Everything runs in this browser. Nothing is uploaded anywhere.
What you are hearing is the best of the population, played live: each upcoming note is read from the current best genome the moment it is scheduled, so the piece corrects itself under your ears as evolution runs, and keeps playing once it has converged.
First minute evolves the opening minute of the file, chords and all — a few hundred genes, roughly two minutes at Normal speed. First two minutes is about three times the work.
The two rolls. Above, the section from your MIDI, every voice, withheld until you reveal it. Below, the best candidate. Each gene is one note: pitch, length, loudness, and the wait until the next note starts — zero means “together with the next one”, which is how a chord is written in the genome. A note prints in black when all four are exactly right and stays red while it is wrong. Nothing is ever copied from the target; candidates only receive a score, and the better ones become parents.