Available options: [i] Riding a bike; [ii] environment; [iii] reduced heat; [iv] sleep; [v] competition; [vi] evolution; [vii] Summer; [viii] Energy;

Fact 1: Behaviors that improve fitness increase through natural selection. Fact 2: Natural selection is the mechanism of evolution. Given the two facts above, behaviors that improve fitness improve through what?
Answer:
[vi]