Our all wise Creator built into His creation the means for life to continue and proliferate through variation in the gene pool. He did not build into creation the ability for creatures to change from species to species. For instance a dog can be a tiny tea cup Chihuahua or a Great Pyrenees but they are both still dogs. A snake may be a tiny Thread Snake about the size of a worm or a 13 foot Boa Constrictor but they are both snakes. God is the ultimate creative force who loves variation as an expression of Himself, His awesome abilities and creativity. The ability for creatures to often survive because of variation within a specie's gene pool exhibits the care of the LORD for His creation.
And yet mutation (20 kinds are so) still occur, and new species arise, right in fornt of our eyes.
Try searching on "ring species" or polyploidy, or nylon eating bacteria.
Incidently, bacteria can adapt entirely through mutation. Do it all the time.