As your quote of me makes clear, I never said *stuff happens* is the explanation to how things happen. I said in the same way that we don't invoke angels to explain the workings of gravity, there is no need to invoke them to explain how the sum total of an organism's environment constrains it's reproductive success.
Okay. Fair enough. So, using "Natural Selection" as your guideline, what did the cheetah and the gazelle evolve from and how is it that the cheetah evolved to be faster than the gazelle?