表型可塑性是指生物个体生长发育过程中遭受不同环境条件作用时产生不同表型的能力。进化的发生有赖于自然选择对种群遗传可变性产生的效力以及各基因型的表型可塑性。有足够的证据说明表型可塑性的可遗传性,它实际上是进化改变的一个成分。一般通过优化模型、数量遗传模型和配子模型来研究表型可塑性的进化。植物的构型是相对固定的,并未完全抑制表型可塑性。克隆植物因其双构件性而具有更广泛的、具有重要生态适应意义的表型可塑性。构件性使克隆植物具有以分株为基本单位的等级结构,从而使克隆植物的表型选择也具有等级性。构件等级一般包含基株、克隆片段或分株系统以及分株3个典型水平。目前认为克隆植物的自然选择有两种模式,分别以等级选择模型和基因型选择模型表征。等级选择模型认为:不同的等级水平同时也是表型选择水平,环境对各水平具有作用,各水平之间也有相互作用,多重表型选择水平的净效应最终通过繁殖水平——分株传递到随后的世代中。基因型选择模型指出:克隆生长引起分株的遗传变异,并通过基株内分株间以及基株间的非随机交配引起种子库等位基因频率的改变,产生微进化。这两种选择模式均突出强调了分株水平在自然选择过程中的变异性以及在进化中的重要性,强调了克隆生长和种子繁殖对基株适合度的贡献。基因型选择模型包含等级选择模型的观点,是对等级选择模型的重要补充。克隆植物的表型可塑性表现在3个典型等级层次上,由于各层次对自然选择压力具有不同的反应,其表型变异程度一般表现出“分株层次>分株片段层次>基株层次”的等级性反应模式。很多证据表明,在构件有机体中构件具有最大的表型可塑性,植物的表型可塑性实际上是构件而非整个遗传个体的反应。这说明克隆植物的等级反应模式可能具有普适性。如果该反应模式同时还是构件等级中不同“个体”适应性可塑性反应的模式,那么可以预测:1)在克隆植物中,分株层次受到的自然选择强度也最大,并首先发生适应性可塑性变化,最终引起克隆植物微进化;2)由于较弱的有性繁殖能力,克隆植物在进化过程中的保守性可能大于非克隆植物。克隆植物等级反应模式的普适性亟待验证。

Phenotypic plasticity is the ability of a genotype to produce distinct phenotypes when exposed to different environments during its ontogeny. There is new strong evidence for the heritable nature of phenotypic plasticity, and variation in plasticity should thus be recognized as an integral component of evolution, but not as a factor that buffers and thereby constrains the action of selection. Clonal plants have both clonal modularity and organismic modularity. As a result, phenotypic selection in clonal plants is hierarchical. Modular hierarchy of clonal plants consists of three levels, the genet, ramet fragment and ramet levels. Modes of hierarchical selection and genotypic selection have been considered in studying natural selection in clonal plants. The hierarchical selection model treats each hierarchical level as a single level of phenotypic selection, and selection effects are determined by both the selective pressure on each level and ioteractions among levels. Net effects of multi-level phenotypic selection are transmitted finally to the succeeding generations through the ramet level. The genotypic selection model puts forward that clonal growth can lead to genetic variances between ramets within a genet. Then allele frequencies will change because of non-random mating within or among genets, leading to microevolution. In clonal plants, ramets may have the greatest phenotypic variation because it is the fundamental unit of sexual and asexual reproduction, whereas genets may have the lowest phenotypic variation because it is a relative stable unit. It is termed as the mode of hierarchical responses of phenotypic plasticity. The module of a plant has the greatest phenotypic plasticity, and phenotypic plasticity is not a whole-plant response but a property of module triggered by local environmental conditions. Therefore, the mode of hierarchical responses of phenotypic plasticity in clonal plants may be universal. If the mode indicates a response mode of adaptive plasticity of different `individuals' in modular hierarchy, we can predict that: 1) the effects of natural selection on clonal plants should be shown first at ramet level and will ultimately result in microevolution as predicted by both hierarchical selection and genotypic selection models and 2) the conservativeness in evolutionary changes may be greater in clonal plants than in non-clonal plants because of lower ability of sexual reproduction and high degree of physiological integration. The most promising directions for future research include areas such as those demonstrating the universality of hierarchical responses to natural selection in clonal plants and revealing the mechanisms of hierarchical responses.