Evolution is the changing of organisms through time. This is the answer an examiner is looking for if you get a one-mark question asking “What is evolution?” (it’s a favourite question for Evolution 101!). This is how we interpret evolution today under the Modern Synthesis – contrast with its original meaning with the homunculi in development.
This change that organisms experience is not one that takes them from “primitive” to “higher” forms, as was thought back in pre-Darwinian days, when ideas concerning the scala naturae dominated. In fact, anything that is still alive today is “as evolved” as the rest. If this was not the case, then we wouldn’t have bacteria, sponges, algae, or even pine trees around.
Change in organisms takes place through mutation in DNA. The mutation rate can be influenced by several extraneous factors, such as UV light and radioactivity, but mutations are also an inevitable part of genetics and life itself, arising easily when DNA is copied, despite the presence of error-correcting mechanisms. This is also why evolution is an intrinsic part of the definition for life – variability will always be found, and so evolution will always occur. The longer the DNA strand, the more mutations will creep through. These are generally the major source of mutation in asexual organisms. Sexual organisms have an extra source of genetic variation. Because sex is all about the fusion of two gene sets, new combinations of genes arise.
Many books have purported “proofs” for evolution, the more common ones being coevolutions, the universality of the genetic code, evo-devo, or atavisms. These are all problematic for a variety of reasons I will explore in a later post. If you need a proof for evolution, there really is only a single foolproof a priori one, and it’s what I have already talked about above: the changeability of the genetic code, and the variations inherited by successive generations. It gains this status because it describes the process of evolution, while other purported proofs are merely observations that can potentially have different explanations; we just fit them a posteriori as proofs for evolution.