Because I am so tired of having to explain this over and over, here it is on one handy-dandy spot. Evolution 101. (To all you science folks, I am going for the 3rd grade version, so I know I’m oversimplifying.)
It’s just this easy: Reproduction, Mutation, Selection-An Idiot’s Guide to Evolution.
1. Reproduction: All organisms reproduce, producing offspring that share DNA from both parents. Something that does not make copies of itself where DNA is transmitted cannot evolve. This means all living things (and possibly some very complex proteins, because they do this with a sort of proto-DNA) can and do evolve.
2. Mutation: Imagine hand-copying a page of writing; sometimes mistakes happen at random. You could copy the same page 100 times and make at least one mistake every time, but only a few times would you make the same mistake. It’s the same with genetic information. (And yes, you could introduce new information; ever found yourself writing down a word you hear in the background while you’re trying to write something down? It’s all the same letters, but grouped in new ways.) Mutations happen in the genetic code with repeated reproduction. That’s mutation.
3. Selection: Some mutations happen to be beneficial, some happen to be harmful, and some make no difference at all.
In most species, more individuals are born than will survive, so an individual that happens to have a mutation, however minor, may have a slightly improved chance of reproducing. Notice that’s MAY, not WILL. Selection is comprised of a lot of different things at once, so no one mutation will make or break a species. Maybe the bird that eats insects has a little bit longer beak, so it can dig out a few more insects, get a little more food, and maybe live longer, and maybe have more offspring. If that only happens once, nothing changes. If the same mutation shows up a few times, and those who have it breed more often, the mutation will turn up more and more often. This takes a LONG time. Similarly, maybe a fish has a weird coloration that just happens to make it harder for predators to see it. It won’t always mean that individual produces more offspring, but over time, since similar mutations occur a few times in every generation, the individuals with a mutation that happens to be beneficial will tend, over time, to reproduce more often, passing on their DNA, with its particular mutation, than those that don’t. That’s selection.
4. Evolution: Over hundreds of generations, that beneficial trait would become the dominant trait and those without it would either be affected by a different mutation and change in a different way or die out. Either way, once enough divergence occurs that the two varieties can no longer interbreed, it is considered a new species. This process is constant and ongoing, and it is never happening with only one genetic trait at a time. And we do see it happen in real-time – that’s why we have the threat of antibiotic-resistant bacteria – the individuals that had some mutation that made them slightly more resistant to antibiotics reproduced more often, and those that didn’t died out.
And that, simply put, is evolution. (And I’m not even a scientist, I’m a medievalist.)
(Incidentally, we live with the results of man-made selection every day. Chihuahuas and Great Danes have each been selectively bred to specific traits. Bananas have been selectively bred to be the fruit we see at the store, which is radically different from what a ‘wild’ banana is. We do this in agriculture all the time. Kale, broccoli, Brussels sprouts, and half a dozen other vegetables were bred from a single species. The only difference is people intentionally selecting, vs. selection happening because of environmental factors.)