And this prevents macroevolution how? If a dog is born in the wild with a mutation that causes his blood to clot more slowly, is it less likely that he'll survive long enough to have a bunch of little puppies? And if the clotting problem is hereditary, will the puppies be better off or will they die quickly? What about a dog who was born with a slightly better digestive tract letting him absorb more nutrients from food; he can survive longer with less food. If the trait is hereditary, which dog's offspring will be more likely to survive? Is it the one that can bleed out more easily or the one that can survive with less food? Which one will be able to pass on his genes to more offspring?

We must remember that anything with chemoreceptors can tell the difference between negative and positive physical effects since it is actually evident that every animal and human naturally thinks "this bad, that good" thinking. In truth, there is something wrong with your conundrum. We, or more specifically the atheistic community or Darwin lobby, are dependent on the non-substantive or circumstantial reasoning with these "if" scenarios when in reality, nature goes against the logic and evidence that actually occurs in nature (e.g., no new species, organic manifestation, etc). Your scenarios present these cases as "what if" when actually, we should be thinking with the mentality of "since so-and-so occurs, we will be under the reasonable conclusion that so-and-so will happen." Sure, your cases may actually occur, but it doesn't cause that particular dog species to develop into a new species. That's the biggest issue with evolution. The Bible presents an origin and evidence for that origin with some support to the thinking that some of the science of evolution is justified; however, the parts that aren't are not in scripture, are impossible to observe, and can be reasoned to have been thought up as being illogical.