If you ask an evolutionary biologist – by definition the only person qualified to answer the question – she will tell you that evolution is simply a change of gene frequencies over time. This may sound rather simple and philosophically uninteresting, but it is in line with what science is all about: seeking answers to specific questions, not to questions of ultimate meaning. The theory of genetic changes in natural populations is very well understood by a branch of biology called population genetics, and modern molecular biology provides direct evidence that gene frequencies do indeed change under our very nose. Examples are abundant and are found in all classes of living organism (humans included, of course).
An exceedingly well understood, and therefore undeniable, current example of evolution by changes at the genetic level is provided by the HIV virus, the causal agent of AIDS. This kind of virus, which causes a disease of the immune system, has evolved for a long time using vertebrates as its vehicle. Closely related forms of the virus attack closely related types of vertebrates, as is predicted by evolutionary theory. For example, one form attacks dolphins, but it is quite distinct from the human form, which is instead much more similar to chimpanzee forms. This is because chimps and humans are more closely related and more similar to each other than either is to dolphins or other cetaceans. Since viruses evolve gradually (like anything else), they are more likely to attack new hosts that are similar to their old, familiar host.
Evolution has produced at least two major forms of HIV in humans, and possibly a third one is in the process of emerging. The history of one of these, HIV-1, has been particularly well characterized. We know that it is very close to a chimp virus and, according to estimates based on molecular data, it passed from chimps to humans in West Africa in the 1940s. From there, it was picked up by a Norwegian sailor who brought it to Europe, where it started spreading in the 1970s.
Interestingly, and again in perfect harmony with the theory of evolution, the HIV virus is changing rapidly, in front of our very eyes. This is because new mutations occur in populations of the virus and these are selected to adapt the virus to changes in its own environment. The environment is constituted by the human body and by our sexual behavior. Different types of HIV are now known that specialize for transmission through different sexual practices, depending on which of these are adopted by particular human populations. Unfortunately, it is this rapid evolution that makes it very difficult to find a cure for AIDS: the target keeps literally shifting in front of us as a result of mutations and selection. This makes the point that evolutionary theory is not just academic; it is of vital importance for the survival and welfare of human beings.