A lot of what is called evolution today is really genetics. For something to truly evolve, it has to acquire genetic material that its parents or other ancestors didn't already have. It has to acquire new genetic material that isn't already in its genes. For a reptile to evolve into a bird, it would need to acquire the directions to grow wings, in addition to the DNA it already has.
For example, all dogs are the same species, Canis familiaris. If we didn't already know that, they might be labeled as different species because they all look so different. Poodles, St. Bernards, Great Danes, German Shepherds and Old English Sheepdogs would all be labeled as different species but, we know that they really are the same species, they just look different.
I'm a bird watcher. There are many different looking sparrows, each in it's own habitat. They each look a teensy bit different. But, they are all still sparrows. The ones that have the best combination of genes to survive in a particular climate will survive. That is just variability within a species. It doesn't prove that all animals evolved from one-celled animals. That is a monstrous leap of logic.
As a creationist, I believe that God created the basic types of plants and animals, and then genetic variation went on from there. For example, God made the basic types of birds including: hummingbird, woodpecker, parrot, pigeon, sparrow, duck, and so forth. Each type had a lot of capacity for genetic variability as we see in dogs so we have different types of hummingbirds, woodpeckers, parrots, and so on like we have different types of dogs. This genetic variability works within fixed limits. These animals will never turn into anything else, they will just develop into different varieties of the same thing.