Data from: Quantitative genetic variance and multivariate clines in the Ivyleaf morning glory, Ipomoea hederacea

Clinal variation is commonly interpreted as evidence of adaptive differentiation, although clines can also be produced by stochastic forces. Understanding whether clines are adaptive therefore requires comparing clinal variation to background patterns of genetic differentiation at presumably neutral markers. Although this approach has frequently been applied to single traits at a time, we have comparatively fewer examples of how multiple correlated traits vary clinally. Here, we characterize multivariate clines in the Ivyleaf morning glory, examining how suites of traits vary with latitude, with the goal of testing for divergence in trait means that would indicate past evolutionary responses. We couple this with analysis of genetic variance in clinally varying traits in 20 populations to test whether past evolutionary responses have depleted genetic variance, or whether genetic variance declines approaching the range margin. We find evidence of clinal differentiation in five quantitative traits, with little evidence of isolation by distance at neutral loci that would suggest non-adaptive or stochastic mechanisms. Within and across populations, the traits that contribute most to population differentiation and clinal trends in the multivariate phenotype are genetically variable as well, suggesting that a lack of genetic variance will not cause absolute evolutionary constraints. Our data are broadly consistent theoretical predictions of polygenic clines in response to shallow environmental gradients. Ecologically, our results are consistent with past findings of natural selection on flowering phenology, presumably due to season-length variation across the range.渐变变异（clinal variation）通常被解读为适应性分化的证据，尽管渐变群（clines）也可由随机演化作用力产生。因此，要明确渐变群是否具有适应性，需将渐变变异与假定中性标记（neutral markers）下的遗传分化本底模式进行比对。尽管该方法常被单次应用于单一性状，但关于多组相关性状的渐变变异模式的研究案例相对匮乏。本研究以常春藤牵牛花（Ivyleaf morning glory）为研究对象，刻画其多变量渐变群特征，探究性状组合随纬度的变化规律，旨在检验性状均值的分化情况——此类分化可指示过往的演化响应。我们结合20个种群中渐变变异性状的遗传方差分析，以此检验两类假说：一是过往的演化响应是否已耗尽遗传方差，二是遗传方差是否会随着种群趋近分布范围边缘而下降。本研究在5个数量性状（quantitative traits）中发现了渐变分化的证据，而中性基因座（neutral loci）上几乎未检测到距离隔离（isolation by distance）的信号——此类信号本可提示非适应性或随机性的演化机制。在种群内部与种群间，对多变量表型的种群分化及渐变趋势贡献最大的性状同样具有遗传变异，这表明遗传方差的缺失不会造成绝对的演化限制。我们的数据与多基因渐变群响应微弱环境梯度的理论预测大体相符。从生态学视角来看，本研究结果与过往关于开花物候（flowering phenology）受自然选择作用的研究发现一致，该现象推测与本研究分布范围内的季长差异有关。