Fishbein Mark, Venable D Lawrence
Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona, 85721.
Evolution. 1996 Dec;50(6):2165-2177. doi: 10.1111/j.1558-5646.1996.tb03607.x.
Very low fruit set in milkweeds and other flowering plants often has been attributed to greater sexual selection on inflorescence size via male, rather than female, reproductive success. Although this explanation has been generally accepted, alternate explanations have been presented, and recently the "male function" or "pollen donation" hypothesis has been sharply criticized. In this paper, we make the distinction between selection on total flower number and on the size of inflorescence units, both of which have been termed "inflorescence size." We present an ESS model for the evolution of inflorescence design that considers reproductive success through male and female function. The model predicts that selection will balance the proportional changes in female and male reproductive success resulting from changes in inflorescence-unit size. We conducted a field study of selection on the size of inflorescence units (umbels) by manipulating umbel size and number in a natural population of Asclepias tuberosa, in southeastern Arizona, during two reproductive seasons. We found that the male fitness function reached a maximum at an intermediate umbel size in both years (although not significantly different from the smallest umbel size in either year), whereas the female fitness function was highest for the smallest umbel size in one year, but was constant across umbel sizes in the other year. We also found that pollinator visitation rate corresponded well with male, but not female, function, and that between-year variation in the male reproductive success of different umbel sizes corresponded with variation in the composition of the pollinator pool. Our empirical results, when inserted in the model, predict ESS umbel sizes similar to those observed in the study population and the species throughout its range.
马利筋及其他开花植物极低的结实率常常被归因于通过雄性而非雌性繁殖成功对花序大小进行的更强的性选择。尽管这一解释已被普遍接受，但也有人提出了其他解释，并且最近“雄性功能”或“花粉捐献”假说受到了尖锐批评。在本文中，我们区分了对总花数的选择和对花序单位大小的选择，这两者都被称为“花序大小”。我们提出了一个花序设计进化的ESS模型，该模型考虑了通过雄性和雌性功能实现的繁殖成功。该模型预测，选择将平衡因花序单位大小变化而导致的雌性和雄性繁殖成功的比例变化。我们在亚利桑那州东南部两个繁殖季节期间，通过在块根马利筋的一个自然种群中操纵伞形花序的大小和数量，对花序单位（伞形花序）大小的选择进行了一项实地研究。我们发现，在这两年中，雄性适合度函数在中等大小的伞形花序处达到最大值（尽管与这两年中最小的伞形花序大小没有显著差异），而雌性适合度函数在一年中对于最小的伞形花序大小最高，但在另一年中在不同伞形花序大小之间保持不变。我们还发现传粉者访花率与雄性而非雌性的功能有很好的对应关系，并且不同伞形花序大小的雄性繁殖成功的年际变化与传粉者群体组成的变化相对应。当将我们的实证结果代入模型时，预测的ESS伞形花序大小与在研究种群以及该物种整个分布范围内观察到的大小相似。
