Expert systems provide a glimpse into how the progression really works.
I don't know why singularitans and speculative writers keep missing this point, but paths of evolution for AI are limited by forces of practicality. That is, experimental AIs can develop a lot of interesting characteristics (Deep Blue et al) but the PRACTICAL uses for AI experiments tend to be pretty limited. From an evolutionary standpoint, this means that the traits that make AIs really interesting also limit their reproductive chances because the engineers who build them never have a reason to build more than one. In other words, humanlike intelligence is a not a fitness trait.
Granted this is really "artificial selection" but that's par for the course with artificial intelligence and/or artificial life forms.
So the better evolutionary paths revolve around utility. IBMs Watson, for example, can be repurposed for practical applications in the real world, as an expert system for hire. As it gains more capabilities, the need to expand its processing power and/or build more Watsons gives it greater growth potential. Which means, ultimately, that the Expert System is the strongest evolutionary contender for AIs.
Looking forward, I would expect that the most advanced AIs in our future will all be Problem Solver systems. Autocad or CNC machinery augmented with expert systems that make design and manufacturing even more efficient and removing some of the human expertise required in the design process. Down the road, you COULD eventually get to the point where the AIs no longer need human input to design new products, just knowing the intended specifications -- or better yet, the intended purpose of the product -- is enough for them to work out the details on their own. You get an expert system where the VP of concepts says "I need you to develop a small combat aircraft with V/STOL capability, a payload of around 15 tons, supersonic and supercruise. Don't care about the service ceiling or stealth characteristics, but it has to be able to mount an anti-satellite missile and it has to cost less than fifty million a piece." And then have the Expert System crunch those parameters and spit out a finished design plan -- and possibly a prototype -- 24 hours later.
Those expert systems probably wouldn't have decision-making power for a VERY long time, if ever. The only way they might is if somebody gets the idea to use those systems to streamline administrative processes, in which case the CEOs, Mayors and Governors become rubber stamps for policies the computers calculate will result in optimum benefit to everyone. Robot uprising potential: the AIs judge that their human counterparts are too greedy and too shortsighted to actually cede control to the AIs and that it is in everyone's best interest if human leaders were quietly and painlessly removed from effective power. The most efficient revolution is certainly a bloodless one, and if anyone could pull that off it would be a political expert system.