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- Jackson, Illiam, 1986-
(author)
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Morphometric analysis of Cambrian fossils and its evolutionary significance
- 2017
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Doctoral thesis (other academic/artistic)abstract
- The Extended Evolutionary Synthesis (EES) is currently emerging as a theoretical alternative to the Modern Synthesis (MS) in which to frame evolutionary observations and interpretations. These alternative frameworks differ fundamentally in their understanding of the relative roles of the genotype, phenotype, development and environment in evolutionary processes and patterns. While the MS represents a gene-centred view of evolution, the EES instead emphasizes the interactions between organism, development and environment. This novel theoretical framework has generated a number of evolutionary predictions that are mutually incompatible with the equivalent of the MS. While research and empirical testing has begun on a number of these in a neontological context, the field of palaeontology has yet to contribute meaningfully to this endeavour. One of the reasons for this is a lack of methodological approaches capable of investigating relevant evolutionary patterns in the fossil record. In this thesis morphometric methods capable of providing relevant data are developed and employed in the analysis of Cambrian fossils. Results of these analyses provide empirical support for the process of evolution through phenotypic plasticity and genetic assimilation hypothesized by the EES. Furthermore, theoretical revision to the species concept in a palaeontological context is suggested. Finally, predictions of the EES specific to the fossil record are made explicit and promising directions of future research are outlined.
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| 2. |
- Jackson, Illiam S. C., 1986-, et al.
(author)
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Genetic assimilation in the fossil record: phenotypic plasticity and later accommodation in Cambrian arthropods
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Other publication (other academic/artistic)abstract
- Genetic assimilation is a hypothesised process in which an initially plastic developmental phenotypic response of an organism to the environment is fixed genetically, i.e. assimilated into its genome 1,2. One central prediction of genetic assimilation is that “phenotypic accommodation can precede, rather than follow, genetic change, in adaptive evolution”3. Here we test this prediction in the fossil record. Agnostus pisiformis, a Cambrian Series 3 trilobite-like arthropod, has been shown to exhibit subtly different patterns of pygidial morphological variation across coeval assemblages4, varying chiefly in the degree to which the axial lobe dominates the pygidium. We demonstrate that this morphological variation as well as that of the slightly younger closely related Homagnostus obesus is significantly correlated with geochemical indicators of dysoxia/euxinia and thus stressed environments5-7. In addition, the variances of high and low-stress assemblages also differ significantly, suggesting that the morphological variability of the different assemblages is induced by environmental stress and can be understood as a reaction norm8. We include in our analysis the younger relative Trilobagnostus holmi and interpret its morphology, which has a strongly reduced variance, as representing a more canalized9 or stabilized1 stage of the assimilation process. Thus our data contain all stages of adaptation via phenotypic plasticity and genetic assimilation10 and support the main predictions of the ‘Extended Evolutionary Synthesis’3.
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| 3. |
- Jackson, Illiam S. C., 1986-, et al.
(author)
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Intraspecific morphological variation of Agnostus pisiformis, a Cambrian Series 3 trilobite-like arthropod
- 2017
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In: Lethaia. - : Scandinavian University Press / Universitetsforlaget AS. - 0024-1164 .- 1502-3931. ; , s. 467-485
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Journal article (peer-reviewed)abstract
- The study of evolution in a palaeontological context is chiefly the study of change in shape and form. This requires data sets that quantify morphology and morphological variation. Historically morphology has been described using discrete characters or more recently using various morphometric approaches. Elliptical Fourier analysis (EFA) is an approach to quantifying morphology that results in the production of large data sets of elliptical Fourier descriptors (EFDs), which are highly suitable to multivariate analysis. EFA is used in this paper to quantify the shape and describe the ontogeny of Agnostus pisiformis (Wahlenberg 1818: Nova Acta Regiae Societatis Scientiarum Upsaliensis 8, 1), a trilobite-like arthropod of Cambrian Series 3, from three coeval localities in Sweden. An ontogenetic difference was detected between geographically distant populations from Västergötland and Skåne in Sweden. These differences are probably the result of environmental dysoxic stress leading to increasing phenotypic variation. These findings illustrate the utility of EFA applied to the study of fossil organisms; permitting studies of such high resolution that multiple assemblages of the same species can be comparatively studied to achieve a more detailed understanding of their morphological and ontogenetic variation.
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| 4. |
- Jackson, Illiam S. C., 1986-, et al.
(author)
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The Extended Evolutionary Synthesis in the fossil record
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Other publication (other academic/artistic)abstract
- The Extended Evolutionary Synthesis (EES) has recently emerged as a theoretical alternative or complement to the Modern Synthesis (MS) in which to frame evolutionary observations and interpretations. The theoretical framework of the EES places a greater evolutionary significance on a number of controversial hypotheses and processes, such as phenotypic plasticity and genetic assimilation, and makes a number of empirical predictions based thereon. Investigation and empirical exploration of these predictions has begun in a neontological context yet the field of palaeontology has not yet been instrumental in developing or testing these predictions. Here the hypothetical process of evolution via phenotypic plasticity and genetic assimilation is explained and contextualised to the fossil record. A prediction of the EES specific to palaeontological data is formulated and methods suitable for its empirical testing are suggested.
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