Only thing, I think, is that it's not entirely clear what exactly triggers mutation. Or at least it isn't to me. I doubt it's about 'defects' in genetic material alone.

It's fairly well understood, at least in a broad sense. Some categories, and analogies to copying a book:

Typos: The easiest measurement of mutation rates are simply copying mistakes by the enzymes that handle replicating genetic material. Even the humble E. Coli uses at least 20 different proteins to copy its DNA, and depending on growth conditions it will make an error every 10⁸ to 10¹⁰ base pairs or about 1 error per 1,000 new bacteria. (Remarkably, the only reason it's that good is because some of those enzymes crawl along completed DNA and effectively edit out mismatches that cause distortions in the double helix.)

Finding a smudged word: Beyond that, pathogen, chemical, or radiation damage to genetic material can up that rate. If a given base pair has been modified (oxidized, etc.) it may not get 'read' correctly by the team of proteins, and the next generation may perpetuate that mistake. In the worst case scenario, the replication proteins get to the mistake and effectively fill in random junk.

Skipping a page by accident: There's also a host of mechanisms by which organisms (particularly those with high reproduction rates, like bacteria) cut, paste, shuffle, and otherwise modify their genome beyond simple copying errors. Most of these changes end up being lethal (and thus removed from the gene pool), but at a high enough reproduction rate it's still a sustainable strategy that can increase overall fitness-- every once in a while one of these mutant bacteria acquires a novel trait that allows it to either outcompete its ancestors or else occupy a niche unavailable to them.

Also, some macro-level mutations (eye color, some diseases, whatever) are actually the result of many genes interacting. There, even if the copying is perfect, just the usual shuffling of genes during sexual reproduction will occasionally produce a novel result from old pieces. Genetics is full of examples where the sum is very different than the parts.

valguarnera@carrionfields.com