Evolution is usually considered to be an historic process in which the living organisms found today have been shaped as a result of diversification from ancestor life forms in the time range of many millions of years. Most people, even many biologists, are not aware that evolution is a very important phenomenon in biology responsible for forming and altering populations of some types of cells and organisms in observable time spans. Evolutionary processes in viruses are at the root of their pathogenic potential. We have witnessed the emergence of as many as forty new pathogenic viruses in the last few decades, including the deadly human immunodeficiency virus (HIV) in our generation. As Charles Darwin showed, evolutionary adaptation to the environment takes place by selection of stochastically generated variant forms of existing individuals. When populations of an organism and its mutation rates are sufficiently large, the adaptation process is rapid enough to be observed in the field and in the laboratory, and to have
important biological consequences. In these cases Darwinian evolution need not be inferred since it can be witnessed and even manipulated.