natural selection IS due to random genetic mutations. Genes mutate randomly. Sure, the genes that are present and which mutate are there due to past random mutations and those that tended to be there as the result of natural selection. The genes that are there that mutate are from parents breeding and passing on their genes.
"ensures" is not right - it's more survival of the fittest. and it's not breeding success - it's SURVIVAL adaptations. "Ensures" in the sense that breeding is required to pass on genes so those that breed most successfully are the ones that will be the most successful at passing on their genes. Survival of the fittest refers to those that have the genetic makeup to give them advantages over others in passing on their genes - in other words in breeding success.
