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Chapter 19 : Do Bacteria Have Sex?
Bacteria do not reproduce sexually; they have one set of genes rather than two, and their cells simply divide-they never fuse with each other like eggs and sperm. Short pieces of the DNA that genes are made of sometimes move from one bacterial cell to another, and this can randomly mix their different versions of genes if it happens often enough, just like real sexual reproduction does. For a long time microbiologists assumed that these parasites were a bacterial way of having sex-that is, that the evolutionary forces responsible for our sexual reproduction had indeed acted on bacteria too, producing ways to move DNA between cells. Nutrients and temperature conditions are usually very different from those the bacteria experience in their natural environments, partly because they are more convenient and partly because one really does not know what the natural conditions are. But studying bacteria under such unnatural conditions makes it very easy to misinterpret what they do. Most bacteria use nutritional signals, just as the DNA-equals-food explanation predicts. Just as people who read only the newspaper headlines think every lottery ticket is a big winner, a scientist who reads only the research papers about bacterial genome sequences could easily conclude that every new gene is an improvement. In either case, the discovery that bacteria do not have sex means that a big key to the puzzle of the evolution of sex will come from finding out why bacteria need so much less mixing than plants and animals.