Nosil P, Sandoval C P, Crespi B J
Department of Biosciences, Simon Fraser University, Burnaby BC, Canada.
J Evol Biol. 2006 May;19(3):929-42. doi: 10.1111/j.1420-9101.2005.01035.x.
Divergent habitat preferences can contribute to speciation, as has been observed for host-plant preferences in phytophagous insects. Geographic variation in host preference can provide insight into the causes of preference evolution. For example, selection against maladaptive host-switching occurs only when multiple hosts are available in the local environment and can result in greater divergence in regions with multiple vs. a single host. Conversely, costs of finding a suitable host can select for preference even in populations using a single host. Some populations of Timema cristinae occur in regions with only one host-plant species present (in allopatry, surrounded by unsuitable hosts) whereas others occur in regions with two host-plant species adjacent to one another (in parapatry). Here, we use host choice and reciprocal-rearing experiments to document genetic divergence in host preference among 33 populations of T. cristinae. Populations feeding on Ceanothus exhibited a stronger preference for Ceanothus than did populations feeding on Adenostoma. Both allopatric and parapatric pairs of populations using the different hosts exhibited divergent host preferences, but the degree of divergence tended to be greater between allopatric pairs. Thus, gene flow between parapatric populations apparently constrains divergence. Host preferences led to levels of premating isolation between populations using alternate hosts that were comparable in magnitude to previously documented premating isolation caused by natural and sexual selection against migrants between hosts. Our findings demonstrate how gene flow and different forms of selection interact to determine the magnitude of reproductive isolation observed in nature.
不同的栖息地偏好会促进物种形成，这在植食性昆虫对寄主植物的偏好中已有观察。寄主偏好的地理变异能为偏好进化的原因提供见解。例如，只有当当地环境中有多种寄主时，针对不适应的寄主转换的选择才会发生，并且这可能导致在有多种寄主与只有单一寄主的区域之间出现更大的分化。相反，即使在使用单一寄主的种群中，寻找合适寄主的成本也可能导致偏好的产生。克氏蒂美弄蝶（Timema cristinae）的一些种群出现在仅有一种寄主植物的区域（异域分布，被不适合的寄主包围），而其他种群则出现在两种寄主植物相邻的区域（邻域分布）。在这里，我们通过寄主选择和互养实验，记录了克氏蒂美弄蝶33个种群在寄主偏好上的遗传分化。取食鼠李属植物的种群对鼠李属植物的偏好强于取食腺柳属植物的种群。使用不同寄主的异域和邻域种群对都表现出不同的寄主偏好，但异域种群对之间的分化程度往往更大。因此，邻域种群之间的基因流动显然限制了分化。寄主偏好导致使用不同寄主的种群之间的交配前隔离水平，其程度与先前记录的因对寄主间迁移者的自然选择和性选择导致的交配前隔离相当。我们的研究结果表明基因流动和不同形式的选择如何相互作用，以确定在自然界中观察到的生殖隔离程度。
