3. “Humans defeat germs!” (but not for long)

In bacteria, one mutated cell appears for every 100 million normal cells. Because some bacteria reproduce as quickly as every 20 minutes, new populations of mutants emerge literally overnight. Most mutations give no discernible advantages or disadvantages to the cell. Unfavorable mutations make bacteria vulnerable to other microbes or the environment, and these cells and their genes disappear forever. On rare occasions a mutation gives a bacterial cell a favorable characteristic called a trait that enables the bacterium to outperform others.

Most people remember from Biology class that a favorable mutation appears only because of a random event. “The survival of the fittest” comes not by plan but ...

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