The brain is composed of different parts, each of which has a different cell composition, connects with different areas, and – in general – performs different functions. If we look at the broader subdivisions of the brain, they are present in all vertebrates. Since not all vertebrates possess the same behavioral functions – either from a qualitative or a quantitative point of view -, this conservatism begs the question of whether or not brain regions can evolve isolate from each other.

This is the problem of mosaic vs. concerted brain evolution. In the case at hand, Mosaic evolution refers to evolutionary changes in a given brain region (either its size, its cellular composition, its connections, or other trait) that are not accompanied by changes in other brain regions. Concerted evolution refers to the opposite situation: those cases in which every evolutionary change in a single trait is accompanied by a change in another trait.
Both processes are important for the topic at hand because the “late equals large” rule predicts that those brain regions which are “born” later in development become larger than those which are born earlier – and, more importantly, altering the “schedules” of region birth alters its relative size. That means that a requirement of the “late equals large” rule is that brain regions evolve in a mosaic pattern. How common is that pattern? That is what we will see in the next post.