Many believe that evolution is just an "idea" and is not proven or is on shaky ground. However, a theory is much more than just a "guess" as so many mistakenly think. A theory is a hypothesis, supported by facts to substantiate it. Evolution is one of the most substantiated theories in science and with recent discoveries, is not even questionable any more.
When people question evolution, it is mostly because they know nothing about it, only what they've been told about it, usually by someone as equally uneducated on the subject. The evidence for evolution however, is massive. There is evidence in the fields of paleontology, anthropology, cosmology, biology and not just ancillary evidence, but observable, testable, statistical evidence. The main areas of evidence however, are in the fossil record and genetics.
The fossil record, although very compelling to science, is for some reason dismissed by creationists and other skeptics as either not evidence at all, or non-existent. For example, science has very detailed records of fossils showing the evolution of primates, horses, whales, dolphins, dinosaurs, amphibians, reptiles, birds and many other species. There are so many fossils that haven't even been classified yet. Literally there are thousands of intermediate forms that have been identified that it takes a true blind eye to dismiss them. These fossils, all dated, show clear transitions from fish to amphibian, amphibian to reptile, reptile to bird, etc. The truth is, no matter how many fossils are found, creationists will always dismiss them because basically they believe for some reason that evolution is against their bible and their god, so they will stop at nothing to discredit it. The fossil record however, is very complete considering how difficult it is for a fossil to even form and be preserved.
The strongest evidence for evolution however is in the field of genetics. Just about everyone has heard that humans are 99% similar genetically to chimpanzees, 96% similar to Gorillas and have other strong genetic similarities to other modern day primates. Creationists will say that it indicates a common designer, however if this was the case, then we would be 99% similar to all other species and there would be little to no variation. This however is clearly not the case. The best evidence however, came from the decoding of the human and chimpanzee genome. You can literally see, line by line in our DNA, our differences and see where we diverged as species. There was also the discovery of why we have two fewer chromosomes than primates. This was a strong discovery for science and evolution, because it one of the last strong contingent points for skeptics and one that could have potentially overturned evolution as a legitimate theory.
Another discovery that came out of study in the field of genetics came from the discover of atavisms. An atavism is basically something that occurs during the development of an embryo, which causes what is normally referred to as a "mutation", such as an extra toe, extra tusks on a elephant, legs on a dolphin, etc. However, these are not mutations. What was discovered in all these cases is that instead of the DNA acting abnormally and creating something extra, what really happened was is that genes in DNA that were supposed to be inactive, were activated for some reason. So in other words, in the case of the dolphin that was found with legs, it wasn't a mutation, it was genes that were already there from its evolutionary history that are not supposed to be active. Atavisms actually show the evolutionary history of that species. All of these atavisms also match with the fossil record which confirms that dolphins were once land walking mammals and that elephants did have 4 tusks instead of just 2 at one point in their evolutionary history.
The last nail in the coffin for proving evolution however, came in the form of a discovery in the human genome. The human and chimpanzee genome are composed of about 3 billion base pairs of genes. Upon examination of the human genes, certain genes looked to be abnormal, with markers on them. In particular 16 different genes out of the 3 billion pairs, had distinct markers on them that were each unique. What was discovered is that this was a rare occurrence in which a virus passed from the parent to the offspring through conception and that if the parent had a strong virus, this virus would become imprinted on a gene. This gene would then in turn be passed on through the generations and become a part of the normal genome. This anomaly was coined ERV, or Endogenous Retro Viruses. With the discover of this, more questions were raised that could potentially solidify common descent and evolution, or again potentially throw a wrench in evolution. If we as humans share a common ancestor with chimpanzees, then their genes should wield some of the same ERV's as us. Not only the same ERV, but in exactly the same location in the gene sequence. Statistically speaking, the odds of just one ERV matching out of 3 billion base pairs of genes, is 1:000016, or a .000016% chance of having just one ERV match without common descent. After examining both the human genome and the chimpanzee, 3 matching ERV's were found, that is to say the exact same ERV, with the same pattern and in the exact same location in both genomes. Statistically the chances of this being random and not due to common descent is a .00000000016% chance. Not many people are aware of this discovery mainly because of ignorance and non-willingness to explore the facts.
If this isn't enough for you, there is one final proof that also definitively proves evolution. It comes from the earlier proof discussed pertaining to atavisms. Sci-Fi movies have always created wonder, such as Jurassic Park on if we could bring long since dead animals back to life using ancient DNA. Scientists have been working on this for years and have been unsuccessful. However, with the discovery of atavisms, scientists sparked new ideas on possibilities, especially since our DNA has our evolutionary history already there. The idea was to reverse engineer an embryo during development of a bird and find a way to activate pseudogenes (or inactive genes) and this would produce a de-evolved bird that would reflect something from the fossil record. This experiment was actually conducted in 2007 and they were able to identify the genes that control feathers. We know from the fossil record that dinosaurs at the end of the Jurassic had what were called proto-feathers, which are an early form of feathers, very much more rudimentary than modern day feathers. After the experiment, the bird embryo produced proto-feathers, instead of modern feathers, which was a massive success for science.
Now in 2008 scientists are working on identifying the genes that control the beak of the bird. Bird embryos show during development an atavism that produces teeth briefly and then reverts and becomes a beak. Scientists are working now to activate those genes so that the embryo will produce those teeth, like they did with the proto-feathers. The eventuality of all this is that they can reverse engineer a modern day bird back to being what was a dinosaur at the end of the Jurassic. Many people think this is science fiction, however it is actually happening. Although the end result probably won't be produced for some time, maybe even up to 25-50 years, It shows that evolution is not only present, but can be observed physically. This is the evidence that creationists avidly claim doesn't exist.
The statement, "Evolution has never been observed" is no longer true. This is real evidence of the likes that should convince even the most stubborn skeptic. However, there will forever be skeptics and those that for religious reasons will not accept the truth. Science has come so far on this issue that to deny that evolution happened is simply thatdenial, but that doesn't change the truth and if people would simply research the issue, we wouldn't have controversy surrounding an issue that is no longer a debate.