Ecogeographic variation in human nasal passages

Theoretically, individuals whose ancestors evolved in cold and/or dry climates should have greater nasal mucosal surface area relative to air volume of the nasal passages than individuals whose ancestors evolved in warm, humid climates. A high surface-area-to-volume (SA/V) ratio allows relatively more air to come in contact with the mucosa and facilitates more efficient heat and moisture exchange during inspiration and expiration, which would be adaptive in a cold, dry environment. Conversely, a low SA/V ratio is not as efficient at recapturing heat and moisture during expiration and allows for better heat dissipation, which would be adaptive in a warm, humid environment. To test this hypothesis, cross-sectional measurements of the nasal passages that reflect surface area and volume were collected from a sample of CT scans of patients of European and African ancestry. Results indicate that individuals of European descent do have higher SA/V ratios than individuals of African descent, but only when decongested. Otherwise, the two groups show little difference. This pattern of variation may be due to selection for different SA/V configurations during times of physical exertion, which has been shown to elicit decongestion. Relationships between linear measurements of the skeletal nasal aperture and cavity and cross-sectional dimensions were also examined. Contrary to predictions, the nasal index, the ratio of nasal breadth to nasal height, is not strongly correlated with internal dimensions. However, differences between the nasal indices of the two groups are highly significant. These results may be indicative of different adaptive solutions to the same problem.     

Genetic, geographic, and environmental correlates of human temporal bone variation

Temporal bone shape has been shown to reflect molecular phylogenetic relationships among hominoids and offers significant morphological detail for distinguishing taxa. Although it is generally accepted that temporal bone shape, like other aspects of morphology, has an underlying genetic component, the relative influence of genetic and environmental factors is unclear. To determine the impact of genetic differentiation and environmental variation on temporal bone morphology, we used three-dimensional geometric morphometric techniques to evaluate temporal bone variation in 11 modern human populations. Population differences were investigated by discriminant function analysis, and the strength of the relationships between morphology, neutral molecular distance, geographic distribution, and environmental variables were assessed by matrix correlation comparisons. Significant differences were found in temporal bone shape among all populations, and classification rates using cross-validation were relatively high. Comparisons of morphological distances to molecular distances based on short tandem repeats (STRs) revealed a significant correlation between temporal bone shape and neutral molecular distance among Old World populations, but not when Native Americans were included. Further analyses suggested a similar pattern for morphological variation and geographic distribution. No significant correlations were found between temporal bone shape and environmental variables: temperature, annual rainfall, latitude, or altitude. Significant correlations were found between temporal bone size and both temperature and latitude, presumably reflecting Bergmann's rule. Thus, temporal bone morphology appears to partially follow an isolation by distance model of evolution among human populations, although levels of correlation show that a substantial component of variation is unexplained by factors considered here.     

Spatial patterns of variation in color and spine shape in the sea urchin Heliocidaris erythrogramma

Abstract. Here we report on the first quantitative survey of morphological variation in the sea urchin Heliocidaris erythrogramma within Western Australia and distinguish between two subspecies found to co‐occur in this region. We surveyed urchins at multiple spatial scales along the Western Australian coastline to assess variation in dermis and spine color and, using landmark‐based geometric morphometrics, spine morphology. Both color and morphology proved to be useful for separating subspecies within Western Australia. There were four major color morphs: red dermis/violet spines (56%), red/violet‐green (23%), red/green (7%), and white/green (10%). Members of the first two color morphs had bulbous spines with wide, flattened tips, a morphology that is unique to Western Australia and characteristic of H. e. armigera, and members of the latter two consistently exhibited the narrow, pointed spines typical of specimens of H. e. erythrogramma, which has a broader distribution. In Western Australia, H. e. armigera was relatively abundant both within and among sites, but H. e. erythrogramma was found only in a few localized patches. Shifts in the relative abundance of these two subspecies occurred at fine spatial scales (<5 km), although environmental correlates of these transitions were unclear. Contrary to expectations, neither dermis color nor spine morphology varied with relative wave exposure: individuals with a red dermis or thickened spine morphology occurred at most sites regardless of exposure, and while white dermis and thinner spines only occurred at high‐exposure sites, these features were not common across the majority of exposed sites. Both color morph frequencies and spine morphology remained stable within sites over the 3‐year duration of this study. While the ecological significance of this morphological variation remains unclear, the consistency of the association between color and spine morphology, occurring across fine spatial scales, suggests that strong environmental or genetic factors are involved in maintaining morphological differentiation between these two subspecies.     

Ontogenetic variation in the mandibular ramus of great apes and humans

Considerable variation exists in mandibular ramus form among primates, particularly great apes and humans. Recent analyses of adult ramal morphology have suggested that features on the ramus, especially the coronoid process and sigmoid notch, can be treated as phylogenetic characters that can be used to reconstruct relationships among great ape and fossil hominin taxa. Others have contended that ramal morphology is more influenced by function than phylogeny. In addition, it remains unclear how ontogeny of the ramus contributes to adult variation in great apes and humans. Specifically, it is unclear whether differences among adults appear early and are maintained throughout ontogeny, or if these differences appear, or are enhanced, during later development. To address these questions, the present study examined a broad ontogenetic sample of great apes and humans using two‐dimensional geometric morphometric analysis. Variation within and among species was summarized using principal component and thin plate spline analyses, and Procrustes distances and discriminant function analyses were used to statistically compare species and age classes. Results suggest that morphological differences among species in ramal morphology appear early in ontogeny and persist into adulthood. Morphological differences among adults are particularly pronounced in the height and angulation of the coronoid process, the depth and anteroposterior length of the sigmoid notch, and the inclination of the ramus. In all taxa, the ascending ramus of the youngest specimens is more posteriorly inclined in relation to the occlusal plane, shifting to become more upright in adults. These results suggest that, although there are likely functional influences over the form of the coronoid process and ramus, the morphology of this region can be profitably used to differentiate among great apes, modern humans, and fossil hominid taxa. J. Morphol. 275:661–677, 2014. © 2013 Wiley Periodicals, Inc.     

Morphological convergence of shell shape in distantly related scallop species (Mollusca: Pectinidae)

Morphological convergence is a central concept in evolutionary biology, but convergent patterns remain under‐studied in nonvertebrate organisms. Some scallop species exhibit long‐distance swimming, a behaviour whose biomechanical requirements probably generate similar selective regimes. We tested the hypothesis that shell shape similarity in long‐distance swimming species is a result of convergent evolution. Using landmark‐based geometric morphometrics, we quantified shell shape in seven species representing major behavioural habits. All species displayed distinct shell shapes, with the exception of the two long‐distance swimmers, whose shells were indistinguishable. These species also displayed reduced morphological variance relative to other taxa. Finally, a phylogenetic simulation revealed that these species were more similar in their shell shape than was expected under Brownian motion, the model of character evolution that best described changes in shell shape. Together, these findings reveal that convergent evolution of shell shape occurs in scallops, and suggest that selection for shell shape and behaviour may be important in the diversification of the group. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 163 , 571–584.     

Morphological variation of the thoracolumbar vertebrae in Macropodidae and its functional relevance

The present study was designed to investigate how the form of the marsupial thoracolumbar vertebrae varies to cope with the particular demands of diverse loading and locomotor behaviors. The vertebral columns of 10 species of Macropodidae, with various body masses and modes of locomotion, together with two other arboreal marsupials, koala and cuscus, were selected. Seventy-four three-dimensional landmark coordinates were acquired on each of the 10 last presacral vertebrae of the 70 vertebral columns. The interspecific variations of the third lumbar vertebra (L3, which approximates the mean) and the transitional patterns of the thoracolumbar segments were examined using the combined approach of generalized Procrustes analysis (GPA) and principal components analysis (PCA). The results of analyses of an individual vertebra (L3) and of the transitional patterns indicate significant interspecific differences. In the L3 study the first PC shows allometric shape variation, while the second PC seems to relate to adaptation for terrestrial versus arboreal locomotion. When the L3 vertebrae of the common spotted cuscus and koala are included for comparison, the vertebra of the tree kangaroo occupies an intermediate position between the hopping kangaroo and these arboreal marsupials. The L3 vertebrae in the arboreal marsupials possess a distinct dorsoventrally expanded vertebral body, and perpendicularly orientated spinous and transverse processes. The results of the present study suggest that vertebral shape in the kangaroo and wallaroos provides a structural adaptation to hopping through a relatively enlarged loading area and powerful lever system. In contrast, the small-sized bettongs (or rat kangaroos) have a relatively flexible column and elongated levers for the action of back muscles that extend and laterally flex the spine. The complex pattern of vertebral shape transition in the last 10 presacral vertebrae was examined using PCAs that compare between species information about vertebral shape variation along the thoracolumbar column. The results reinforce and emphasize important aspects of the patterns of variation seen in the detailed analysis of the third lumbar vertebra. The results also imply that size, spinal loading pattern, and locomotor behavior exert an influence on shaping the vertebra. Further, the morphological adaptations are consistent within these marsupials and this opens up the possibility that this kind of analysis may be useful in making functional inferences from fossil material.     

Host‐plant dependent wing phenotypic variation in the neotropical butterfly Heliconius erato

Most phytophagous insects feed on a single plant during development, and this may influence not only performance‐linked traits, but also more subtle morphological differences. Insect–plant interactions are thus valuable for studying environmental influences on phenotypes. By using geometric morphometrics, we investigated the variation in forewing size and shape in the butterfly Heliconius erato phyllis reared on six species of passion vines (Passiflora spp.). We detected wing shape sexual dimorphism, for which the adaptive significance deserves further investigation. There was size as well as wing shape variation among individuals fed on different hosts. These subtle differences in shape were interpreted as environmental effects on development, which should be under weak natural selection for these traits, and therefore not strongly canalized. This result reinforces the role of plasticity on host‐plant use, as well as the corresponding consequences on developmental variability among phytophagous insects. We propose that this variation can be an important factor in resource specialization and partner recognition, possibly triggering reproductive isolation and sympatric speciation in phytophagous insects. This interaction also shows itself as a good model for studying the role of environmental and interaction diversity in evolution. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 765–774.     

Variation in morphology and functional performance across distinct evolutionary lineages of the Moorish gecko (Tarentola mauritanica) from the Iberian Peninsula

Deciphering the mechanisms that underlie morphological and functional diversity is essential for understanding how organisms adapt to their environment. Interestingly, phenotypic divergence does not necessarily correspond to the geographic and genetic separation between populations. Here, we explored the morphological and functional divergence among populations of two genetically differentiated clades of the Moorish gecko, Tarentola mauritanica. We used linear and geometric morphometrics to quantify morphological variation and investigated how it translates into biting and CLIMBING PERFORMANCE, to better understand the mechanisms potentially underlying population and lineage divergence. We found marked morphological differences between clades, both in body size and head shape. However, much of this differentiation is more strongly related to local variation between populations of the same clade, suggesting that recent ecological events may be more influential than deep evolutionary history in shaping diversity patterns in this group. Despite a lack of association between morphology and functional diversification in the locomotor system of the Moorish gecko, straightforward links are observed between head morphology and biting performance, providing more hints on the possible underlying causes. Indeed, variation in bite force is mostly determined by size variation and sexual dimorphism, and differences between the two clades concern how sexual variation is expressed, reinforcing the idea that both social and ecological factors contribute in shaping differentiation. Interestingly, the individuals from the islets off the coast of Murcia exhibit particular morphological and functional traits, which suggests that the ecological conditions related to insularity may drive the phenotypic differentiation of this population.     

Pectoral girdle and forelimb variation in extant Crocodylia: the coracoid–humerus pair as an evolutionary module

To date, all statements about evolutionary morphological transformation in Crocodylia have essentially been based on qualitative observations. In the present study, we assessed the morphological variation and covariation (integration) between the scapula, coracoid, humerus, radius, and ulna of 15 species of Crocodylidae, Alligatoridae, and Gavialis + Tomistoma using three‐dimensional geometric morphometrics. The results obtained reveal that the variation of elements within species (intraspecific) is large. However, despite this variability, variation across species (interspecific) is mainly concentrated in two dimensions where the disparity is constrained: ‘robusticity’ and ‘twist’ (forelimbs) and ‘robusticity’ and ‘flexion’ (pectoral girdle). Robusticity (first dimension of variation) embodies a set of correlated geometrical features such as the broadening of the girdle heads and blades, or the enlargement of proximal and distal bone ends. The twist is related to the proximal and/or distal epiphyses in the forelimb elements, and flexion of the scapula and coracoid blades comprises the second dimension of variation. In all crocodylians, forelimb integration is characterized by the strong correlations of a humerus–ulna–radius triad and by a radius–ulna pair, thus forming a tight forelimb module. Unexpectedly, we found that the humerus and coracoid form the most integrated pair, whereas the scapula is a more variable and relatively independent element. The integration pattern of the humerus–coracoid pair distinguishes a relatively robust configuration in alligatorids from that of the remainder groups. The patterns of variation and integration shared by all the analyzed species have been interpreted as an inherited factor, suggesting that developmental and functional requirements would have interacted in the acquisition of a semi‐aquatic and versatile locomotion at the Crocodylia node at least 65 Mya. Our findings highlight the need to incorporate the humerus–coracoid pair in biodynamic and biomechanical studies. © 2012 The Linnean Society of London     

Body shape differences in a pair of closely related Malawi cichlids and their hybrids: Effects of genetic variation,phenotypic plasticity,and transgressive segregation

Phenotypic differences may have genetic and plastic components. Here, we investigated the contributions of both for differences in body shape in two species of Lake Malawi cichlids using wild‐caught specimens and a common garden experiment. We further hybridized the two species to investigate the mode of gene action influencing body shape differences and to examine the potential for transgressive segregation. We found that body shape differences between the two species observed in the field are maintained after more than 10 generations in a standardized environment. Nonetheless, both species experienced similar changes in the laboratory environment. Our hybrid cross experiment confirmed that substantial variation in body shape appears to be genetically determined. The data further suggest that the underlying mode of gene action is complex and cannot be explained by simple additive or additive‐dominance models. Transgressive phenotypes were found in the hybrid generations, as hybrids occupied significantly more morphospace than both parentals combined. Further, the body shapes of transgressive individuals resemble the body shapes observed in other Lake Malawi rock‐dwelling genera. Our findings indicate that body shape can respond to selection immediately, through plasticity, and over longer timescales through adaptation. In addition, our results suggest that hybridization may have played an important role in the diversification of Lake Malawi cichlids through creating new phenotypic variation.     

The role of head shape and trophic variation in the diversification of the genus Herichthys in sympatry and allopatry

In the present study we evaluated the putative cases of sympatric speciation in the genus Herichthys by studying the variation in head shape using principal component analysis, phylomorphospace and reconstructions of the ancestral states of feeding preferences. Herichthys includes both allopatric and sympatric sister species, as well as sympatric unrelated species and thus offers great potential for evolutionary studies of putatively sympatric speciation. Herichthys is the northernmost group of cichlids in America and one of the most ecologically disparate genera within Middle American cichlids. Fifteen anatomical points were recorded on the heads of 293 specimens of the 11 species recognized within the genus. The results show that in spite of having wide variation in consumed diets, most species of Herichthys are close in morphospace. However, morphological variation was great among the two pairs of sympatric sister species in agreement with the suggested sympatric model of speciation.     

Geographic variation in the long-nosed snake, Rhinocheilus lecontei (Colubridae): beyond the subspecies debate

Scalation, colour pattern, linear and geometric morphometrics were used to quantify geographical differentiation in the long-nosed snake, Rhinocheilus lecontei, and to test the hypothesis that all four subspecies are morphologically distinct. Also investigated were potential associations between morphological (scalation, colour pattern, linear measurements) and environmental variables (climate, vegetation, soil). Sexual dimorphism was weakest for geometric and strongest for linear morphometric variables. Morphological variables differed widely in their ability to differentiate subspecies. Linear morphometric variables achieved the most statistically significant pairwise Mahalanobis distances between subspecies, while geometric morphometrics largely failed to differentiate them. Colour pattern showed the strongest and linear morphometrics the weakest correlation with environment. Several characters varied continuously along latitudinal or longitudinal gradients, such that, in some cases, the clines for closely related traits were discordant. No one subspecies was consistently divergent in all analyses, leading to the conclusion that the three mainland subspecies are not sufficiently distinct to warrant separate subspecies status. The island subspecies, though not always statistically distinct, is geographically separate from other populations and differs in characters related to size. Given the small number of specimens available, a decision regarding its taxonomic status (i.e. elevation to species level) is best deferred until additional specimens can be examined and data on molecular variation can be analysed.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 83 , 65–85.     

Cranial shape variation and molecular phylogenetic structure of crested newts (Triturus cristatus superspecies: Caudata,Salamandridae) in the Balkans

In the present study, we investigated the degree of congruence between phylogeny, as inferred from mitochondrial (mt)DNA sequences, and cranium shape variation of crested newts (Triturus cristatus superspecies) in the Balkans. These newts belong to four phylogenetic clades defined by mtDNA analysis, and significantly differed in cranial shape. Allometry explained a high percentage of shape variation in crested newts. The clade‐specific allometric slopes significantly diverged for both the ventral cranium and dorsal cranium, indicating that differences in shape between clades could not be a simple consequence of their difference in size. The analysis of hierarchical and spatial variation showed similarity in the patterns of global and spatially localized hierarchical variation of cranial shape. We also found significant congruence between the pattern of cranial shape variation and molecular phylogeny. The differences in morphology of Triturus dobrogicus in comparison to other crested newt clades, including marked differences in cranium shape, is discussed in the context of the evolution and ecology of crested newts. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 348–360.