Angert A L, Schemske D W
Department of Plant Biology, Michigan State University, East Lansing 48824, USA.
Evolution. 2005 Aug;59(8):1671-84.
Every species occupies a limited geographic area, but it remains unclear why traits that limit distribution do not evolve to allow range expansion. Hypotheses for the evolutionary stability of geographic ranges assume that species are maladapted at the range boundary and unfit beyond the current range, but this assumption has rarely been tested. To examine how fitness varies across species' ranges, we reciprocally transplanted two species of monkeyflowers, Mimulus cardinalis and M. lewisii, within and beyond their present elevation ranges. We used individuals of known parentage from populations collected across the elevation ranges of both species to examine whether populations are adapted to position within the range. For both species we found the greatest average fitness at elevations central within the range, reduced fitness at the range margin, and zero or near-zero fitness when transplanted beyond their present elevation range limits. However, the underlying causes of fitness variation differed between the species. At high elevations beyond its range, M. cardinalis displayed reduced growth and fecundity, whereas at low elevations M. lewisii experienced high mortality. Weak differences in performance were observed among populations within each species and these were not related to elevation of origin. Low fitness of both species at their range margin and weak differentiation among populations within each species suggest that adaptation to the environment at and beyond the range margin is hindered, illustrating that range margins provide an interesting system in which to study limits to adaptation.
每个物种都占据着有限的地理区域，但目前尚不清楚为何限制分布的性状没有进化以实现分布范围的扩展。关于地理分布范围进化稳定性的假说认为，物种在分布范围边界处适应不良，且在当前分布范围之外不适合生存，但这一假设很少得到验证。为了研究适合度如何在物种的分布范围内变化，我们在两种猴面花（Mimulus cardinalis和M. lewisii）当前海拔范围之内和之外进行了相互移栽实验。我们使用了从这两个物种整个海拔范围内收集的具有已知亲本的个体，来检验种群是否适应其在分布范围内的位置。对于这两个物种，我们发现分布范围中部海拔处的平均适合度最高，分布范围边缘处的适合度降低，而当移栽到当前海拔范围极限之外时，适合度为零或接近零。然而，这两个物种适合度变化的潜在原因有所不同。在其分布范围之外的高海拔地区，M. cardinalis表现出生长和繁殖力下降，而在低海拔地区，M. lewisii死亡率很高。在每个物种的种群之间观察到表现上的微弱差异，且这些差异与起源海拔无关。两个物种在其分布范围边缘处适合度较低以及每个物种内种群之间的微弱分化表明，对分布范围边缘及以外环境的适应受到阻碍，这说明分布范围边缘提供了一个研究适应极限的有趣系统。
