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Hageman, S.J. 1994 10 15: Microevolutionary implications of chal variation in the Paleozoic bryozoan Streblotrypa Lethaia

Ideally, studies of microevolution, including those of patterns and rates of speciation, need to account for features of geographic and ecophenotypic morphologic variation. These factors can provide primary sources of variation for evolution to act upon. They can also, however, produce variation among characters that are not directly related to speciation events, thus confounding recognition of larger evolutionary patterns. Because of limitations of geologic settings, features of morphologic variation associated with geographic and ecophenotypic variation are difficult to constrain and are often neglected. Consequences of ignoring potential geographic variation in microevolutionary studies are varied. The likelihood of observing stasis or gradation may increase or decrease depending on initial assumptions. A series of tests that place reasonable limits on potential errors, however, can strengthen conclusions from empirical microevolutionary studies. Late Paleozoic cyclothems of the North American Midcontinent provide an excellent opportunity to study patterns of microevolution and concomitant ecophenotypic and geographic variation. Based on multivariate analysis of 28 morphometric characters, specimens of the rhabdomesine bryozoan genus Streblotrypa, collected from eight coeval localities distributed along a 300 km transect, exhibit a morphologic cline associated with thickening of the skeleton from south to north. Additional specimens were collected from five younger localities to test for temporal and ecologic variation. The range of observed morphologic variation from a single horizon was as great as that observed over the entire stratigraphic sequence from varied lithologies. This indicates a pattern of overall morphological stasis through the interval studied. Bryozoa, clines, microevolution, variability, geographic variation, Carboniferous.