Evolution is an incremental mechanism, which is to say that it preserves most of what it has previously done (DNA encoded) and just makes small changes on top of that. IOW it is essentially structure preserving, and any change that undoes anything that still has value is likely to be maladaptive and not preserved (as opposed to repurposing of gills into ears, etc, where the original function is not being used).

Of course evolution is also messy and isn't operating out of a playbook of decomposable single-function parts. Experiments with evolving electronic hardware have resulted in circuits taking advantage of all sorts of nasty non-linear analog effects, as you might expect.

Still, given the inherently incremental nature of evolution, it is highly likely to result in a system of parts operating with some degree of independence to each other. While there are still many aspects of the brain's functioning we don't understand, it's pretty apparent that it is composed of functional parts like this - cortex, hippocampus, cerebellum, basal ganglia, etc.