Microevolution in Two Lineages of Upper Ordovician (Cincinnatian) Bryozoans

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Pachut 9506003 This project proposes to study microevolution among fossil organisms from a new perspective: that of the genetics, heritability, and selection of characters within preserved lineages. Such a study would be unique in paleontology because it could separate observed temporal variation into heritable and non-heritable components, estimate temporal changes in levels of heterozygosity, and separate stabilizing from directional selection. Observed microevolutionary phenomena such as stasis, gradualism, or lineage splitting could be assessed in terms of their underlying genetic basis. Fossil bryozoans, because they form colonies, are among the few groups in the fossil record in which this type of analysis is possible. The only other abundant groups in which a comparable analysis could be carried out are corals and graptolites. Secondly, bryozoans are very abundant on a bed-by-bed basis over lengthy stratigraphic intervals in the Edenian, Maysvillian, and Richmondian Stages of the Cincinnatian Series in its type area. The two generic lineages chosen, Peronopora and Parvohallopora, both have continuous distributions with numerous branching points over a duration exceeding 10 million years. Branching patterns within the lineages are resolvable using both cladistic techniques and chronostratigraphy based on both conodonts and sequence boundaries. Testing several specific microevolutionary hypotheses in these lineages. The first of these is that stabilizing selection underlies observed stasis, even stasis lasting millions of years. The second hypothesis is that speciation, or lineage branching, is unrelated to directional selection preceding the branching point, a key test of punctuated equilibrium. The third hypothesis is that heterozygosity levels may be reduced prior to a branching event, producing a genetic bottleneck that may break out of stasis. The main disadvantage of this study is that estimates of genetic parameters must be based on morphological data; however, r ecent studies of genetic heritabilities and covariance and morphological variability in living bryozoan species justify the substitution of estimates, calculated from partitioned within- and among-colony phenotypic variances, for genetic parameters. The tradeoff, that more than balances this "disadvantage" is that these parameters can be studied in two well-sampled branching lineages enduring over a 10 million year interval.