Campbell Diane R, Waser Nickolas M, Wolf Paul G
Department of Ecology and Evolutionary Biology, University of California, Irvine, California, 92697.
Rocky Mountain Biological Laboratory, Crested Butte, Colorado, 81224.
Evolution. 1998 Dec;52(6):1602-1611. doi: 10.1111/j.1558-5646.1998.tb02241.x.
Models of hybrid zones differ in their assumptions about the relative fitnesses of hybrids and the parental species. These fitness relationships determine the form of selection across the hybrid zone and, along with gene flow, the evolutionary dynamics and eventual outcome of natural hybridization. We measured a component of fitness, export and receipt of pollen in single pollinator visits, for hybrids between the herbaceous plants Ipomopsis aggregata and I. tenuituba and for both parental species. In aviary experiments with captive hummingbirds, hybrid flowers outperformed flowers of both parental species by receiving more pollen on the stigma. Although hummingbirds were more effective at removing pollen from anthers of I. aggregata, hybrid flowers matched both parental species in the amount of pollen exported to stigmas of other flowers. These patterns of pollen transfer led to phenotypic stabilizing selection, during that stage of the life cycle, for a stigma position intermediate between that of the two species and to directional selection for exserted anthers. Pollen transfer between the species was high, with flowers of I. aggregata exporting pollen equally successfully to conspecific and I. tenuituba flowers. Although this study showed that natural hybrids enjoy the highest quality of pollinator visits, a previous study found that I. aggregata receives the highest quantity of pollinator visits. Thus, the relative fitness of hybrids changes over the life cycle. By combining the results of both studies, pollinator-mediated selection in this hybrid zone is predicted to be strong and directional, with hybrid fitness intermediate between that of the parental species.
杂交区域模型在关于杂种和亲本物种相对适合度的假设上存在差异。这些适合度关系决定了整个杂交区域的选择形式，并且与基因流一起，决定了自然杂交的进化动态和最终结果。我们测量了草本植物聚合红钟花和细管红钟花之间的杂种以及两个亲本物种在单次传粉者访花时花粉的输出和接受情况，以此作为适合度的一个组成部分。在对圈养蜂鸟进行的鸟舍实验中，杂种花朵在柱头上接受了更多花粉，表现优于两个亲本物种的花朵。尽管蜂鸟从聚合红钟花的花药上移除花粉的效率更高，但杂种花朵向其他花朵柱头输出的花粉量与两个亲本物种相当。这些花粉转移模式在生命周期的那个阶段导致了表型稳定选择，使得柱头位置处于两个物种之间的中间状态，并导致对伸出花药的定向选择。物种间的花粉转移量很高，聚合红钟花的花朵向同种和细管红钟花的花朵成功输出花粉的情况相同。尽管这项研究表明自然杂种享有最高质量的传粉者访花，但之前的一项研究发现聚合红钟花接受传粉者访花的数量最多。因此，杂种的相对适合度在生命周期中会发生变化。通过结合两项研究的结果，预计在这个杂交区域中传粉者介导的选择将是强烈且定向的，杂种适合度介于亲本物种之间。
