Behaviors that improve fitness increase through natural selection.
Natural selection is the mechanism of evolution.
Behaviors that improve fitness increase through evolution.
How do behaviors that improve fitness increase?

[a] because of Darwin
[b] competition
[c] due to fertilization
[d] due to asexual reproduction
[e] through evolution
[f] the body's largest organ
[g] Earth orbiting the Sun
[h] Riding a bike