Nosil P, Crespi B J
Department of Biosciences, Behavioral Ecology Research Group, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada.
Evolution. 2004 Jan;58(1):102-12. doi: 10.1111/j.0014-3820.2004.tb01577.x.
Population differentiation often reflects a balance between divergent natural selection and the opportunity for homogenizing gene flow to erode the effects of selection. However, during ecological speciation, trait divergence results in reproductive isolation and becomes a cause, rather than a consequence, of reductions in gene flow. To assess both the causes and the reproductive consequences of morphological differentiation, we examined morphological divergence and sexual isolation among 17 populations of Timema cristinae walking-sticks. Individuals from populations adapted to using Adenostoma as a host plant tended to exhibit smaller overall body size, wide heads, and short legs relative to individuals using Ceonothus as a host. However, there was also significant variation in morphology among populations within host-plant species. Mean trait values for each single population could be reliably predicted based upon host-plant used and the potential for homogenizing gene flow, inferred from the size of the neighboring population using the alternate host and mitochondrial DNA estimates of gene flow. Morphology did not influence the probability of copulation in between-population mating trials. Thus, morphological divergence is facilitated by reductions in gene flow, but does not cause reductions in gene flow via the evolution of sexual isolation. Combined with rearing data indicating that size and shape have a partial genetic basis, evidence for parallel origins of the host-associated forms, and inferences from functional morphology, these results indicate that morphological divergence in T. cristinae reflects a balance between the effects of host-specific natural selection and gene flow. Our findings illustrate how data on mating preferences can help determine the causal associations between trait divergence and levels of gene flow.
种群分化通常反映了趋异自然选择与使基因流同质化以削弱选择效应的机会之间的平衡。然而，在生态物种形成过程中，性状分化导致生殖隔离，并成为基因流减少的原因而非结果。为了评估形态分化的原因及其生殖后果，我们研究了17个克氏蒂美步甲种群之间的形态分化和性隔离。相对于以加州鼠李为寄主植物的个体，适应以腺叶石楠为寄主植物的种群个体往往整体体型较小、头部较宽且腿部较短。然而，寄主植物物种内的种群之间在形态上也存在显著差异。根据所使用的寄主植物以及从使用替代寄主的相邻种群大小和基因流的线粒体DNA估计推断出的使基因流同质化的可能性，可以可靠地预测每个单一种群的平均性状值。在种群间交配试验中，形态并未影响交配的概率。因此，基因流的减少促进了形态分化，但形态分化并未通过性隔离的进化导致基因流的减少。结合表明大小和形状具有部分遗传基础的饲养数据、寄主相关形态平行起源的证据以及功能形态学的推断，这些结果表明克氏蒂美步甲的形态分化反映了寄主特异性自然选择和基因流效应之间的平衡。我们的研究结果说明了关于交配偏好的数据如何有助于确定性状分化与基因流水平之间的因果关联。
