This question came up during a class I took during my masters studies at the ETH Zurich. What is the maximum speed of evolution? On what factors does it depend? We discussed the idea that selection curbs the speed of evolution, acting as a brake by removing some phenotypes from the population and preventing evolution from continuing down those trajectories.
In that case, evolution without brakes means evolution in the absence of selection. All individuals reproduce in a statistically equal way, and individuals carrying mutations that in the real world would be lethal have just as many offspring as anyone else.
If we think of evolution in terms of travel through sequence space, then we can ask what affects the speed of that travel. Imagine that we start with a single reproducing genotype GREEN of length K in an environment that imposes no restrictions on population size or anything else.