If an animal is prey to a fast predator then
any change that reduces the prey's speed is under intense selective pressure and those changes can not spread in the population.
If the prey animal lives in a warmer climate that is starting to cool then increased body fat would be another answer to the new selective pressure of the new environment. However, this would slow the prey down and the predator selective pressure will maintain the body form of the prey.
This is an example of one selection mechanism remaining the same and another changing. The result may be extinction but if not some characteristics of the animal must be maintained.
Another form of intense selective pressure is any constant environment. Over time an animal becomes more and more finely tuned to this environment. Then
any change is moving from the
local optimum that has been achieved. The constant environment supplies the selective pressure and stasis must be maintained.
Note that this is a
local optimum. Accidents of history establish the current situation and may exclude "more optimum" solutions.
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