Griffith Timothy M, Watson Maxine A
Department of Biology, Indiana University, Bloomington, Indiana 47405, USA.
Am Nat. 2006 Feb;167(2):153-64. doi: 10.1086/498945. Epub 2005 Dec 14.
Ecologists often consider how environmental factors limit a species' geographic range. However, recent models suggest that geographic distribution also may be determined by a species' ability to adapt to novel environmental conditions. In this study, we empirically tested whether further evolution would be necessary for northern expansion of the weedy annual cocklebur (Xanthium strumarium) in its native North American range. We transplanted seedlings beyond the northern border and photoperiodically manipulated reproductive timing, a trait important for adaptation to shorter growing seasons at higher latitudes within the range, to determine whether further evolution of this trait would result in a phenotype viable beyond the range. Earlier reproductive induction enabled plants to produce mature seeds beyond the range and to achieve a reproductive output similar to those grown within the range. Therefore, evolution of earlier reproduction in marginal populations would be necessary for northward range expansion. This study is the first to empirically show that evolution in an ecologically important trait would enable a species to survive and reproduce beyond its current range. These results suggest that relatively few traits may limit a species' range and that identifying evolutionary constraints on such traits could be important for predicting geographic distribution.
生态学家常常思考环境因素如何限制物种的地理分布范围。然而，最近的模型表明，地理分布也可能由物种适应新环境条件的能力所决定。在本研究中，我们通过实证检验了，在其原生北美分布范围内，杂草一年生苍耳（苍耳属）向北扩张是否需要进一步进化。我们将幼苗移植到分布范围的北界之外，并对繁殖时间进行光周期调控，繁殖时间是该物种适应分布范围内较高纬度地区较短生长季节的一个重要性状，以此来确定该性状的进一步进化是否会产生一种在分布范围之外也能存活的表型。更早的繁殖诱导使植物能够在分布范围之外产生成熟种子，并实现与在分布范围内生长的植物相似的繁殖产出。因此，边缘种群中更早繁殖的进化对于向北的分布范围扩张是必要的。本研究首次通过实证表明，一个具有生态重要性的性状的进化将使一个物种能够在其当前分布范围之外生存和繁殖。这些结果表明，相对较少的性状可能会限制物种的分布范围，并且识别此类性状的进化限制对于预测地理分布可能很重要。
