Amir suffered a severe brain injury in a motorcycle accident and was partially paralyzed on the left side of his body. after several months of intensive physical therapy, he gradually regained the use of his left leg and arm. this example best illustrates the principle of:

  Plasticity

Plasticity or brain  (neural)  plasticity applies to injury. When we are injured, we use some movement patterns of our legs and arm to avoid strain on damaged tissues and to keep us going. But gradually, we are back to our original movement.

Amir gradually regained the use of his leg and arm due to the principle of plasticity (the capability of the central nervous system to let us adopt from its functional and structural organization to our current requirements). 

genetic variation will increase because of a new habitat and food source.

explanation: