Wright's shifting balance theory and the diversification of aposematic signals

Despite accumulating evidence for selection within natural systems, the importance of random genetic drift opposing Wright's and Fisher's views of evolution continue to be a subject of controversy. The geographical diversification of aposematic signals appears to be a suitable system to assess the factors involved in the process of adaptation since both theories were independently proposed to explain this phenomenon. In the present study, the effects of drift and selection were assessed from population genetics and predation experiments on poison-dart frogs, Ranitomaya imitator, of Northern Peru. We specifically focus on the transient zone between two distinct aposematic signals. In contrast to regions where high predation maintains a monomorphic aposematic signal, the transient zones are characterized by lowered selection and a high phenotypic diversity. As a result, the diversification of phenotypes may occur via genetic drift without a significant loss of fitness. These new phenotypes may then colonize alternative habitats if successfully recognized and avoided by predators. This study highlights the interplay between drift and selection as determinant processes in the adaptive diversification of aposematic signals. Results are consistent with the expectations of the Wright's shifting balance theory and represent, to our knowledge, the first empirical demonstration of this highly contested theory in a natural system.

尽管学界已积累了大量关于自然系统中自然选择作用的实证证据，但与莱特（Wright）和费希尔（Fisher）的进化观相悖的随机遗传漂变（random genetic drift）的重要性，至今仍是颇具争议的核心议题。警戒色信号（aposematic signals）的地理分化现象，恰好是评估适应过程相关影响因素的理想模型系统，因为上述两种进化理论均被独立提出以解释该现象。本研究针对秘鲁北部的模仿箭毒蛙（Ranitomaya imitator）开展种群遗传学分析与捕食实验，以量化评估遗传漂变与自然选择的效应。我们特别聚焦于两种截然不同的警戒色信号之间的过渡区域。与那些捕食压力较高、维持单型警戒色信号的区域不同，该过渡区域的特征为选择压较低且表型多样性较高。因此，表型分化可能在无需显著适合度（fitness）损失的情况下通过遗传漂变得以发生。若这些新生表型能被捕食者成功识别并规避，便可能在替代生境中完成定植。本研究强调了遗传漂变与自然选择之间的相互作用，将其视为警戒色信号适应性分化的决定性过程。研究结果与莱特移位平衡理论（Wright's shifting balance theory）的预期完全相符，据我们所知，这也是首个在自然系统中证实这一极具争议性理论的实证研究。