Data from: Genetic and morphological differentiation in Populus nigra L.: isolation by colonization or isolation by adaptation?

Identifying processes underlying the genetic and morphological differences
among populations is a central question of evolutionary biology. Forest
trees typically contain high levels of neutral genetic variation, and
genetic differences are often correlated with geographic distance between
populations [isolation by distance (IBD)] or are due to historic
vicariance events [isolation by colonization (IBC)]. In contrast,
morphological differences are largely due to local adaptation. Here, we
examined genetic (microsatellite) and morphological (from a common garden
experiment) variation in Populus nigra L., European black poplar,
collected from 13 sites across western Europe and grown in a common garden
in Belgium. Significant genetic differentiation was observed, with
populations from France displaying greater admixture than the distinct
Spanish and central European gene pools, consistent with previously
described glacial refugia (IBC). Many quantitative traits displayed a
bimodal distribution, approximately corresponding to small-leaf and
large-leaf ecotypes. Examination of nine climatic variables revealed the
sampling locations to have diverse climates, and although the correlation
between morphological and climatic differences was significant, the
pattern was not consistent with strict local adaptation. Partial Mantel
tests based on multivariate summary statistics identified significant
residual correlation in comparisons of small-leaf to large-leaf ecotypes,
and within the small-leaf samples, but not within large-leaf ecotypes,
indicating that variation within the small-leaf morphotype in particular
may be adaptive. Some small-leaf populations experience climates very
similar to those in large-leaf sites. We conclude that adaptive
differentiation and persistent IBC acted in combination to produce the
genetic and morphological patterns observed in P. nigra.

解析种群间遗传与形态差异的内在机制，是进化生物学的核心研究议题。林木通常携带高水平的中性遗传变异，种群间的遗传差异常与地理距离呈相关性[即距离隔离（isolation by distance, IBD）]，或由历史地理隔离事件所致[即定植隔离（isolation by colonization, IBC）]。与之相对，形态差异主要由局部适应所驱动。
本研究以欧洲黑杨（Populus nigra L.）为研究对象，对其遗传（微卫星（microsatellite）标记）与形态（来自共同花园试验（common garden experiment））变异展开分析。研究共采集西欧13个样点的材料，并将其种植于比利时的共同花园中。
研究观测到显著的遗传分化，法国种群相较于特征鲜明的西班牙与中欧基因库，表现出更高的遗传混合度，这与此前报道的冰期避难所（IBC）模式相一致。诸多数量性状呈现双峰分布，大致对应小叶生态型与大叶生态型。
对9项气候变量的分析显示，采样样点的气候类型多样；尽管形态差异与气候差异间存在显著相关性，但该模式并不符合严格的局部适应。基于多变量汇总统计量的偏曼特尔检验（partial Mantel test）显示，小叶与大叶生态型间的比较以及小叶种群内部均存在显著的剩余相关性，而大叶种群内部则未观测到该现象，这表明尤其是小叶形态型内的变异可能具有适应性。部分小叶种群所处的气候与大叶种群样点的气候极为相似。
本研究认为，适应性分化与持续的定植隔离（IBC）共同作用，塑造了欧洲黑杨中观测到的遗传与形态格局。