with each other and negative evolutionary correlation with measurements of the inter-ceratobranchial ligament (ICB ligament). Using diet data from published literature, we found extensive dietary convergence within Leuciscidae. The most common transitions we found were between herbivorous and invertivorous taxa and between insectivore types (aquatic vs. terrestrial). We document a trade-off in which herbivorous leuciscids have large teeth, short ICB ligaments, and large muscle attachment areas, whereas insectivorous leuciscids showed the opposite pattern. Inverse patterns of morphological integration between the ICB ligament the rest of the pharyngeal jaw correspond this dietary trade-off, which indicates that coordinated evolution of morphological traits contributes to functional diversity in this clade. However, these patterns only emerge in the context of phylogeny, meaning that the pharyngeal jaws of North American leuciscids converge by similar means (structural changes in response to dietary demands), but not necessarily to similar ends (absolute phenotype).

Morphological integration in the gorilla, chimpanzee, and human neck.

Although integration studies are important to understand the evolution of organisms' traits across phylogenies, vertebral integration in primates is still largely unexplored. Here we describe and quantify patterns of morphological integration and modularity in the subaxial cervical vertebrae (C3-C7) in extant hominines incorporating the potential influence of size. Three-dimensional landmarks were digitized on 546 subaxial cervical vertebrae from 141 adult individuals of Gorilla gorilla, Pan troglodytes, and Homo sapiens. Integration and modularity, and the influence of size effects, were quantified using geometric morphometric approaches. All subaxial cervical vertebrae from the three species show a strong degree of integration. Gorillas show the highest degree of integration; conversely, humans have the lowest degree of integration. Analyses of allometric regression residuals show that size is an important factor promoting integration in gorillas, with lesser influence in chimpanzees and almost no effect in humans. Results point to a likely ancestral pattern of integration in non-human hominines, whereby the degree of integration decreases from cranial to caudal positions. Humans deviate from this pattern in the cranialmost (C3) and, to a lesser extent, in the caudalmost (C7) vertebrae, which are less integrated. These differences can be tentatively related to the emergence of bipedalism due to the presence of modern human-like C3 in australopiths, which still preserve a more chimpanzee-like C7.

Morphological Integration and Alternative Life History Strategies: A Case Study in a Facultatively Paedomorphic Newt.

Tetrapod limbs are serially homologous structures that represent a particularly interesting model for studies on morphological integration, i.e. the tendency of developmental systems to produce correlated variation. In newts, limbs develop at an early larval stage and grow continuously, including after the habitat transition from water to land following metamorphosis. However, aquatic and terrestrial environments impose different constraints and locomotor modes that could affect patterns of morphological integration and evolvability. We hypothesize that this would be the case for alternative heterochronic morphs in newts, i.e. aquatic paedomorphs that keep gills at the adult stage and adult metamorphs that are able to disperse on land. To this end, we analyzed patterns and strengths of correlations between homologous skeletal elements of the fore- and hindlimbs as well as among skeletal elements within limbs in both phenotypes in the alpine newt, Ichthyosaura alpestris. Our results showed that metamorphs and paedomorphs had similar, general patterns of limb integration. Partial correlations between homologous limb elements and within limb elements were higher in paedomorphs when compared to metamorphs. A decrease in partial correlation between homologous limb elements in metamorphs is accompanied with a higher evolvability of the terrestrial morph. All these results indicate that environmental demands shaped the patterns of morphological integration of alpine newt limbs and that the observed diversity in correlation structure could be related to a qualitative difference in the modes of locomotion between the morphs.

Patterns of morphological integration between parietal and temporal areas in the human skull.

Modern humans have evolved bulging parietal areas and large, projecting temporal lobes. Both changes, largely due to a longitudinal expansion of these cranial and cerebral elements, were hypothesized to be the result of brain evolution and cognitive variations. Nonetheless, the independence of these two morphological characters has not been evaluated. Because of structural and functional integration among cranial elements, changes in the position of the temporal poles can be a secondary consequence of parietal bulging and reorientation of the head axis. In this study, we use geometric morphometrics to test the correlation between parietal shape and the morphology of the endocranial base in a sample of adult modern humans. Our results suggest that parietal proportions show no correlation with the relative position of the temporal poles within the spatial organization of the endocranial base. The vault and endocranial base are likely to be involved in distinct morphogenetic processes, with scarce or no integration between these two districts. Therefore, the current evidence rejects the hypothesis of reciprocal morphological influences between parietal and temporal morphology, suggesting that evolutionary spatial changes in these two areas may have been independent. However, parietal bulging exerts a visible effect on the rotation of the cranial base, influencing head position and orientation. This change can have had a major relevance in the reorganization of the head functional axis.

Patterns of morphological integration in the dental arches of individuals with malocclusion.

In humans, there is a large range of variation in the form of the maxillary and mandibular dental arches. This variation can manifest as either prognathism or retrognathism in either or both arches, which can cause malocclusion and lead to abnormal masticatory function. This study aims to identify aspects of variation and morphological integration existing in the dental arches of individuals with different types of malocclusion. Coordinate landmark data were collected along the gingival margins of 397 scanned dental casts and then analyzed using geometric morphometric techniques to explore arch form variation and patterns of morphological integration within each malocclusion type. Significant differences were identified between Class II forms (increased projection of upper arch relative to the lower arch) and Class III forms (lower arch projection beyond the upper arch) in symmetrical shape variation, including anteroposterior arch discrepancies and abnormal anterior arch divergence or convergence. Partial least squares analysis demonstrated that Class III dental arches have higher levels of covariance between upper and lower arches (RV = 0.91) compared to the dental arches of Class II (RV = 0.78) and Class I (RV = 0.73). These high levels of covariance, however, are on the lower end of the overall range of possible masticatory blocks, indicating weaker than expected levels of integration. This study provides evidence for patterns of variation in dental arch shape found in individuals with Class II and Class III malocclusions. Moreover, differences in integration found between malocclusion types have ramifications for how such conditions should be studied and treated. Am. J. Hum. Biol. 28:879-889, 2016. © 2016Wiley Periodicals, Inc.

Morphological integration and changes in pelvic dimensions with age in adult female humans

This study examines morphological integration (MI) in the human pelvis with respect to sex and age. Anatomists and biological anthropologists have often cited the flair of the iliac blades, sub‐pubic angle, and breadth between the ischia among the morphologies that distinguish females and males, though the magnitude of difference between the sexes varies among groups. Despite these differences, it is assumed that patterns of MI within the human pelvis are the same between the sexes; the few studies of evolvability and integration in the pelvis to date remove effects of sexual dimorphism on the variance‐covariance matrix. In addition, the age of adult individuals is never considered a factor in quantitative genetic studies of the evolution of the pelvis, as the pelvis is thought to reach its definitive morphology with the cessation of primary growth. However, studies by Tague (1994) and Auerbach et al. (in review) show that older adult females have mediolaterally wider pelvic outlets (MLPO) and anteroposteriorly deeper pelvic inlets (APPI), while no age‐related differences in pelvic dimensions are observed among males. Though covariation within the pelvis should be similar between the sexes and among age groups within the sexes, no assessment of overall integration within the pelvis has been compared in light of these factors.In this study, 18 linear dimensions were measured from the rearticulated pelves of 327 adult human skeletons (188 females, 139 males) recovered from archaeological sites in North America dating from the last millennium. Fusion of the iliac crest was the criterion for inclusion. We aged the skeletons based on diagnostic changes in pelvic articular morphology, and placed the skeletons into two groups: “Young” (<~25 years) and “Not Young” (>~25 years). MI was measured by comparing relative eigenvalue variances between sex and age‐and‐sex groups, following Pavličev et al. (2009. Evol Biol 36:157–170), as well as by comparing mean‐scaled covariance matrices of the traits within each group. Relative eigenvalue variances have a scale of zero to one and higher values indicate more integration.Results indicate that MI differs notably between age groups within sex, but not between the sexes. Both females and males have similar relative eigenvalues (females = 0.162; males = 0.172). The slightly lower value for females may be driven by the influence of the Young female group, which has a relative eigenvalue variance of 0.139. In contrast, the Not Young female group has relative eigenvalue variance of 0.202. This is a stark difference, and indicates that younger females have less integration than older females. Examination of the covariance matrices indicates lower covariances among most traits, but especially between MLPO and APPI. This indicates that covariation of traits within the pelvis continue to change in adults, and older females have more integrated pelves. Further developmental and evolutionary implications are considered.Support or Funding InformationNational Science Foundation, BCS Grant #0962752

Modularity and cranial integration across ontogenetic stages in Martino’s vole, Dinaromys bogdanovi

We explored modularity and morphological integration of the ventral cranium during postnatal ontogeny in Martino’s vole (Dinaromys bogdanovi). Two closely related phylogenetic groups, originating from the Central and Southeastern part of the species range in the western Balkans, were considered. As expected, both phylogroups had similar patterns of ontogenetic changes in cranial size and shape variation, modularity and integration. At the level of within individual variation, the hypothesis that the viscerocranial and neurocranial regions are separate modules was rejected, indicating that the hypothesized modules are not developmental, but rather functional. At the level of among individual variation, the viscerocranium and the neurocranium could not be recognized as separate modules at the juvenile stage. The strength of association between the hypothesized modules becomes lower with age which finally results in a clear 2-module organization of the ventral cranium at the adult stage. On the other hand, patterns of morphological integration for the cranium as a whole, the viscerocranium and the neurocranium stay consistent across ontogenetic stages. The developmental mechanism producing integration of the cranium as a whole, as well as integration of the neurocranium, varies throughout postnatal ontogeny. In contrast, we detected the ontogenetic stability of the mechanism responsible for covariation of viscerocranial traits which could provide ongoing flexibility of the viscerocranial covariance structure for high functional demands during lifetime. Findings from our study most likely support the idea of the ‘palimpsest-like’ model of covariance structure. Moreover, similarity or dissimilarity in the patterns of within and among individual variation in different sets of analyzed traits and comparisons across ontogenetic stages demonstrate how studies on small mammals other than mice can give new insights into postnatal cranial development.

Cranial base topology and basic trends in the facial evolution of Homo

Facial prognathism and projection are important characteristics in human evolution but their three-dimensional (3D) architectonic relationships to basicranial morphology are not clear. We used geometric morphometrics and measured 51 3D-landmarks in a comparative sample of modern humans (N = 78) and fossil Pleistocene hominins (N = 10) to investigate the spatial features of covariation between basicranial and facial elements. The study reveals complex morphological integration patterns in craniofacial evolution of Middle and Late Pleistocene hominins. A downwards-orientated cranial base correlates with alveolar maxillary prognathism, relatively larger faces, and relatively larger distances between the anterior cranial base and the frontal bone (projection). This upper facial projection correlates with increased overall relative size of the maxillary alveolar process. Vertical facial height is associated with tall nasal cavities and is accommodated by an elevated anterior cranial base, possibly because of relations between the cribriform and the nasal cavity in relation to body size and energetics. Variation in upper- and mid-facial projection can further be produced by basicranial topology in which the midline base and nasal cavity are shifted anteriorly relative to retracted lateral parts of the base and the face. The zygomatics and the middle cranial fossae act together as bilateral vertical systems that are either projected or retracted relative to the midline facial elements, causing either midfacial flatness or midfacial projection correspondingly. We propose that facial flatness and facial projection reflect classical principles of craniofacial growth counterparts, while facial orientation relative to the basicranium as well as facial proportions reflect the complex interplay of head-body integration in the light of encephalization and body size decrease in Middle to Late Pleistocene hominin evolution. Developmental and evolutionary patterns of integration may only partially overlap morphologically, and traditional concepts taken from research on two-dimensional (2D) lateral X-rays and sections have led to oversimplified and overly mechanistic models of basicranial evolution.

Shifts in subsistence type and its impact on the human skull's morphological integration.

Here we evaluate morphological integration patterns and magnitudes in different skull regions to detect if shifts in morphological integration are correlated to the appearance of more processed (softer) diets. To do so, three transitional populations were analyzed, including samples from groups that inhabited the same geographical region and for which the evidence shows that major changes occurred in their subsistence mode. Ninety three-dimensional landmarks were digitized on 357 skulls and used as the raw data to develop geometric morphometric analyses. The landmark coordinates were divided into several different regions of biomechanical interest, following a three-level hierarchically nested scheme: the whole skull, further subdivided into neurocranium (divided into the vault and basicranium), the facial (divided into the lower and upper facial), and the masticatory apparatus (divided into alveolar, temporal, and temporo-mandibular joint). Our results indicate that the morphological integration and variability patterns significantly vary across skull regions but are maintained across the transitions. The alveolar border and the lower facial are the regions manifesting greater value of morphological integration and variability, while the upper facial, the temporo-mandibular joint, and the basicranium are highly integrated and poorly variable. The transition to softer diets increased morphological variation across cranial regions that are more exposed to masticatory strains effects.

Head, Body and Fins: Patterns of Morphological Integration and Modularity in Fishes

Actinopterygians demonstrate high levels of morphological disparity, especially in the variation of fin positions, sizes and shapes. One hypothesis to explain the diversity of fin morphologies is that it is facilitated by a modular organization. According to this hypothesis, fin modules would be quasi-independent during ontogeny or evolution, facilitating their evolvability. We investigated variational modularity of fins in two cyprinid species, the zebrafish (Danio rerio) and the Northern redbelly dace (Chrosomus eos), to determine which subsets of fins are quasi-independent and which are most highly integrated in positioning. Hypotheses of modularity were evaluated using a combination of methods suitable for analyses of landmarks. The hypothesis that the dorsal and anal fins belong to a posterior trunk and tail module is strongly supported, a finding that can be explained by the use of subcarangiform locomotion in these two species. There is also some support for the hypothesis that the paired fins and head region each constitute variational modules. The support for fin variational modules is weaker than expected considering the wealth of developmental evidence supporting fin modularity. This might be related to a dissociation of the fin positioning modules during actinopterygian evolution, a process that had already been suggested for the dorsal and anal fins. Alternatively, the fin modules inferred from developmental data might not directly translate into variational modules: variational modules can incorporate the signals from numerous partially overlapping developmental processes so that one to one correspondence between developmental and variational modules is not always expected.

Patterns of morphological integration in the appendicular skeleton of mammalian carnivores.

We investigated patterns of evolutionary integration in the appendicular skeleton of mammalian carnivores. The findings are discussed in relation to performance selection in terms of organismal function as a potential mechanism underlying integration. Interspecific shape covariation was quantified by two-block partial least-squares (2B-PLS) analysis of 3D landmark data within a phylogenetic context. Specifically, we compared pairs of anatomically connected bones (within-limbs) and pairs of both serially homologous and functional equivalent bones (between-limbs). The statistical results of all the comparisons suggest that the carnivoran appendicular skeleton is highly integrated. Strikingly, the main shape covariation relates to bone robustness in all cases. A bootstrap test was used to compare the degree of integration between specialized cursorial taxa (i.e., those whose forelimbs are primarily involved in locomotion) and noncursorial species (i.e., those whose forelimbs are involved in more functions than their hindlimb) showed that cursors have a more integrated appendicular skeleton than noncursors. The findings demonstrate that natural selection can influence the pattern and degree of morphological integration by increasing the degree of bone shape covariation in parallel to ecological specialization.

The Evolution of Morphological Integration in the Ruminant Skull

Patterns of morphological integration have the potential to influence evolutionary trajectories. However, this influence depends on the stability of the integration pattern relative to the rate of evolutionary change. Studying the evolution of integration over large phylogenetic scales is complicated by its multivariate nature and the need to have both large sample sizes and a comprehensive taxon coverage. As a result, the question of how integration evolves over long time scales is still poorly understood. In this study I examined the evolution of integration across the phylogeny of extant ruminants, as reflected in their within-population covariation structure. I analyzed interlandmark distances from 2,054 skulls, representing 47 out of the 200 extant species of ruminants, including all major subfamilies of bovids and cervids. I estimated the within-population covariance matrix for each species, and compared them using multidimensional scaling and phylogenetic comparative methods. Results show that closely-related taxa differ substantially from each other in their integration pattern. However, the differences among higher-level clades still reflect their history of common descent. Differences between bovids and cervids involve mainly the oral and nasal regions, in accord with their different feeding and locomotion adaptations. Thus, the effect of both natural selection and phylogenetic history can be detected in the ruminant skull even though integration varies considerably among closely-related species.