Giraffes don't turn into elephants and dogs don't turn into cats. Claims that if evolution is true then giraffes should turn into elephants and that dogs should turn into cats are straw man arguments of young earth creationists. They are the very arguments used in some creationist literature. For example, the evolutionist (though not a scientist and though not believing that natural selection is the cause of macroevolution) Francis Hitching's Neck of the Giraffe book contains of an illustration from such creationist literature which show the following misrepresentations of evolution: an animal with the main body of an elephant and the head of a giraffe; an animal with the main body of a giraffe and the head of elephant; an animal with the main body of a dog and the head of cat; an animal with the main body of a cat with the head of a dog. I know that because I have Hitching's book, checked out from the library. The creationist illustration shown in the book (In "chapter five: Creation v. evolution") is accompanied by the following words.
"People who believe in evolution say that one animal can change into another. The Bible says they can't. Have you ever seen any of these animals? Have you ever seen a 'giraffant' or an "eleraff'? How abut 'pigdeer' or a 'catdog'?"
Giraffes evolved (by descent with modification) from some animals which were not elephants, elephants evolved from some animals which were not giraffes, cats evolved from some animals which were not dogs, and dogs evolved from some animals which were not cats (recent scientific evidence indicates that dogs evolved from grey wolves). But, the known fossil record clearly shows the following:
anatomically modern humans evolved from humans who were less human looking than modern day humans;
the earliest ape-like hominid beings existed before the first humans (the ones which are less anatomically modern looking humans than modern humans) [in this example I am defining human as those who are assigned to the genus Homo];
the earliest apes existed before the earliest ape-like hominids;
an ape species (Aegyptopithecus zeuxis) existed with some monkey-like features (including a tail as an adult) before the first full apes existed;
the first primates (ones who were not monkeys) existed before the first monkeys (monkeys, apes, and humans are some types of primates);
the first mammals existed before the first primates existed;
the first mammal-like animals (synapsids, which used to be commonly called mammal-like reptiles since in a number of ways they resembled reptiles) existed before the first mammals;
the first four legged terrestrial egg laying animals existed before the first mammal-like animals;
the first amphibious four legged animals with webbed feet existed before the first four legged terrestrial animals;
the first fish with leg-like limbs and webbed feet existed before the first fully amphibious four legged animals with webbed feet;
the first fish with lobe fins as limbs and paddle-like digits existed before the first fish with leg-like limbs and webbed feet;
the first fish with a nasal passage to the throat existed before the first lobe finned fish existed (lobe finned fish, amphibians, reptiles, and mammals have a nasal passage to the throat);
the first fish with jaws existed before the first fish with both jaws and the the nasal passage to the throat;
the first fish without jaws existed before the first fish with jaws;
the first vertebrate existed before the first fish;
etc.
I know the above about the fossil record because in science books I have seen illustrations of discovered fossils of that which I stated above.