Oxford G S
Department of Biology (Area 18), University of York, P.O. Box 373, York YO10 5YW, United Kingdom.
Evolution. 2005 Oct;59(10):2170-84. doi: 10.1554/05-046.1.
The candy-stripe spider, Enoplognatha ovata, exhibits a striking color polymorphism comprising three morphs. A number of lines of evidence strongly suggest that this polymorphism is maintained by natural selection: its presence in a sister species, E. latimana; the physical nature of the variation; the virtual lack of monomorphic populations; the highly consistent rank-order of morphs within populations; and the presence of large-scale clines associated with climatic variables. However, the absence of selection is equally strongly suggested by very local surveys of morph frequencies over space and time, perturbation experiments, and a variance in morph frequency between populations that is virtually independent of spatial scale. In addition, local spatial patterns in one study site (Nidderdale, Yorkshire, England) have been explained in terms of intermittent drift over half a century ago, a hypothesis supported here by the distributions of four other genetic markers (two allozyme and two visible polymorphisms). A heuristic model is suggested that reconciles these apparently contradictory messages regarding the importance of drift and selection in this system. It is proposed that when allele frequencies of the color morph redimita lie between approximately 0.05 and 0.3, the deltaq on q plot is very shallow, so that within this region, where the majority of populations lie, selection is weak and drift is the major force determining local morph frequencies. However, outside this range of frequencies, powerful selection acts to protect the polymorphism. This model may apply to polymorphisms in other species and explain why evidence of selection in natural populations is often elusive.
花条蜘蛛（Enoplognatha ovata）表现出显著的颜色多态性，由三种形态组成。大量证据有力地表明，这种多态性是由自然选择维持的：它存在于姐妹物种E. latimana中；变异的物理性质；几乎不存在单态种群；种群内形态的高度一致的等级顺序；以及与气候变量相关的大规模渐变群的存在。然而，通过对形态频率在空间和时间上的非常局部的调查、扰动实验以及种群间形态频率的差异实际上与空间尺度无关，同样有力地表明不存在选择。此外，在一个研究地点（英国约克郡尼德代尔）的局部空间模式已根据半个多世纪前的间歇性漂变来解释，这里有另外四个遗传标记（两个等位酶和两个可见多态性）的分布支持这一假设。提出了一个启发式模型，该模型调和了关于该系统中漂变和选择重要性的这些明显矛盾的信息。有人提出，当颜色形态redimita的等位基因频率在大约0.05到0.3之间时，q值变化图上的deltaq非常浅，因此在这个大多数种群所在的区域内，选择作用较弱，漂变是决定局部形态频率的主要力量。然而，在这个频率范围之外，强大的选择作用来保护这种多态性。这个模型可能适用于其他物种的多态性，并解释了为什么在自然种群中选择的证据往往难以捉摸。
