جڏهن جاندار شين جو هڪ ننڍڙو گروه (هڪ ننڍي آبادي) اهم آبادي کان جدا ٿي وڃي جتان اهي آيا هئا (جهڙوڪ اهي ڪنهن پهاڙ يا ندي تي وڃن ٿا يا پوءِ اهي ڪنهن نئين ٻيٽ تي هليا وڃن ٿا تہ جيئن اهي آساني سان واپس نہ اچي سگهن) اهي اڪثر پنهنجي پاڻ کي پهرين کان مختلف ماحول ۾ ڏسندا.
When a small group of living things (a small population) gets separated from the main population that they came from (like if they move over a mountain range or a river, or if they move to a new island so that they can't easily move back) they will often find themselves in a different environment than they were in before.