Music that has sex

by Paul Curzon, Queen Mary University of London

Music is a creative part of our culture but what drives that creativity? Music changes over time as new sounds are invented and old ones fall from favour. Where does the creativity come from though? Is it from the 'creative people': the musicians? Or is it you and me that really drive the creative changes, just by the music we choose to listen to? Could a machine make creative new music we love without an actual composer? Researchers based in London and Japan teamed up to investigate. They did it by creating a Darwinian music engine - a musical machine that has sex!

Well ok. It doesn't really have sex, but it does do a software equivalent. Life evolved through natural selection and sex is central to that. When we have a baby, we are mixing up the genes of the parents giving that child qualities from both. Your DNA is a complex code storing all the information about how to make you. DNA determines the colour of your eyes, how many legs you have, even whether you have a shell or not! It is unique to you - no living thing is exactly the same as you so (twins aside), nothing else has exactly the same DNA to describe them. Sex is just a way to split two creature's DNA in half and pair up the two halves to make new DNA and so a new creature.

Natural selection then works because the fittest creatures, most able to survive the conditions they find themselves born into, survive to have babies of their own and so pass their DNA on. That DNA contains information about what made them special enough to do so well, increasing the chances that the baby does well too.

You can model this in a computer. First create a code to represent the properties of the thing you want to evolve - its 'genome'. It can just be a string of numbers, one for each property. Next, create a random set of these strings to start things off. Then the sex begins. Pairs of strings are split in half and joined back together. Extra random changes to the strings add mutation to the process. Test the newly created population, keeping the best and destroying the worst. For that you need a 'fitness' function to decide what is good and bad. In nature the fitness function is your ability not to die. In the software version it can be anything that captures your idea of good and bad.

The researchers investigating musical culture, did this: their 'creatures' were bits of sound. Their artificial 'DNA' - the strings of numbers - represented different music. Each encoded a computer program. When that program is executed it plays a short, seamlessly looping sound sequence. The genome/program determines things like where notes are placed and instrumentation, though other things like the tempo are identical for every loop. Sexual combination and mutation mimic the fusion of existing musical motifs, rhythms, and harmonies, and the invention of novel ones.

Their music engine, 'DarwinTunes', started with a group of short audio loops that played random noise. These loops paired up, sexually reproduced and mutated, creating new loops of music. They were left to evolve over 2,500 generations of musical 'creatures' with daughters replacing their parents on each round. The twist was that the selection was based on the likes and dislikes of thousands of people who rated the music clips for how much they liked them. Only the top 100 survived in any round.

The loops of random sounds quickly evolved into music. This was partly because pleasing chords and rhythms used in western music started to evolve. Later, however, the amount of evolution slowed and there was little improvement after 600 odd generations. This pattern of fast then slow evolution is actually seen in the real world: in the wild, the fossil record and in lab experiments. To work out what was going on with the music the team carried out other experiments using methods devised by biologists studying the evolution of bacteria. It turned out the slow-down was mostly because of a decrease in the accuracy of how the music was transmitted. A similar thing arises in early musical cultures when, as musicians learn existing complex musical themes they make mistakes so the original themes are lost. Once the evolution got to a certain point, favoured but complicated innovations were being lost, so improvements could no longer build well on those that came before.

The main aim of the experiment was to understand how musical culture develops. It also shows though that by using a Darwinian process machines can make pleasing music without a composer. However there is more to it than just natural selection - to be as creative as human composers driving musical cultural change something else is needed. It is more than just sex and mutation! Creative machines will need some other spark that composers have.