Schluter Dolph
Zoology Department and Centre for Biodiversity Research, University of British Columbia, Vancouver, British Columbia V6T 1Z4 Canada.
Evolution. 2003 May;57(5):1142-50. doi: 10.1111/j.0014-3820.2003.tb00323.x.
We know little about how natural selection on a species is altered when a closely related species consuming similar resources appears in its environment. In a pond experiment with threespine sticklebacks I tested the prediction that divergent natural selection between competitors is frequency-dependent, changing with the distribution of phenotypes in the environment. Differential growth and survival of phenotypes in a target stickleback population were contrasted between two treatments. In one treatment an offshore zooplankton feeder (the limnetic stickleback species) was added to the same pond as the target. In the other treatment I added the benthic stickleback instead, a species adapted to feeding on invertebrates from sediments and inshore vegetation. The target population was ecologically and morphologically intermediate with phenotypic variance artificially inflated by hybridization. Growth rates of phenotypes within the target population differed between treatments as predicted by character displacement. The impact of adding a second species always fell most heavily on those phenotypes in the target population resembling the added species most closely. However, those individuals in the target population that most resembled the added species did not experience reduced survival. Instead, consistent survival differences between populations suggested the presence of an inshore-offshore gradient in mortality risk. These results provide further support for the hypothesis of character displacement in sympatric sticklebacks. They suggest that displacement along the resource gradient also led to divergence in vulnerability to agents of mortality, probably including predation.
当一个消耗相似资源的近缘物种出现在某一物种的环境中时，我们对该物种的自然选择是如何改变的知之甚少。在一项针对三刺鱼的池塘实验中，我检验了这样一个预测：竞争者之间的趋异自然选择是频率依赖的，会随着环境中表型的分布而变化。在两种处理方式下，对比了目标刺鱼种群中不同表型的生长和存活情况。在一种处理方式中，将一种以离岸浮游动物为食的鱼（即湖沼型刺鱼物种）与目标鱼放入同一个池塘。在另一种处理方式中，我放入了底栖型刺鱼，该物种适应于以沉积物和近岸植被中的无脊椎动物为食。目标种群在生态和形态上处于中间状态，其表型变异通过杂交被人为扩大。正如性状替换所预测的那样，目标种群中不同表型的生长速率在不同处理方式下存在差异。添加第二个物种的影响总是对目标种群中与添加物种最相似的那些表型最为严重。然而，目标种群中那些与添加物种最相似的个体并没有经历存活率的降低。相反，种群之间一致的存活差异表明在死亡风险方面存在近岸 - 离岸梯度。这些结果为同域分布的刺鱼中性状替换的假说提供了进一步的支持。它们表明沿着资源梯度的替换也导致了对死亡因素（可能包括捕食）的易感性差异。
