Adaptation without specialization early in a host shift: fecundity data

Students of speciation debate the role of performance trade-offs across different environments early in speciation. We tested for early performance trade-offs with a host shift experiment using a member of the Enchenopa binotata species complex of treehoppers (Hemiptera: Membracidae). In this clade of plant-feeding insects, different species live on different host plants and exhibit strong behavioral and physiological host specialization. After five generations, the experimental host shifts resulted either in no adaptation or in adaptation without specialization. The latter result was more likely in sympatry; in allopatry, populations on novel host plants were more likely to go extinct. We conclude that in the early stages of speciation, adaptation to novel host plants does not necessarily bring about performance trade-offs on ancestral environments. Adaptation may be facilitated rather than hindered by gene flow, which prevents extinction. Additional causes of specialization and assort...物种形成研究领域的学者们长期争论，物种形成早期不同环境间的性能权衡（performance trade-offs）究竟发挥着怎样的作用。本研究以二斑恩蝉（*Enchenopa binotata*）物种复合体中的一员角蝉为实验对象，该类群隶属于半翅目（Hemiptera）角蝉科（Membracidae），通过寄主转移（host shift）实验检验早期性能权衡效应。该植食性昆虫类群内的不同物种分别栖息于专属寄主植物，并表现出强烈的行为与生理寄主特化现象。经过五代实验周期后，实验性寄主转移最终仅出现两种结果：未引发适应性演化，或仅形成适应性却未产生寄主特化。后一种结果在同域分布（sympatry）条件下更易出现；而在异域分布（allopatry）条件下，寄居于新型寄主植物的种群更易走向灭绝。本研究得出结论：在物种形成的早期阶段，对新型寄主植物的适应性演化未必会导致祖先寄主环境下的性能权衡。基因流（gene flow）可通过阻止种群灭绝来促进适应性演化，而非阻碍这一过程。寄主特化与选型……