[Junior]

Not necessarily. For instance, if you have a population covering a large area, say a super continent, and then the area is split into one or more smaller areas, each of those populations becomes isolated. If the environment changes in any of those new areas, the population will change to adapt to that change. And, if any of the new environments remains stable with regard to the previous environment, the populations therein will remain stable. This ain't rocket science.

[AndrewC]

If the environment changes in any of those new areas, the population will change to adapt to that change.

Of course, it ain't rocket science. That is its problem. Why go to all of the problem of restating what I stated. A stable population remains stable unless a change occurs. The more stable the population, the bigger the change required to make it something else.