When Charles Darwin saw the Galapagos Islands finches with their varied bill sizes, he assumed they had a common ancestor. This creationists believe, too. But with tiny changes in those sizes and lots of time (which was just being suggested in British science) large changes could potentially happen. And this is what Darwin proposed. Large scale changes in plant and animal types, creating new taxonomic classifications, is supposedly what has happened over billions of years.
However, observational science on even the Galapagos finches records changes in bill sizes that are apparently reversible as climate changes occur on the islands. Thus no “net evolution” takes place. Instead there seems to be a God-designed flexibility in species to adapt a little (which we shall call “microevolution”) to living conditions over time.