In attempt to clarify the significance of interspecific interactions in evolutionary ecology, the growth characteristics of bacterial populations sampled from ecological microcosms which act as fairly realistic models of natural ecosystems were investigated, with a particular emphasis on the ability of a system to remain reasonably stable in the genetic composition in spite of the occurrences of various mutants from native strains. Newly-emerged mutants in a community are inhibited in their multiplication, or excluded by a network of many elementary interactions between the different species of the populations, thus preserving the traits of the parental strains in a community. The interactions in the form of a network may be viewed as evidence for a maintenance of stability in a community.


(Communicated by Hiroo IMURA, M.J.A.)