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?