Right Before Our Eyes
Sometimes evolution happens overnight. Unlike most larger organisms, viruses and bacteria reproduce very rapidly--they simply split in two every few minutes or hours, with both offspring continuing to divide and multiply. Such short spacing between generations can help speed up the evolutionary process. These organisms have an enormous potential to acquire advantageous adaptations quickly. A mutation that gives an advantage in reproduction can dominate a population in a few days.
Like all viruses, influenza, or "the flu," can evolve very rapidly. Each year scientists study flu viruses from around the world in order to predict how they might evolve. Scientists then create a vaccine designed to help the body's immune system ward off the most dangerous of the upcoming year's varieties, a process that has saved countless lives. But since any one vaccine can help immune systems fight only some varieties of flu, others survive, reproduce and evolve. Thus, new flu vaccines are issued every year not because the old ones have worn off, but to fight newly evolved or re-emergent varieties of the virus.
Sometimes, viruses can change so dramatically that human immune systems are nearly powerless to fight them. In 1918, an unfamiliar flu swept the globe. Despite precautions, such as those taken by U.S. Army soldiers in Seattle, that flu virus killed between 25 and 50 million people worldwide.
An Antibiotic Arms Race
Antibiotics such as penicillin became widely available more than 60 years ago, so you might think infections and bacterial disease would be problems of the past. Think again.
When people or animals take antibiotics, most bacteria cells in their bodies die. But of those millions of bacteria, a few might have mutations that make them resistant to any one particular antibiotic. It is those cells that survive and multiply--a dramatic example of natural selection in action. Over time, resistant bacteria strains replace nonresistant ones, and soon that initial antibiotic becomes ineffective. Researchers are forced to develop new ones. But the bacteria evolve in response to these new drugs--creating in effect an antibiotic arms race. To combat quickly evolving bacteria, many doctors prescribe multiple antibiotics to be taken at the same time. Such drug "cocktails" greatly reduce the chances that any bacteria cells will survive and multiply.