A proposed model allowed the analysis of the competition for saprophytic living in soil and nodulation in the evolution of nodule bacteria (Rhizobium). The model involves parameters, which characterize these two types of the interstrain competitionin Rhizobium populations interacting with host plants. Variation in the parameters of saprophytic competitiveness was shown to ensure the formation of a genetically polymorphic bacterial population, but to have a little effect on the rate of its evolution. In contrast, variation in the parameters of nodulation competitiveness can alter the evolution rate of a Rhizobium population and increase its polymorphism owing to the reproduction of rare genotypes. Analysis of the model with the parameters of a legume-Rhizobium system similar to natural ones indicated that the interstrain competition for nodulation is a major factor ensuring the high variation of Rhizobium populations and the possibility for new forms of plant symbionts to appear in evolution.
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