Answer:
Environmental factors, and here's why.
Explanation:
Natural selection plays around with our evolution in many ways. Natural selection via. breeding is the conscious choosing of mates who have more desirable traits (eg. Stronger bones, better speed, intelligence) and can help populations that are in need of certain traits.
Environmental factors are, in my opinion, more effective. Without this, the other would not be present. Different biomes, such as forests, mountains, underwater, and savannah all require different traits. This helps cull out those that would not benefit the population by using their negative traits against them. A kangaroo born without a tail would not survive long due to the decrease in speed, therefore eliminating it from the breeding pool.
