yes, that is true. However even if you take a single group in one environment and gradually change it over long period of time you can end up with something completely different even though the individual changes were extremely small.
Imagine a RGB LED that very slowly goes through the full color spectrum over a year. From day to day the colors would look almost identical to the point that you could swap them and no one would notice. You would probably call all the colors during January "red". Once you got into July you would call the colors "green" and would never be able to swap those hues with their "ancestors" from January.
This was a convoluted analogy to explain the concept of "gradual change", but hopefully it makes sense.
Imagine a RGB LED that very slowly goes through the full color spectrum over a year. From day to day the colors would look almost identical to the point that you could swap them and no one would notice. You would probably call all the colors during January "red". Once you got into July you would call the colors "green" and would never be able to swap those hues with their "ancestors" from January.
This was a convoluted analogy to explain the concept of "gradual change", but hopefully it makes sense.