Evolvability of switch networks

In addition to having a coexpression behaviour which is easily tunable by one parameter, the generic bHLH network studied here has the advantage of being perhaps more easily evolvable than a network in which every element actively represses all others: the interaction needs only take place between every element and a common activator (which requires $n$ interactions, instead of $n \left(n-1\right)/2$). bHLH networks have been suggested to evolve mainly by single-gene duplication events (Amoutzias et al., 2004), maintaining a topology in which every element of the network interacts with a restricted number of "hubs".