Sometimes people criticize evolution on the grounds that "it's all about randomness". They ask questions like this:
If I spread a bunch of airplane parts on a football field, and a tornado comes around and stirs up all the parts, what is the likelihood that the result will be a correctly assembled, functioning airplane? This is the same likelihood that the human body (or the eye, or the brain, or the hand) could have arisen out of evolution, a process characterized entirely by randomness.Evolution consists of two parts. One is variation, which can be random but need not be, and the other is selection, which is not random at all. The part of evolution that is random, the point mutations and crossovers among chromosomes, is not where its explanatory power resides. If that were the whole story, then complex forms really would be no more likely than working airplanes popping out of tornadoes. These random bits of variation merely supply the variety upon which the filter of selection operates.
It is in selection that the explanatory power of evolution resides. Selection is the non-random part of evolution, where the signal (this trait works) is separated from the noise (that trait doesn't work). It is because selection is consistent and non-random that we see the re-appearance of traits at very different times and places in the history of life. Tyrannosaurus Rex and my cat both have sharp claws. Are cats direct ancestors of T. Rex, and did those sharp-claw genes somehow survive tens of millions of years unmodified? No, but they are both hunting predators faced with problems that sharp claws solve. Likewise, complex eyes with focusing lenses have independently evolved dozens of times, because clear vision is useful.
Randomness in the physical world is of two types, fundamental randomness and consequent randomness. Fundamental randomness is the stuff of quantum mechanics. When particles appear to act randomly, are there hidden variables which, if we could see them, we'd be able to see through the apparent randomness to an underlying determinism? If there aren't, then the universe includes a component of fundamental randomness -- some things are just random and there's nothing you can do about it. My understanding is that it's still an open question among physicists whether fundamental randomness exists in the universe, but the weight of opinion favors it, as experimentation has ruled out local hidden variables and only non-local hidden variables remain as a possibility.
Consequent randomness is the appearance of randomness among things that are individually deterministic. A cryptographic hash algorithm is a good example. If we feed this deterministic process with a deterministic input sequence (e.g. 1, 2, 3, 4, 5...) what we get is an output sequence of large integers that look entirely random. They pass every statistical test of randomness with flying colors. Yet in some important sense they aren't random at all, because we can start the input sequence over and we get exactly the same output sequence repeated. So we have apparent randomness arising from deterministic pieces in a complicated Rube Goldberg fashion.
Consequent randomness can easily arise where there is a mixing of data with different explanations or from different domains. Peoples' cholesterol levels and their telephone numbers are unrelated, so if telephone numbers are put in order of the person's cholesterol level, the sequence appears random.
Often people object to evolution on the grounds that it requires fundamental randomness, and these same people often find the notion of fundamental randomness personally abhorrent, and so they accept this situation as a disproof of the theory of evolution. In fact, evolution works just fine when variation is driven by consequent randomness. All genetic algorithms running on computers work this way.
In his talk above, Dawkins discusses a much better potential disproof of evolution, for which he thinks the creationists ought to be scrambling to find evidence. If we found fossils in the wrong geological strata, for instance a rabbit fossil among dinosaur fossils or trilobite fossils, then the case for evolution would be significantly weakened. Such fossils, which Dawkins calls "anachronistic", have never been found among the many hundreds of thousands of fossils recorded in natural history museums and universities around the world. While we may find gaps in the fossil record, we never find such temporal discrepancies.
So why do I personally believe in evolution? I have two answers to that. The first is that the entire process can be done on a computer. It's a standard thing, people have been doing it for years, it reliably solves hard problems, it's a classic technique in computer science. For me to disbelieve in the efficacy of genetic algorithms would be akin to an auto mechanic whose personal convictions prevent him believing in the inflation of tires, while his colleagues inflate tires on a daily or weekly basis in the shop around him.
Second, the fundamental idea of evolution is so simple. It has so few parts. There are only a very small number of places it could possibly go wrong. If it went wrong in one of those places, no malicious cabal or conspiracy of evolutionary biologists could cover up its failure for long. The logic of evolution is simply too simple and too compelling to be incorrect.
Objectors to evolution are sometimes motivated by the fear that it rules out the possibility of an afterlife. Having lost loved ones and myself being mortal, I have some appreciation for this concern. Personally I cannot rule out the possibility of a universe of Cartesian dualism, and in fact I very much hope it's the case. As far as I am aware the strongest arguments against dualism are Occam's razor and Dennett's objection that in a dualist universe, a philosopher like himself would have no hope of understanding or explaining anything because everything would be arbitrary. I also appreciate Dennett's position, but it seems to me to lack imagination -- perhaps there is a dualism that is lawful, understandable, and explainable, and which could ultimately become part of science, but which also allows for some piece of a person's mind or personality that outlasts the physical body. Then it might be possible that such minds and physical bodies might undergo parallel processes of evolution as organisms increase in complexity over billions of years.