Indeed, some species may remain unchanged for long periods of time, as Darwin noted. Nautilus, Lingula, and other so-called “living fossils” are Darwin’s examples of organisms that have remained unchanged in their appearance for millions of years.

Evolution affects all aspects of an organism’s life: morphology (form and structure), physiology (function), behavior, and ecology (interaction with the environment). Underlying these changes are changes in the hereditary materials. Hence, in genetic terms, evolution consists of changes in the organisms’ hereditary makeup.

Evolution can be seen as a two-step process. First, hereditary variation arises by mutation; second, selection occurs by which useful variations increase in frequency and those that are less useful or injurious are eliminated over the generations. “Useful” and “injurious” are terms used by Darwin in his definition of natural selection. The significant point is that individuals having useful variations “would have the best chance of surviving and procreating their kind” (Darwin, 1859b, p. 81). As a consequence, useful variations increase in frequency over the generations, at the expense of those that are less useful or injurious.

The process of mutation provides each generation with many new genetic variations, in addition to those carried over from previous generations. Thus, it is not surprising to see that, when new environmental challenges arise, species are able to adapt to them. More than 200 insect and rodent species, for example, developed resistance to DDT, Warfarin, and other pesticides in places where spraying was intense. Although these animals had never before encountered these synthetic compounds, mutations allowed some individuals to survive in their presence. These individuals reproduced and, thus, the mutations providing resistance increased in frequency over the generations, so that eventually the population was no longer susceptible to the pesticide. The adaptation had come about by the combined processes of mutation and natural selection.

The resistance of disease-causing bacteria and parasites to antibiotics and other drugs is a consequence of the same process. When an individual receives an antibiotic that specifically kills the bacteria causing a disease—say, tuberculosis—the immense majority of the bacteria die, but one in several million may have a mutation that provides resistance to the antibiotic. These resistant bacteria survive, multiply, and spread from individual to individual. Eventually, the antibiotic no longer cures the disease in most or all people because the bacteria are resistant. This is why modern medicine treats bacterial diseases with cocktails of antibiotics. If the incidence of a mutation conferring resistance to a given antibiotic is