TWO PHASES. “The first, innovative phase of evolution is temporally brief, engenders dramatic genomic and phenotypic perturbations, and is linked to population bottlenecks. The second, reductive phase that represents “evolution as usual” is protracted in time, is facilitated by the deletion bias that seems to be a general feature of genome evolution.”
http://onlinelibrary.wiley.com/doi/10.1002/bies.201300037/full