Hunt Gene
Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, MRC 121, P.O. Box 37012, Washington, DC, 20013-7012, USA.
Evolution. 2007 Jul;61(7):1560-76. doi: 10.1111/j.1558-5646.2007.00129.x.
Trait variation and covariation are understood to influence the response of populations to natural selection on generational time scales, but their role, if any, in shaping long-term macroevolutionary divergence is still unclear. The present study uses the rich fossil record of the ostracode genus Poseidonamicus to reconstruct in great detail the evolutionary history of a set of landmark-based morphometric characters. This reconstruction included two kinds of evolutionary inferences: ancestor-descendant transitions among populations repeatedly sampled at the same location and divergence between lineages measured as independent contrasts on a phylogeny. This reconstructed history was then used to test if evolutionary changes were concentrated in directions (traits or combinations of traits) with high phenotypic variance. Two different statistics of association between evolution and variation tested the null hypothesis that evolutionary changes occur in random directions with respect to trait variability. The first of these measured the similarity between the directions of evolutionary change and the axis of maximum variance, and the second measured the degree to which evolutionary changes were concentrated in directions of high phenotypic variation. Randomization tests indicated that both kinds of evolutionary inferences (ancestor-descendant and phylogenetic contrasts) occurred preferentially in directions of high phenotypic variance (and close to the axis of maximal variation), suggesting that within-population variation can structure long-term divergence. This effect decayed after a few million years, but at least for one metric, never disappeared completely. These results are consistent with Schluter's genetic constraints model in which evolutionary trajectories on adaptive landscapes are deflected by variation within and covariation among traits.
性状变异和协变被认为会在代际时间尺度上影响种群对自然选择的响应，但其在塑造长期宏观进化分歧中所起的作用（如果有的话）仍不明确。本研究利用丰富的海相介形虫属波塞冬介形虫化石记录，非常详细地重建了一组基于地标点的形态测量特征的进化历史。这种重建包括两种进化推断：在同一地点重复采样的种群之间的祖先-后代转变，以及作为系统发育树上独立对比测量的谱系间分歧。然后，利用这个重建的历史来检验进化变化是否集中在具有高表型方差的方向（性状或性状组合）上。进化与变异之间的两种不同关联统计检验了零假设，即进化变化相对于性状变异性在随机方向上发生。其中第一种测量了进化变化方向与最大方差轴之间的相似性，第二种测量了进化变化集中在高表型变异方向上的程度。随机化检验表明，两种进化推断（祖先-后代和系统发育对比）都优先发生在高表型方差方向（且接近最大变异轴），这表明种群内变异可以构建长期分歧。这种效应在几百万年后衰减，但至少对于一个指标来说，从未完全消失。这些结果与施卢特的遗传约束模型一致，在该模型中，适应景观上的进化轨迹会因性状内部的变异和性状之间的协变而发生偏转。
