Abstract:

Population distribution of a species in space can be generally classified two types: discrete and continuous distribution, but the genetic variation underlying this physical distribution is very complicated. How does the genetic variation partition between and within populations? How can this genetic variation be maintained in real world? Thus, knowledge of population genetic structure may help us to understand the evolutionary process of the species and assist us in decision-making on conservation of genetic resources. So far insight into population genetic structure is very restricted, and theoretical studies are mainly confined to the three classical models of genetic structure: island model (Wright,1931), stepping-stone model (Kimura,1953) and isolation by distance model (Wright,1943). The first two models can be used to address the case of discrete distribution in space, and the third one to the case of continuous distribution. Since the introduction of these three models, many limitations involved in them have been relaxed and their variants have been developed and analyzed using a variety of statistical genetic methods. In this paper, the three models and their variants and cline theory, a specific genetic structure of populations in terms of the change of genetic variation with geographical distance, were remarked in detail, including their kernel ideas, application limitations and relaxation, development, and testing of our understanding using the naturally occurring genetic markers within taxa. In the end, the author highlighted the requirement of exploring appropriate theory suitable for genetic structure of plant populations because there are many obvious differences between plant and animal population genetic structure. Firstly, vectors of gene flow in plant species are different from those in animals. Secondly, the migration rate contained in the formulae of traditional population structure models cannot be substituted linearly by seed and pollen flow if rates of seed and pollen flow are not too small, which is highly likely in many plant species. Thirdly, population genetic structures of three plant genomes with contrasting modes of inheritance are different. Fourthly, differences in population structure among three plant genomes can provide important information on estimation of seed and pollen flow, on inferring colonization history, etc.. The integration of the impacts of seed and pollen flow with the interaction between cytonuclear genes probably can gain deep insight into the genetic structure of natural plant populations.

Key words: Population genetic structure, Island model, Stepping-stone model, Isolation by distance model, Cline theory