"I'm inclined to think that this isn't true of the universe (or evolution)… for example, a rock rolling down a hill has a far greater probability of happening than one rolling up… clearly not equal. This is pretty much true of all spontaneous reactions vs. non-spontaneous ones." - S.H.Le
Probabilities, (as any other outcome of inerplay of events) srictly depend on boundary and initial conditions. For example, when you shake bricks inside a big box in a hope to get one unique configuration we call a house (which is only ONE out of multi-zillion others equally possible, and thus happening rarely as you may calculate by dividing one by many zillions), you will NEVER get a single brick OUTSIDE the box. But if you make a hole in the box, then from time to time you have a brick or few out, and the bigger the hole, the higher probability of having bricks out. Now, suppose you shake the box with a hole near some mass (Earth). Now initial conditions include gravity, and more bricks will move down than up. With time, regardless their chaotic collisions with each other, most of bricks end up outside of box on the ground. So, you may see that changing initial conditions by including gravity, you make probability of moving down higher than up. Weak gravity - slight difference in probability, stronger gravity - larger difference. You see how everything obeys math?
Evolution is not exception, of course. Take an aquarium and put in it all kind of fish: gold fish, arctic fish, tropical fish and whatever else fish you like. Now drop the temperature of water below 60, and the tropical fish will die out. Drop it more, and gold fish is gone - only arctic stuff survives and happily multiply occupying as much room and eating as much food as is available. That is the essence of evolution: not adaptation but dying out of misfits. Wanna create new species? Heat or x-chemical resistant?Start with the same aquarium and rise the temperature to 70 - all arctic stuff is gone. Rise it to 80 - gold fish is also "out of luck" in survival of fittiest, but tropical will florish. Elevate it to 90, and only some "heat resistant" will survive. Put x-chemical in the water, and you may kill most of the fish. But strangely enough, those who survives will give the offspring unsensitive to that chemical (clue: some small fraction of all fish ALREADY had strange "x-taste": due to constant mutations not all fish is born equal). Now be proud - you have just "created" new specie of "heat and x-chemical resistant" fish which will enjoy hot x-chemical water and florish in it. (Now remove the chemical - and most of this fish may die out in such unsalty water).
Again, there is nothing here except math (=physics=chemistry=life).