Martin Fowler on the will to design:
In order to work, evolutionary design needs a force that drives it to converge. This force can only come from people—somebody on the team has to have the determination to ensure that the design quality stays high.
This will does not have to come from everyone (although it's nice if it does), usually just one or two people on the team take on the responsibility of keeping the design whole. This is one of the tasks that usually falls under the term 'architect'.
This responsibility means keeping a constant eye on the code base, looking to see if any areas of it are getting messy, and then taking rapid action to correct the problem before it gets out of control. The keeper of the design doesn't have to be the one who fixes it—but they do have to ensure that it gets fixed by somebody.