Swain Douglas P
Department of Zoology, University of British Columbia, Vancouver, BC, V6T 1W5, CANADA.
Evolution. 1992 Aug;46(4):998-1013. doi: 10.1111/j.1558-5646.1992.tb00615.x.
Variation in the number of vertebrae is widespread in fishes, and is partly genetic in origin. The adaptive significance of this variation was tested by exposing larvae of the threespine stickleback (Gasterosteus aculeatus) to predation by sunfish (Lepomis gibbosus). Two vertebral characters were considered: the total number (VN) and the ratio of abdominal to caudal vertebrae (VR). Predation was selective for both characters, but selection was more directly related to VR than to VN. The direction of selection depended on larval length: as length increased, optimal VR decreased. Total selection for VR was a combination of direct selection and an indirect effect of selection acting on a correlated trait, the ratio of precaudal to caudal length. Direct and indirect selection were in opposing directions at a given larval length. Variation in vertebral number may be maintained in populations partly because the strength of selection is reduced by opposing directions between direct and indirect selection, and between total selection at different larval lengths.
脊椎骨数量的变异在鱼类中广泛存在，且部分起源于遗传。通过将三刺鱼（Gasterosteus aculeatus）的幼鱼暴露于太阳鱼（Lepomis gibbosus）的捕食之下，对这种变异的适应性意义进行了测试。研究考虑了两个脊椎特征：总数（VN）以及腹部与尾椎骨的比例（VR）。捕食对这两个特征都具有选择性，但选择与VR的关联比与VN的关联更为直接。选择的方向取决于幼鱼的长度：随着长度增加，最优VR降低。对VR的总体选择是直接选择与作用于相关性状（尾前与尾长的比例）的选择间接效应的组合。在给定的幼鱼长度下，直接选择和间接选择方向相反。脊椎骨数量的变异可能在种群中得以维持，部分原因是直接选择与间接选择之间以及不同幼鱼长度下的总体选择之间的相反方向降低了选择强度。
