We present a suite of tools under development for music creation and performance which combines an evolving music generation algorithm with an evolving motion-sensitive interface. The system can be used as a dance-driven musical instrument that allows an immediate physical interaction with music, because the evolutionary algorithm generates live code, that evolves during the performance. The classification of the physical moves required to control the music also evolve, leading to a performance both in sound and in motion. To reach this goal we introduce "commensal computation", a way of organising artificially evolving system inspired by symbiotic relationships observed in biology. As a consequence the system is split in two 'organisms': a 'host' which generates music from input moves; and a 'symbiont' which simplifies the representation of the input moves for the host. We describe how it allows evolution, and sketch the artistic possibilities.