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Biomedical subjects

Rebecca Rogers Ackermann

Publications and source records attributed to Rebecca Rogers Ackermann.

7 recordsLinked to original sources
PubMed2006

Identifying the morphological signatures of hybridization in primate and human evolution.

Recent studies point to contact and possible admixture among contemporaneous hominin species during the Plio-Pleistocene. However, detection of hybridization in fossils-and especially fossil hominins-is contentious, and it is hindered in large part by our lack of understanding about how morphological hybridity is manifested in the primate skeleton. Here, we report on a study of known-pedigree, purebred yellow and olive baboons (n = 112) and their hybrids (n = 57), derived from the baboon colony of the Southwest Foundation for Biomedical Research. The hybrids were analyzed in two different groups: (1) F1 = olive x yellow first-generation hybrids; (2) B1 = olive x F1 backcross hybrids. Thirty-nine metric variables were tested for heterosis and dysgenesis. Nonmetric data were also collected from the crania. Results show that these primate hybrids are somewhat heterotic relative to their parental populations, are highly variable, and display novel phenotypes. These effects are most evident in the dentition and probably indicate the mixing of two separately coadapted genomes and the breakdown in the coordination of early development, despite the fact that these populations diverged fairly recently. Similar variation is also observed in museum samples drawn from natural hybrid zones. The results offer a strategy for detecting hybrid zones in the fossil record; implications for interpreting the hominin fossil record are discussed.

Analysis of Variance↗
PubMed2005

Ontogenetic integration of the hominoid face.

By investigating similarity in cranial covariation patterns, it is possible to locate underlying functional and developmental causes for the patterning, and to make inferences about the evolutionary forces that have acted to produce the patterns. Furthermore, establishing where these covariation patterns may diverge in ontogeny can offer insight into when selection may have acted on development. Here, covariation patterns are compared among adult and non-adult members of the African ape/human clade, in order to address three questions. First, are integration patterns constant among adult African apes and humans? Second, are they are constant in non-adults--i.e. throughout ontogeny? Third, if they are not constant, when do they diverge? Measurements are obtained from 677 crania of adult and non-adult African apes and humans. In order to address the first two questions, correlation matrices and theoretical integration matrices are compared using matrix correlation methods. The third question is evaluated by comparing correlation and variance/covariance patterns, using matrix correlation and random skewers methods, respectively, between adjacent age categories within each species, and between equivalent age categories among the four species. Results show that the hominoids share a similar pattern of ontogenetic integration, suggesting that common developmental/functional integrative processes may play an important role in keeping covariance structure stable across this lineage. However, there are some important differences in the magnitude of integration and in phenotypic covariance structure among the species, which may provide some insight into how selection acted to differentiate humans from the great apes.

Adolescent↗
PubMed2004

Detecting genetic drift versus selection in human evolution.

Recent paleoanthropological discoveries reveal a diverse, potentially speciose human fossil record. Such extensive morphological diversity results from the action of divergent evolutionary forces on an evolving lineage. Here, we apply quantitative evolutionary theory to test whether random evolutionary processes alone can explain the morphological diversity seen among fossil australopith and early Homo crania from the Plio-Pleistocene. We show that although selection may have played an important role in diversifying hominin facial morphology in the late Pliocene, this is not the case during the early evolution of the genus Homo, where genetic drift was probably the primary force responsible for facial diversification.

Animals↗
PubMed2002

Common patterns of facial ontogeny in the hominid lineage.

Recent evaluation of Neanderthal and modern human ontogeny suggests that taxon-specific features arose very early in development in both lineages, with early, possibly prenatal, morphological divergence followed by parallel postnatal developmental patterns. Here we use morphometric techniques to compare hominoid facial growth patterns, and show that this developmental phenomenon is, in fact, not unique to comparisons between Neanderthals and modern humans but extends to Australopithecus africanus and to the hominoid lineage more broadly. This finding suggests that a common pattern of juvenile facial development may be more widespread and that the roots of ontogenetically early developmental differentiation are deep-perhaps predating the ape/human split of 6+ million years ago.

Animals↗
PubMed2002

Discerning evolutionary processes in patterns of tamarin (genus Saguinus) craniofacial variation.

Quantitative genetic theory specifies evolutionary expectations for morphological diversification by genetic drift in a monophyletic clade. If genetic drift is responsible for the evolutionary morphological diversification of a clade, patterns of within- and between-taxon morphological variance/covariance should be proportional. We tested for proportionality of within- and between-species craniofacial morphological variation in 12 species of tamarins (genus Saguinus). We found that within- and between-taxon morphological variations across the entire genus were not proportional, and hence not likely to be due to genetic drift alone. The primary deviation from proportionality is that size and size-related shape in the cranium is more variable relative to other aspects of cranial morphology than expected under genetic drift, suggesting differential size selection between the two major clades, the small-bodied and large-bodied tamarins. Within each of these major clades, most of the interspecific variation is consistent with the pattern expected under genetic drift, although specific contrasts may indicate the involvement of differential selection. Morphological distances among taxa do not correspond very closely to the phylogeny derived from mtDNA. In particular, S. oedipus and S. geoffroyi are very distinct morphologically from the rest of the tamarins, although they are phylogenetically the sister clade to a clade containing S. midas and S. bicolor. Morphological similarity is not a good guide to phylogenetic affinity in the tamarins, especially with regard to deeper nodes in the phylogenetic tree.

Animals↗
PubMed2002

Patterns of covariation in the hominoid craniofacial skeleton: implications for paleoanthropological models.

Living species are often used as analogues for fossil ones. When this is done, the implicit assumption is made that hominids and living hominoids vary in the same way. This paper addresses the validity of this assumption by comparing patterns of facial variation among humans and African apes. In particular, it addresses three major questions that underlie approaches to reconstructing hominid relationships. First, is phenotypic variation similar between closely related species? Second, if it is dissimilar, why? Third, is it feasible to use analogue species for modeling purposes? Measurements are obtained from 542 crania of adult apes and humans. Care is taken to choose homologous data, and account for differences in population size and structure. Variance/covariance and correlation matrices among the species are compared using common principal component (CPC) analysis, random skewers methods and matrix correlations. Morphological distances (D(2)) are calculated between population means, and between randomized pairs of individuals within each population, to evaluate intraspecific variation. Morphological distances are also calculated between randomized pairs of individuals using the variation patterns of analogue populations, in order to evaluate the efficacy of such substitutions. Results show that while the hominoids share a similar pattern of facial variation overall, the patterns do diverge. This difference generally corresponds to the phylogenetic relationships among these species, suggesting that patterns of variation may have diverged through time in the large bodied hominoids. Because interpretation of relationships in the fossil record is confounded by a lack of understanding of how variation changes through time, exploration of such patterns of divergence can provide important clues to understanding human evolution. Additionally, neglecting to account for this divergence when using living analogues as variation "yardsticks" can give rise to interpretations of the fossil record that are more speciose than is warranted.

Animals↗