of human pathogens has come to pose one of the most serious public health problems now facing human societies. Many strains of bacteria have become increasingly resistant to once-effective antibiotics as natural selection has amplified resistant strains that arose through naturally occurring genetic variation. The microorganisms that cause malaria, gonorrhea, tuberculosis, and many other diseases have demonstrated greatly increased resistance to the antibiotics and other drugs used to treat them in the past. The continued use and overuse of antibiotics has had the effect of selecting for resistant populations because the antibiotics give these strains an advantage over nonresistant strains.4
Similar episodes of rapid evolution are occurring in many different organisms. Rats have developed resistance to the poison warfarin. Many hundreds of insect species and other agricultural pests have evolved resistance to the pesticides used to combat them—and even to chemical defenses genetically engineered into plants. Species of plants have evolved tolerance to toxic metals and have reduced their interbreeding with nearby nontolerant plants—an initial step in the formation of separate species. New species of plants have arisen through the crossbreeding of native plants with plants introduced from elsewhere in the world.
The creation of a new species from a pre-existing species generally requires