Abstract:

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.

Key words: fitness, adaptation, modular hierarchy, genotypic selection, microevolution