Environmental factors and selective breeding can only go so far. A finch remains a finch, and a dog remains a dog. In fact, changes from microevolution are often merely cyclical. For example, when the Galapagos Islands experienced heavy rainfall in the early 1980’s, previously rare small-beaked finches made a comeback, and their numbers and percentage of the overall population increased dramatically.
Mutations can change existing genes, but can’t add new information. So, for example, the Chernobyl nuclear disaster caused sheep to be born with extra legs, but a sheep could not grow wings because it simply doesn’t have that genetic information.