Microevolution in Two Lineages of Upper Ordovician (Cincinnatian) Bryozoans

Pachut 9506003 该项目提出从一个新的角度研究化石生物中的微进化：遗传学，遗传力和保存谱系内特征的选择。 这样的研究在古生物学中是独一无二的，因为它可以将观察到的时间变化分为遗传和非遗传成分，估计杂合性水平的时间变化，并将稳定化与定向选择分开。 观察到的微进化现象，如停滞，渐进，或血统分裂，可以评估其潜在的遗传基础。 化石苔藓虫，因为它们形成群体，是化石记录中少数几个可以进行这种类型分析的类群之一。 唯一可以进行可比分析的其他丰富的群体是珊瑚和笔石。 其次，在其类型区辛辛那提统的Edenian阶、Maysvillian阶和Richmondian阶的漫长地层间隔中，苔藓虫逐层非常丰富。 所选的两个属系，Peronopora和Parvohallopora，都有连续的分布，在超过1000万年的时间内有许多分支点。 分支模式内的谱系是可解析的分支技术和年代地层学的基础上牙形刺和层序边界。 在这些谱系中测试几个特定的微进化假设。 第一个是稳定选择是观察到的停滞的基础，即使是持续数百万年的停滞。 第二个假设是物种形成或谱系分支与分支点之前的定向选择无关，这是间断平衡的关键测试。 第三个假设是，杂合性水平可能在分支事件之前降低，产生可能打破停滞的遗传瓶颈。 本研究的主要缺点是，遗传参数的估计必须基于形态学数据，然而，最近的遗传遗传力和协方差和形态变异性活苔藓虫物种的研究证明了替代估计，计算分区内和外菌落表型方差，遗传参数。 权衡，超过平衡这一“缺点”是，这些参数可以在两个良好的采样分支谱系持续超过1000万年的时间间隔进行研究。

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.