repeating patterns are common due to segmentation and gene duplication. I suspect, and I think a lot of others do as well, that the appearance of complicated new shapes occurs fairly commonly after mutation, as a consequence of the hierarchical structure of developmental gene regulatory networks and the process of cellular differentiation. In a sense, it could be there is a collection of "modules", and successful mutations involve mostly manipulation of the regulatory network of those modules, while the modules themselves don't change very often at all (at least in part because they have so many dependent processes). By combining expression of those modules with spatial patterning.