  In many real-world systems, such as adaptive robotics, achieving a single,
optimised solution may be insufficient. Instead, a diverse set of
high-performing solutions is often required to adapt to varying contexts and
requirements. This is the realm of Quality-Diversity (QD), which aims to
discover a collection of high-performing solutions, each with their own unique
characteristics. QD methods have recently seen success in many domains,
including robotics, where they have been used to discover damage-adaptive
locomotion controllers. However, most existing work has focused on single-agent
settings, despite many tasks of interest being multi-agent. To this end, we
introduce Mix-ME, a novel multi-agent variant of the popular MAP-Elites
algorithm that forms new solutions using a crossover-like operator by mixing
together agents from different teams. We evaluate the proposed methods on a
variety of partially observable continuous control tasks. Our evaluation shows
that these multi-agent variants obtained by Mix-ME not only compete with
single-agent baselines but also often outperform them in multi-agent settings
under partial observability.
