Blain Stephanie A, Chavarie Louise, Kinney Mackenzie H, Schluter Dolph
Department of Zoology and Biodiversity Research Center University of British Columbia Vancouver British Columbia Canada.
Faculty of Environmental Sciences and Natural Resource Management Norwegian University of Life Sciences Ås Norway.
Ecol Evol. 2022 Apr 13;12(4):e8831. doi: 10.1002/ece3.8831. eCollection 2022 Apr.
A solitary population of consumers frequently evolves to the middle of a resource gradient and an intermediate mean phenotype compared to a sympatric pair of competing species that diverge to either side via character displacement. The forces governing the distribution of phenotypes in these allopatric populations, however, are little investigated. Theory predicts that the intermediate mean phenotype of the generalist should be maintained by negative frequency-dependent selection, whereby alternate extreme phenotypes are favored because they experience reduced competition for resources when rare. However, the theory makes assumptions that are not always met, and alternative explanations for an intermediate phenotype are possible. We provide a test of this prediction in a mesocosm experiment using threespine stickleback that are ecologically and phenotypically intermediate between the more specialized stickleback species that occur in pairs. We manipulated the frequency distribution of phenotypes in two treatments and then measured effects on a focal intermediate population. We found a slight frequency-dependent effect on survival in the predicted direction but not on individual growth rates. This result suggests that frequency-dependent selection might be a relatively weak force across the range of phenotypes within an intermediate population and we suggest several general reasons why this might be so. We propose that allopatric populations might often be maintained at an intermediate phenotype instead by stabilizing or fluctuating directional selection.
与通过性状替换向两侧分化的同域竞争物种对相比，孤立的消费者群体常常进化到资源梯度的中间位置，并具有中间平均表型。然而，支配这些异域种群中表型分布的力量却很少被研究。理论预测，广食性物种的中间平均表型应由负频率依赖选择维持，即交替的极端表型受到青睐，因为它们在稀有状态时对资源的竞争减少。然而，该理论做出的假设并不总是成立，中间表型也可能有其他解释。我们在一个中宇宙实验中对这一预测进行了检验，该实验使用了三刺鱼，它们在生态和表型上介于成对出现的更特化的刺鱼物种之间。我们在两种处理中操纵了表型的频率分布，然后测量了对一个焦点中间种群的影响。我们发现对生存有轻微的频率依赖效应，方向符合预测，但对个体生长率没有影响。这一结果表明，频率依赖选择在中间种群的表型范围内可能是一种相对较弱的力量，我们提出了几个可能导致这种情况的一般原因。我们认为，异域种群可能常常通过稳定或波动的定向选择而维持在中间表型。
