[Competitive selection in simple systems].

1. The types of selection in simple population systems (haploid or asexual populations without specific ecological interaction), composed of two components, are analysed according to their mode of reproduction. The terms "expansive", "competitive", and "restrictive" selection are suggested for them. 2. A general mathematical model to describe selection processes in simple systems independently of the mode of reproduction is defined. It relates the parameters of reproductivity, natality, and mortality, selective value, population number, and frequency. 3. The conditions under which the selective value remains constant were investigated. It was shown by the Lottka-Volterra equations that this is only to be expected for expansive and simple competitive selection in identical ecological niches. 4. A simple bacterial system, composed of two E. coli strains, was studied experimentally. It was found, that the selective value undergoes rapid changes during transition from expansive to competitive selection and also during simple competitive selection. Adequate qualitative predictions and some approximate quantitative predictions of these changes could be made by analysis of the factors influencing selection. Effective factors were: natality, mortality (survival factor), carrying capacity of the environment, adaptation and niche differences. The niche differences appear to be due to different oxygen requirements and led to coexistence of the two strains. 5. The results are discussed with regard to the competitive exclusion principle.