Free the Red Queen

When you design an evolutionary algorithm(EA) you are approaching evolution in a different way than how evolution works in nature. Nature doesn't use evolution to solve a problem, evolution is an inherent property of life.

This difference is very apparent when you consider parasites and co-evolution in general. In nature parasites are something bad that organisms need to be able to counter. Matt Ridley devised the Red Queen hypothesis with regard to this. The Red Queen concept is taken from Alice in Wonderland in which Alice meets the Red Queen. The Red Queen is running as fast as she can all the time. The unfortunate Queen is under the assumption that she must run as fast as she can in order to stand still. This is how evolution works in nature. There are several levels of evolution. You have to be able to survive and counter the static effects of nature. Secondly you have to be able to cope with the changing effects, you have to be able to adapt. Thirdly it is beneficial to be able to adapt to adapting counterparts like parasites. You see the point, this reasoning never stops and we are facing the same problem as the Red Queen. Evolution guides us to be able to better take advantage of and counter, evolution.

So designing an EA you have to design an artificial environment. The easiest way is to have a static environment. The function measuring the fitness is set beforehand and is equal from generation to the next. The next stage is to introduce competition among individuals in the population. This is good if you don't have a good understanding about what the goal actually is, but you know that there must be a best way to do it, like playing a game of chess. Here many designers of EA meet a tough challenge. They see that their population converge too fast. After a while all the individuals look the same and only mutations can make progress to better fitness. This is very suboptimal. So how can you keep you artificial population diverse?

Enter the Red Queen. You introduce parasites. Here parasites are good, they force the population to be diverse. The parasites quickly adapt to the host population and prey on the majority. If one parasite evolves that is able to prey on a major sub-population his children will have much easier to find suitable hosts and so they have a clear benefit.

Introducing parasites makes the fitness landscape change. Parasites are able to make valleys less deep and mountains less high in an ever changing way. They can never really turn the landscape inside out but they can make it much easier for sub-populations to escape a local minima and they keep the whole population from settling in one valley.

So, should you introduce the parasites in the same way as you design the fitness landscape? Ideally no. In nature the parasites appear just by themselves because the Red Queen is loose. Co-evolution sometimes give rise to symbiosis like in the case of the mitochondria and sometime give rise to parasites. Ideally you should set the Red Queen free.