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Morphological integration and the anthropoid dentition

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The pattern and strength of genetic covariation is shaped by selection so that it is strong among functionally related characters and weak among functionally unrelated characters. Genetic covariation is expressed as phenotypic covariation within species and acts as a constraint

The pattern and strength of genetic covariation is shaped by selection so that it is strong among functionally related characters and weak among functionally unrelated characters. Genetic covariation is expressed as phenotypic covariation within species and acts as a constraint on evolution by limiting the ability of linked characters to evolve independently of one another. Such linked characters are "constrained" and are expected to express covariation both within and among species. In this study, the pattern and magnitude of covariation among aspects of dental size and shape are investigated in anthropoid primates. Pleiotropy has been hypothesized to play a significant role in derivation of derived hominin morphologies. This study tests a series of hypotheses; including 1) that negative within- and among-species covariation exists between the anterior (incisors and canines) and postcanine teeth, 2) that covariation is strong and positive between the canines and incisors, 3) that there is a dimorphic pattern of within-species covariation and coevolution for characters of the canine honing complex, 4) that patterns of covariation are stable among anthropoids, and 5) that genetic constraints have been a strong bias on the diversification of anthropoid dental morphology. The study finds that patterns of variance-covariance are conserved among species. Despite these shared patterns of variance-covariance, dental diversification has frequently occurred along dimensions not aligned with the vector of genetic constraint. As regards the canine honing complex, there is no evidence for a difference in the pleiotropic organization or the coevolution of characters of the complex in males and females, which undermines arguments that the complex is selectively important only in males. Finally, there is no evidence for strong or negative pleiotropy between any dental characters, which falsifies hypotheses that predict such relationships between incisors and postcanine teeth or between the canines and the postcanine teeth.

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Date Created
2011

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Comparative and experimental investigations of cranial robusticity in mid-Pleistocene hominins

Description

Extremely thick cranial vaults have been noted as a diagnostic characteristic of Homo erectus since the first fossil of the species was identified, but potential mechanisms underlying this seemingly unique trait have not been rigorously investigated. Cranial vault thickness (CVT)

Extremely thick cranial vaults have been noted as a diagnostic characteristic of Homo erectus since the first fossil of the species was identified, but potential mechanisms underlying this seemingly unique trait have not been rigorously investigated. Cranial vault thickness (CVT) is not a monolithic trait, and the responsiveness of its layers to environmental stimuli is unknown. Identifying factors that affect CVT would be exceedingly valuable in teasing apart potential contributors to thick vaults in the Pleistocene. Four hypotheses were tested using CT scans of skulls of more than 1100 human and non-human primates. Data on total frontal, parietal, and occipital bone thickness and bone composition were collected to test the hypotheses: H1. CVT is an allometric consequence of brain or body size. H2. Thick cranial vaults are a response to long, low cranial vault shape. H3. High masticatory stress causes localized thickening of cranial vaults. H4. Activity-mediated systemic hormone levels affect CVT. Traditional comparative methods were used to identify features that covary with CVT across primates to establish behavior patterns that might correlate with thick cranial vaults. Secondly, novel experimental manipulation of a model organism, Mus musculus, was used to evaluate the relative plasticity of CVT. Finally, measures of CVT in fossil hominins were described and discussed in light of the extant comparative and experimental results. This dissertation reveals previously unknown variation among extant primates and humans and illustrates that Homo erectus is not entirely unique among primates in its CVT. The research suggests that it is very difficult to make a mouse grow a thick head, although it can be genetically programmed to have one. The project also identifies a possible hominin synapomorphy: high diploë ratios compared to non-human primates. It also found that extant humans differ from non-human primates in overall pattern of which cranial vault bones are thickest. What this project was unable to do was definitively provide an explanation for why and how Homo erectus grew thick skulls. Caution is required when using CVT as a diagnostic trait for Homo erectus, as the results presented here underscore the complexity inherent in its evolution and development.

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Date Created
2012

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Biomechanical Constraints on Molar Emergence in Primates

Description

Across primates, molar-emergence age is strongly correlated to life-history variables, such as age-at-first-reproduction and longevity. This relationship allows for the reconstruction of life-history parameters in fossil primates. The mechanism responsible for modulating molar-emergence age is unknown, however. This dissertation uses

Across primates, molar-emergence age is strongly correlated to life-history variables, such as age-at-first-reproduction and longevity. This relationship allows for the reconstruction of life-history parameters in fossil primates. The mechanism responsible for modulating molar-emergence age is unknown, however. This dissertation uses a biomechanical model that accurately predicts the position of molars in adults to determine whether molar emergence is constrained by chewing biomechanics throughout ontogeny. A key aspect of chewing system configuration in adults is the position of molars: the distal-most molar is constrained to avoid tensile forces at the temporomandibular joint (TMJ). Using three-dimensional data from growth samples of 1258 skulls, representing 21 primate species, this research tested the hypothesis that the location and timing of molar emergence is constrained to avoid high and potentially dangerous tensile forces at the TMJ throughout growth. Results indicate that molars emerge in a predictable position to safeguard the TMJ during chewing. Factors related to the size of the buffer zone, a safety feature that creates greater stability at the TMJ during biting, account for a large portion of both ontogenetic and interspecific variation in the position of emergence. Furthermore, the rate at which space is made available in the jaws and the duration of jaw growth both determine the timing of molar emergence. Overall, this dissertation provides a mechanical and developmental model for explaining temporal and spatial variation in molar emergence and a framework for understanding how variation in the timing of molar emergence has evolved among primates. The findings suggest that life history is related to ages at molar emergence through its influence on the rate and duration of jaw growth. This dissertation provides support for the functionally integrated nature of craniofacial growth and has implications for the study of primate life history evolution and masticatory morphology in the fossil record.

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Created

Date Created
2017

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Evolutionary Genetics of CORL Proteins

Description

Transgenic experiments in Drosophila have proven to be a useful tool aiding in the

determination of mammalian protein function. A CNS specific protein, dCORL is a

member of the Sno/Ski family. Sno acts as a switch between Dpp/dActivin signaling.

dCORL is involved in

Transgenic experiments in Drosophila have proven to be a useful tool aiding in the

determination of mammalian protein function. A CNS specific protein, dCORL is a

member of the Sno/Ski family. Sno acts as a switch between Dpp/dActivin signaling.

dCORL is involved in Dpp and dActivin signaling, but the two homologous mCORL

protein functions are unknown. Conducting transgenic experiments in the adult wings,

and third instar larval brains using mCORL1, mCORL2 and dCORL are used to provide

insight into the function of these proteins. These experiments show mCORL1 has a

different function from mCORL2 and dCORL when expressed in Drosophila. mCORL2

and dCORL have functional similarities that are likely conserved. Six amino acid

substitutions between mCORL1 and mCORL2/dCORL may be the reason for the

functional difference. The evolutionary implications of this research suggest the

conservation of a switch between Dpp/dActivin signaling that predates the divergence of

arthropods and vertebrates.

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Created

Date Created
2019