Altenberg, L.

his special issue focuses on two emergent properties of artificial evolutionary systems: evolvability and robustness. The three papers presented here expand the subject in distinct directions—two dealing with robustness, and one with evolvability. Short- and long-term evolution depends on the variational properties of the systems: how changes to their structure map to changes in their behavior. Robustness and evolvability are key variational properties that themselves show evolutionary dynamics. This journal’s focus is on artificial evolutionary systems that are active—which take inputs from their environment and act on them to produce their behavior. Active systems are an especially rich domain for the evolution of robustness and evolvability because the size and structure of their data objects are often open-ended.