The correct answer is - theory about distribution of organisms by elevation.
It is easily noticeable if we have a hike and we have a change of few hundred or few thousand meters of elevation from the starting point to the end point, that the plants and the animals are changing as the elevation is becoming bigger.
The reason for this is that the living conditions change as the elevation becomes bigger. The higher the place is, the colder it is, Also, the winds are stronger, and the terrain is usually steeper. That results in plant and animal distribution in accordance to their abilities to survive in certain places. While some plants and animals have evolved in more moderate climate, others have evolved in harsher ones, which makes make different in appearance and having differences in their lifestyles. That also results in borders, which usually are gradual, with having mixture between two separate ecosystems.
