The word evolution (sometimes called Darwinism) has a variety of definitions, from simply “change” to “the natural process by which all life derived from a single ancestor,” and is referred to alternately as “hypothesis,” “theory,” “law,” and “fact.” Because of its imprecise nature, the term is often used ambiguously to imply that the processes we can observe in the present (e.g., natural selection) “prove” that the processes we cannot observe in the past must have happened as well (e.g., the change of dinosaurs into birds). In fact, the term evolution can also be used to denote the philosophy of naturalism, which depends upon unobserved events in the past (including in astronomy, chemistry, and geology).
In scientific terms, evolution generally means the change in genetic material between generations, which is also referred to as “descent with modification.” These changes are attributed to mutations, gene flow and drift, and natural selection, which are examples of observational science and can be shown to occur. However, the other aspect of evolution is the belief that all animals descended from one original ancestor. Evolutionists sometimes claim this “fact” is established in the fossil record, homology (similar structures), and genetic evidence. However, any evidence involving historical science (one-time events that cannot be retested) is subject to interpretational bias on the part of the scientist.
Mutations and genetic drift are often cited as the source of heritable traits from one generation to the next. While mutations do cause changes in the genome and genetic drift changes the frequency of those traits, neither process is capable of changing one kind of animal into another. More often, mutations have either no noticeable impact or cause degeneration.
When evolutionary scientists claim that evolution is a fact, they are relying upon a fallacy known as “bait and switch” (define a term one way, but use it in a completely different way later). Often the claim is that since one can observe natural selection, then descent from a common ancestor must also be true. However, this presupposes that the current processes we observe could cause the origin of completely novel structures (e.g., giving rise to lungs or complex brains). Such a claim is contrary to information theory and the laws of nature.