I lurk these boards for fun and entertainment and occasionally jump in to point out how ignorant some of the people here are. Evolution is obviously not something that you either believe in or you don't... its something you either UNDERSTAND or you DONT.
The evidence for evolution: how do we know it is true? There is an overwhelming body of evidence for evolution. To roughly go over a few...
+The fossil record is one handy piece of evidence. Rocks lower down in the earth are ‘older’ (as more rock piles up over then, they get buried). In these older rocks, deeper in the earth, we find much simpler fossilised organisms, and can observe a change to more complex organisms in the higher up rocks. We know the rocks are older because we have many dating methods, which we can cross-reference when examining a rock. They give the same answer each time, which is strong evidence that the dating methods are accurate.
+Another way we know is by looking at DNA, the stuff that makes us us. Here's a triumphant example. Humans have 23 pairs of chromosomes, but our closest relatives, the great apes - chimpanzees, gorillas and orang-utans - all have 24 pairs of chromosomes. This seems to suggest that the ancestor we all share had 24 pairs of chromosomes too (the great apes are not our ancestors - they are our cousins, like our 3 foxes above were cousins). Where did this chromosome go in humans? This would seem to put the theory of evolution in jeopardy, but no! We have mapped and understood all the chromosomes in both chimpanzees and humans and compared them and... what's this?? One of the pairs of chromosomes in humans is exactly the same as 2 of the chimp chromosomes but fused together! We can perfectly see the exact difference and mechanism by which human chromosomes became different from the other great apes - 2 of them joined together into a single chromosome.
+The life on Earth is evidence of evolution itself. We can see the different stages of evolution in different organisms. Take, for instance, the amazingly complex and clever eye. Our eyes are very well developed compared with most animals (save some birds of prey etc). How could such a complex thing have evolved? Well, we have a pretty good idea how, and we can actually see every stage of eye evolution in other organisms. An eye at its most basic is a light sensitive cell. We can find those in nature. Next is a patch of cells in such a shape that can detect direction of light. We can find those too. Next is a hole of cells creating a simple pin-hole. We see those in nature. And then we find the next step up, creatures with a lens. Then animals with a further step, muscles to focus the lens. Each ‘stage’ of the eye can be found in other animals. We can use this to trace the development of our own eyes.
+The last evidence for evolution I will mention here is observation. Evolution is an ongoing process - everything is still evolving and we can see it evolving. The easiest example is the bacteria and viruses that make us ill. These organisms live, die and reproduce so quickly that they evolve extremely quickly, too. Why do we need to have a new flu vaccination every year? Because the influenza virus evolves. Why do we need to finish a course of anti-biotics if they are prescribed? Because if we only use half of the anti-biotics, we only kill the weakest half of the bacteria making us ill. The strongest half lives on and reproduces even more (because they won't have competition from their weaker brethren). We'd be helping the bacteria to evolve. This experiment is an example of a way that we have actually observed evolution, including a new irreducibly complex adaptation — the ability to digest citric acid.
The mechanism for evolution - natural selection - is simple, logical and effective. The evidence is overwhelming (there is a lot more than what I mentioned above). In fact, there is more evidence for evolution than any other theory in science. Just remember: natural selection, natural selection, natural selection. Random good changes will help an organism have more babies thanks to their environment. Random bad changes will cause an organism to have fewer babies thanks to their environment. Nature naturally selects the best changes! From here it is a numbers game. Things die and things live. The genes of those who live long enough to reproduce are passed on.
