The Role of Phenotypic Plasticity in the Interpretation of Stratophenetic Patterns in the Paleozoic

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The Role of Phenotypic Plasticity in the Interpretation of Stratophenetic Patterns in the PaleozoicBrenda R. Hanke and Andrew J. WebberMany evolutionary studies focus on how populations of organisms adapt and change in the face of environmental changes. The raw material upon which natural selection acts is plasticity, or variability, in an organism?s phenotypes (the physical expression, such as body size, shape, and others, of the genetic make-up of an organism). Nearly all phenotypes will show some plasticity because traits that are expressed result from the interaction of genetics with environment. The relationship between the plasticity of phenotypes and variable environmental conditions, however, still requires further understanding. While numerous biological studies have addressed this relationship with modern organisms, even extending into the young fossil record, this area of research has been lacking in settings that are much older. The power of the fossil record is that it allows us to measure changes within a species over longer time scales; but can paleontology?s contribution to evolutionary biology improve our understanding of the relationship between phenotype and environment over time? There are two primary steps in achieving this goal: 1. the geographic distribution of phenotypic variability within a species and how it corresponds to spatial environmental gradients (known as a reaction norm) must be accurately documented. 2. The reaction norm for a species must then be evaluated over time.The goal of this research is to describe the relationship between paleoenvironmental conditions and variability in the shape of two trilobite species within rocks of the Kope Formation (Ordovician Period, around 450 million years old). In this study, morphology is measured using geometric morphometrics, which quantifies shape change at high resolution. Paleoenvironmental change is measured through gradient analysis, which uses the distribution and abundance of fossil remains to characterize environment. Preliminary research shows a statistically significant portion of shape variability in the cranidium (head) of the trilobite Flexicalymene granulosa is related to the environment in which this species is preserved. This relationship between shape and environment lead us to propose that patterns of change through time in this trilobite species is the result of populations following their preferred environments (also known as environmental tracking). To test our hypothesis, the next step is to systematically track this established relationship through time. To this end, we will compare cranidial shape to general geological interpretations, such as rock type, of environment, and to a quantified measure of environment acquired through gradient analysis in the Kope Formation. Intensive field work conducted over the next three years will provide a comprehensive dataset of trilobite cranidial shape and environment through time and space from which to test our hypothesis.This study has direct implications for understanding how variation within a trilobite species contributes to evolution within trilobites as a group. It compares morphological change to a high-resolution measure of environmental change using a novel approach, which can be used on many organisms in many geological settings. An important component of this study will be the integration of scientific research with science education through a workshop developed at Cincinnati Museum Center designed to provide training for regional science teachers in geology and paleontology.