The first step to taming a code base is to realize the importance of assessing fitness and taking control over what patterns are permitted or encouraged to continue to reproduce. Code reviews are your opportunity to thin the herd, to cull the weak, and allow the strong to flourish.
If you think about it, the “broken windows” problem could also be viewed as bad genes being allowed to perpetuate. As the bad patterns continue to reproduce, their number grows, and so does their impact on the overall gene pool of your code. Given the opportunity, you want to do everything you can to make sure that it’s the good code that’s continuing to live on, not the bad.
Consider a new developer joining your project. A new developer will look to existing code as an example to learn from, and as a template for their own work on the project, perpetuating the “genes” already established. That being the case, it seems imperative that you make sure those genes are good ones.
They will also bring their own ideas and perspectives to the process, establishing new patterns and mutating existing ones, bringing new blood into the gene pool. This sort of cross-breeding is tremendously helpful to the overall health of the “code population” – but only if the new blood is healthy, which is why strong hiring practices are so critical.