Gene flow and variation in stature and craniofacial dimensions among indigenous populations of southern Mexico, Guatemala, and Honduras

Populations with histories of outbreeding tend to be taller even in the face of seemingly unchanged health and nutritional circumstances, while inbreeding generally results in a negative correlation between body dimensions and level of inbreeding. Populations that have experienced a positive secular trend in stature and maturation typically have histories of higher outbreeding rates in combination with improved nutritional, health, and economic conditions, suggesting a genetic-environment interaction. In general, Middle American Indians have not experienced a secular trend in stature, nor a substantial increase in their standard of living, but they have experienced varying degrees of admixture with Spanish and African populations since the Spanish Conquest. The relationship between estimated gene flow and variation in several anthropometric dimensions is thus considered in indigenous Mesoamerican populations. Available data on height, sitting height, craniofacial dimensions, and admixture rate of Indian populations from southern Mexico, Guatemala, and Honduras were assembled from the literature. The population mean data on 3,371 adults from 23 populations for males and 18 for females were analyzed by regression with anthropometry as the dependent variable, and log10 of admixture as the independent variable. Admixture was less than or equal to 28.9% in this study and is suggestive of a primarily traditional Indian cultural and economic base. The results suggest that populations with higher admixture tend to be taller, and that the increase in stature is due to greater subischial length in both sexes. A decrease in nasion-menton height, and an increase in nasal breadth, nasal height, and the nasal index is suggested for the dimensions of the craniofacial complex.(ABSTRACT TRUNCATED AT 250 WORDS)     

NONRANDOM FACTORS IN MODERN HUMAN MORPHOLOGICAL DIVERSIFICATION: A STUDY OF CRANIOFACIAL VARIATION IN SOUTHERN SOUTH AMERICAN POPULATIONS

The causes of craniofacial variation among human populations have been the subject of controversy. In this work, we studied aboriginal populations from southern South America, the last continental region peopled by humans and with a wide range of ecological conditions. Because of these characteristics, southern South America provides a unique opportunity to study the relative importance of random and nonrandom factors in human diversification. Previous craniometric studies recognized remarkable differences among populations from this region, usually resorting to random factors as the main explanation. In contrast, here we suggest, using tests based on quantitative genetic models, that: (1) the rate of craniofacial divergence among these populations is too high and (2) the patterns of variation within and between populations are too different to be explained by genetic drift alone. In addition, the among-sample craniofacial variation is correlated with climate and diet but not with mtDNA variation. We suggest that the influence of nonrandom factors (e.g., plasticity, selection) on human craniofacial diversification in regions with large ecological variation is more important than generally acknowledged and capable to generate large craniofacial divergence in a short period of time. These results bring nonrandom factors into focus for the interpretation of human craniofacial variation.     

Heterozygosity and craniofacial dimensions of Zapotec school children from a subsistence community in the valley of Oaxaca, southern Mexico

Gene flow is associated with differences in craniofacial and postcranial dimensions among indigenous populations of southern Mexico. This study compares four craniofacial dimensions in 322 children from families which have an average inbreeding coefficient of 0.01 and 36 children from families which have an inbreeding coefficient of zero (more heterozygous) in a Zapotec speaking community. In addition, two indices were computed. With sex and chronological age constant, there is a statistically significant difference between more and less heterozygous children in bizygomatic diameter. Differences in biparietal diameter and fronto-occipital length reflect the same tendency. The differences probably reflect more an effect of heterozygosity on these dimensions than facial dissimilarity of the populations from which the new genetic materials were drawn because gene flow was from related groups of Indians (e.g., Mixtec) in southern Mexico. Hence, midface growth and overall normal size appear to be affected by fluctuation in level of heterozygosity.     

Patterns of quantitative genetic variation in multiple dimensions

A fundamental question for both evolutionary biologists and breeders is the extent to which genetic correlations limit the ability of populations to respond to selection. Here I view this topic from three perspectives. First, I propose several nondimensional statistics to quantify the genetic variation present in a suite of traits and to describe the extent to which correlations limit their selection response. A review of five data sets suggests that the total variation differs substantially between populations. In all cases analyzed, however, the “effective number of dimensions” is less than two: more than half of the total genetic variation is explained by a single combination of traits. Second, I consider how patterns of variation affect the average evolutionary response to selection in a random direction. When genetic variation lies in a small number of dimensions but there are a large number of traits under selection, then the average selection response will be reduced substantially from its potential maximum. Third, I discuss how a low genetic correlation between male fitness and female fitness limits the ability of populations to adapt. Data from two recent studies of natural populations suggest this correlation can diminish or even erase any genetic benefit to mate choice. Together these results suggest that adaptation (in natural populations) and genetic improvement (in domesticated populations) may often be as much constrained by patterns of genetic correlation as by the overall amount of genetic variation.     

Variation in soft-tissue thicknesses on the human face and their relation to craniometric dimensions

The average thickness of soft tissues on parts of the face is known, but its variation has not been related to cranial morphology. To investigate this relationship, measurements of facial soft-tissue depths and craniometric dimensions were taken on adult, white Australian cadavers (17 male and 23 female). Significant correlations between many soft-tissue depths and craniometric dimensions were found, suggesting a relationship between the amount of soft tissue present on the face and the size of the underlying bony skeleton. Soft-tissue depths were highly positively correlated with each other; craniometric dimensions were correlated but to a lesser extent. Males had thicker soft tissues and larger craniometric dimensions than females; considerable overlap of ranges was also noted. Multiple regression analysis was used to produce equations predicting the soft-tissue depth at specified areas of the face from craniometric dimensions. A subsample of nine cadavers was examined for the effects of tissue embalming. Embalming caused significant initial increases in facial soft-tissue depths. Cadavers embalmed for less than 6 months had soft-tissue depths significantly greater than for fully embalmed cadavers. The evidence that facial soft-tissue thicknesses vary with craniofacial dimensions has implications for forensic identification, facial aesthetic surgery, and approximation of the facial features of extinct individuals.     

Three-dimensional measurement of the occlusal surfaces of lower first molars of Australian aboriginals

The occlusal surfaces of lower first molars of Australian Aboriginals were measured in three dimensions with the aid of Moiré contourography. Molar cuspal heights in this population were higher than in Japanese (Mongoloid) but lower than in Dutch (Caucasoid) populations. Intercuspal distances were considerably larger than those in the two other populations. Populational differences in occlusal features may influence both craniofacial structures and jaw movements in the three populations. Low correlations between the cuspal heights and the intercuspal distances in the other two populations were also found in this population, indicating that human molar cuspal height is independent of the transverse size of the crown. Mean values for the height of the three principal cusps in lower first molars were less than those in upper ones. However, the height of the hypoconid, which was the highest in the lower molar cusps, showed almost the same mean value as the height of the upper three principal cusps, indicating that the height of the main functional cusp, in both upper and lower first molars, was almost the same.     

Sibling correlations for cranial measurements from serial radiographs

Sibling correlations are presented for nine craniofacial measurements from lateral cephalometric radiographs of participants in the Fels Longitudinal Study of Growth and their siblings. Because the data were collected serially, measurements on sibs at the same age can be compared at one-year intervals. Sibling correlations are generally lowest during the first year of life and highest as adulthood is approached, but no interpretable pattern is established during the intervening years. Most of the correlations are significantly greater than zero, and many exceed 0.5, the value expected for a completely heritable polygenic trait. This suggests that common sibling environment contributes to sibling resemblance for craniofacial dimensions in our sample.     

Family resemblance for components of craniofacial size and shape

Path analysis is used to analyze family resemblance for eight principal components extracted from 30 anthropometric measurements of the craniofacial complex. Based on likelihood ratio tests, the null hypothesis of no assortative mating is rejected for the nasal height component. The null hypothesis of no common sibling environmental effect is rejected for the cranial size, craniofacial breadth, and nasal height components. Finally, the hypothesis that transmission from both mother and father is equal to 1/2, consistent with simple autosomal polygenic inheritance, is rejected for components corresponding to craniofacial breadth and upper facial height, thus implicating some effect of familial environment. Transmissibility is higher for components related to cranial size and facial height than for those related to facial breadth or ear dimensions.     

Family resemblance for cranio-facial measurements in Velanti Brahmins from Andhra Pradesh, India

Maximum likelihood estimates of familial correlations are presented for 12 cranio-facial measurements taken on 399 nuclear families from Southern India. Marital resemblance is significantly different from zero for head circumference, head breadth, minimum frontal breadth, bizygomatic breadth, total facial height, and nasal height, but not for bigonial breadth, nose breadth, nose depth, or ear dimensions. All other familial correlations are significantly greater than zero except for the father-daughter correlation for nasal depth. Path analysis with a TAU transmission model with sex effects reveals that family resemblance for head circumference, head length, bigonial breadth, total facial height, and nasal height, but not for bigonial breadth, nose breadth, nose depth, or ear dimensions. All other familial correlations are significantly greater than zero except for the father-daughter correlation for nasal depth. Path analysis with a TAU transmission model with sex effects breadth, and sex effects in transmission were found for head breadth and nose dimensions. Sex effects in this sample may be due to the fact that different anthropometrists were used for male and female subjects.     

Genetic correlations do not constrain the evolution of sexual dimorphism in the moss Ceratodon purpureus

The trajectory of phenotypic evolution is constrained in the short term by genetic correlations among traits. However, the extent to which genetic correlations impose a lasting constraint is generally unknown. Here, I examine the genetic architecture of life-history variation in male and female gametophytes from two populations of the moss Ceratodon purpureus, focusing on genetic correlations within and between the sexes. A significant negative correlation between allocation to vegetative and reproductive tissue was evident in males of both populations, but not females. All traits showed between-sex correlations of significantly less than one, indicating additive genetic variance for sexual dimorphism. The degree of dimorphism for traits was significantly negatively associated with the strength of the between-sex correlation. The structure of genetic correlations among life-history traits was more divergent between the two populations in females than in males. Collectively, these results suggest that genetic correlations do not impose a lasting constraint on the evolution of life-history variation in the species.     

Morphological differentiation of aboriginal human populations from Tierra del Fuego (Patagonia): implications for South American peopling

This study aims to integrate the craniofacial morphological variation of southern South American populations with the results of mtDNA haplogroup variation, to discuss the South America peopling. Because the causes of morphological differentiation of Fueguian populations are still a controversial subject, the comparison with neutral variation could contribute to elucidate them. Samples of human remains from South America regions were used to analyze the evolutionary relationships. Several craniofacial traits observed in frontal and lateral view were analyzed by means of geometric morphometrics techniques, and the evolutionary relationships based on morphological and molecular data were established in base to ordination analyses. The results from the facial skeleton agree with those obtained from mtDNA haplogroup frequencies, with La Pampa/Chaco samples detached from the Patagonian samples. Hence, the same mechanism that accounts for the pattern of frequency of haplogroups could explain the variation found in facial skeleton among the samples. It is suggested that such geographic pattern of craniofacial and molecular diversity may reflect the effect of genetic drift that occurred in the small founding populations isolated by distance or geographic barriers. Conversely, the results obtained using the traits from the lateral view slightly differ from the molecular results, showing differences between southernmost Patagonian and the other samples. Therefore, mechanisms other than genetic drift (e.g., natural selection) could have acted to shape the pattern observed in some craniofacial structures present in the lateral view, characterized by the fact that the southernmost Patagonian samples display the most robust and dolichocephalic crania.     

A craniofacial growth maturity gradient for males and females between 4 and 16 years of age

Differential growth of the craniofacial complex implies variation in ontogenetic patterns of development. This investigation quantifies the relative maturity—as defined by percent adult status—of nine cephalometric dimensions and stature. Analysis is based on 663 lateral cephalograms from a mixed longitudinal sample of 26 males and 25 females between 4 and 16 years of age. Graphic comparison of maturity status across the age range shows that variation is intergraded between the neural and somatic growth maturity patterns, as described by head height and stature, respectively. The maturity gradient moves from head height through anterior cranial base, posterior cranial base and maxillary length, upper facial height, corpus length, and ramus height to stature. After 9 years of age ramus height is less mature than stature. Anterior maxillary and mandibular heights diminish during transitional dentition and thereafter exhibit maturity patterns that compare to corpus length. Although females are consistently more mature than males, the gradient of variation between dimensions is sex independent.     

Taxonomic,functional, and phylogenetic dimensions of rodent biodiversity along an extensive tropical elevational gradient

Relationships among taxonomic, functional, and phylogenetic dimensions of biodiversity provide insight about the relative contributions of ecological and evolutionary processes in structuring local assemblages. We used data for rodent species distributions from an extensive tropical elevational gradient to 1) describe elevational gradients for each of three dimensions of biodiversity, 2) evaluate the sufficiency of species richness as a surrogate for other dimensions, and 3) quantify the relative support for mechanisms that increase or decrease phylogenetic or functional dispersion. Taxonomic biodiversity was quantified by species richness, as well as by richness, evenness, diversity, dominance, and rarity at generic and familial levels. Morphological and categorical traits were used to estimate functional biodiversity, and an ultrametric mammalian supertree was used as the basis for estimating phylogenetic biodiversity. Elevational gradients of each dimension of biodiversity were strong, with significant linear and non‐linear components based on orthogonal polynomial regression. Empirical linear and non‐linear regression components were consistently different than those expected based on species richness for generic, familial, and phylogenetic biodiversity, but not for functional biodiversity. Nevertheless, the congruence of dimensions of biodiversity based on correlation analyses indicated that any one dimension is a useful surrogate for the other dimensions for rodents at Manu. Given variation in species richness, assemblages from lowland rainforests comprised more biodiversity than expected, whereas assemblages from cloud and elfin forests represented less biodiversity than expected. Warm temperatures, vertical complexity of the vegetation, and high productivity likely facilitate niche differentiation in rainforests, whereas cricetid rodents are competitively superior to other clades in the less structurally complex, less productive, and colder, high elevation habitats.     

Comparing indigenous and introduced populations of Melaleuca quinquenervia (Cav.) Blake: response of seedlings to water and pH levels

Melaleuca quinquenervia is a wetland tree species indigenous to eastern Australia. It was separately introduced to east and west Florida as an ornamental, but has since become invasive, dominating several habitat types. We tested the predictions that (1) Australian populations would exhibit more genetic variation than Florida populations, due to founder effect, and (2) high phenotypic plasticity would be found in all populations, due to the wide range of habitats occupied. We compared the phenotypic plasticity and familial variation among three Australian populations, two east Florida, and two west Florida populations in a greenhouse experiment. We grew seedlings collected from different maternal trees in each population under two water levels and three pH levels, reflecting the natural range of water levels and soil pH in Florida and Australian Melaleuca stands. We measured leaf size and shape, growth rate and above-ground biomass of seedlings and determined the components of phenotypic variance (familial, environmental, and their interaction) using univariate and multivariate analysis of variance. All traits showed significant among-population and among-family variation, as well as significant phenotypic plasticity, in response to both water level and pH level changes. Sensitivity to pH was particularly high, presumably because plants were grown under pHs ranging from 4.7 to 7.4, and because pH can influence nutrient availability. Familial variation contains genetic variation, but it may also be confounded with maternal environmental effects. Comparing Australian to Floridian Melaleuca, amounts of familial variation and phenotypic plasticity varied by trait. Overall, Australian Melaleuca had more among-population variation than Floridian Melaleuca, presumably reflecting the wider latitudinal range and longer time for evolutionary change in Australia, but had similar amounts of among-family variation, within any one population. If maternal effects are strong, among-population differences may merely reflect greater environmental differences among Australian sites than Florida sites. Australian Melaleuca had less phenotypic plasticity, possibly due to founder effects in Florida or to subsequent adaptive evolution of phenotypic plasticity in Floridian populations. Floridian Melaleuca shows little loss of familial variation, compared to indigenous Australian populations, and that, in combination with its high phenotypic plasticity, should allow it to continue to colonize new areas successfully.     

Craniofacial variation and growth in the Prader-Labhart-Willi syndrome

A study of anthropometric variation and craniofacial growth in individuals with the Prader-Labhart-Willi syndrome (PLWS) illustrates the utility of anthropometry in clinical evaluation and research. Anthropometric measurements, including head length and breadth, minimum frontal diameter, and head circumference, were obtained on 38 PLWS individuals (21 with chromosome 15 deletions) with an age range from 2 weeks to 39 years. No anthropometric differences were found between the two chromosome subgroups. A relative deceleration in the growth of certain craniofacial dimensions (head circumference and length) is suggested by the negative correlations between age and Z-scores for the measurements. Raw values for minimum frontal diameter and head breadth were near or below the 5th percentile curve, while almost all values for head length and circumference fell within normal limits. The data support suggestions that dolichocephaly be considered an early diagnostic feature of PLWS. Furthermore, the status of narrow bifrontal diameter as a major feature of PLWS is confirmed.     

Temporal changes in human temporomandibular joint size and shape

Measurements approximating the size of the temporomandibular joint were taken on a series of genetically homogeneous populations from early Nubia, which span almost 10,000 years and embody a shift from a primarily hunting and gathering adaptation to a completely agricultural lifeway. A generalized trend of reduction of temporomandibular joint size was observed. In addition, a decrease in sexual dimorphism was apparent for all measures of joint size; a change primarily mitigated by reductions in male dimensions. The observed variation in size and form of the temporomandibular joint is most likely the result of the reduction in masticatory muscle robusticity and resultant changes in craniofacial form which have been documented for the transition from a hunting and gathering to an agricultural subsistence in Nubia.     

Among‐population variation and correlations in sexually dimorphic traits of Silene latifolia

Abstract The degree of sexual dimorphism in a trait may be determined directly by disruptive selection, as well as by correlations with other traits under selection. We grew seeds from nine populations of the dioecious plant Silene latifolia in a common‐garden experiment to determine whether phenotypic variation and correlations existed for floral, leaf and resource allocation traits, and whether this variation had a genetic component. We also determined the traits which were sexually dimorphic, the degree of dimorphism, and whether it varied among populations. Seven traits exhibited among‐population variation and sexual dimorphism. Variation in the degree of dimorphism occurred only for two traits, suggesting that dimorphism may be evolving more slowly than trait means. Males had more, smaller flowers, shorter leaves, and allocated less of their total biomass to stems and more to leaves than females. Flower production was the most sexually dimorphic trait and was correlated with all measured traits. Most traits exhibited significant correlations between the sexes. The pattern of correlations and the degree of sexual dimorphism among traits lead us to suggest that intrasexual selection for an exaggerated floral display in males has indirectly led to sexual dimorphism in a host of other traits.     

Head posture and craniofacial morphology

The associations between craniofacial morphology and the posture of the head and the cervical column were examined in a sample of 120 Danish male students aged 22–30 years. Two head positions were recorded on lateral cephalometric radiographs, one determined by the subject's own feeling of a natural head balance (self balance position), and the other by the subject looking straight into a mirror (mirror position). Craniofacial morphology was described by 42 linear and angular variables, and postural relationships by 18 angular variables. A comprehensive set of correlations was found between craniofacial morphology and head posture. The correlations were similar for both head positions investigated. Of the postural variables, the position of the head in relation to the cervical column showed the largest set of correlations with craniofacial morphology. Extension of the head in relation to the cervical column was found in connection with large anterior and small posterior facial heights, small antero-posterior craniofacial dimensions, large inclination of the mandible to the anterior cranial base and to the nasal plane, facial retrognathism, a large cranial base angle, and a small nasopharyngeal space. The possible role of functional factors in mediating the relationship between morphology and posture was discussed.     

Morphometric testing of structural hypotheses of the supraorbital region in modern humans

Our understanding of the functional morphology of the primate supraorbital region is based largely on previous morphometric and in vivo mechanical tests of hypotheses in non-human anthropoids. Prior tests of two structural hypotheses explaining morphological variation in the supraorbital region, the craniofacial size hypothesis and the spatial hypothesis, did not fully consider modern humans. We extend these previous findings to include modern humans by conducting morphometric tests of these two hypotheses in a sample of adult Melanesian crania. Morphometric correlates of structural predictions for the craniofacial size and spatial hypotheses were developed and compared to measurements of the supraorbital region via bivariate product-moment correlations. Measurements of the supraorbital region are significantly correlated with a craniofacial size estimate across individuals from this Melanesian sample. This result supports the prediction of the craniofacial size hypothesis that the magnitude of the supraorbital region is proportional to craniofacial size. The predicted link between the degree of neural-orbital disjunction and the magnitude of the supraorbital region, explicated in the spatial hypothesis, receives mixed support in the correlation analysis. These two results agree with previous research indicating that support for the craniofacial size and spatial hypotheses can be found across and within anthropoid primate species, including modern humans. Correlational support for both the craniofacial size and spatial hypotheses suggests multiple factors influence variation in the modern human supraorbital region. Thus, a single hypothesis cannot fully account for modern human variation in this region. The low bivariate correlation coefficients in this study further question whether existing hypotheses can adequately explain morphological variation in the supraorbital region in a primate population sample. Novel functional, structural, behavioral and developmental ideas must be explored if we are to better understand morphological variation in the modern human supraorbital region.     

Linkage disequilibrium analysis identifies an FGFR1 haplotype-tag SNP associated with normal variation in craniofacial shape

Mutations in FGFR1 and TWIST1 have been reported to affect the timing of calvarial suture fusion resulting in craniosynostosis and facial abnormalities. We screened nonpathologic populations for genetic polymorphisms that may associate with normal craniofacial variation. We identified 17 single-nucleotide polymorphisms (SNPs) in FGFR1, 6 of which were novel (g.8591855G-->A, g.8593685G-->A, g.8602303C-->T, g.8602475A-->G (p.Ile293Val), g.8605849C-->T, g.8607868G-->A). No SNPs were found in TWIST1. FGFR1 SNP haplotypes were reconstructed for Caucasian, Asian, Australian Aboriginal, and African American populations. All populations shared two linkage disequilibrium blocks, with one haplotype-tag SNP (htSNP) tagging each block. The htSNP g.8592931G-->C was found to have a significant negative correlation with the cephalic index for all populations (R = -0.187, p = 0.036), with larger correlations in Asians and females. This finding is a starting point in the identification of a set of SNPs that can be genotyped to determine both normal and disease craniofacial phenotypes.