A remark on evolutionary systems

Evolutionary systems have been introduced by Csuhaj-Varjú and Mitrana (Acta Inform. 36 (2000) 913) who proved that two context-sensitive or three context-free components are sufficient to obtain all recursively enumerable languages. We improve these results by showing that two context-free components are sufficient to generate all recursively enumerable languages. Furthermore, we study the power of systems with one component.