Most genes are passed vertically from one generation to another by sexual reproduction within a species or during hybridization between closely related species. Genetic information can also move horizontally among different species of organisms by means other than sexual reproduction. Though much rarer than vertical gene transmission, horizontal gene transfer has occurred many times. It does not alter the process of natural selection, which acts upon genetic variability in populations, whatever its source, but it does alter the scientific understanding of where genetic variability comes from. Most evolutionary explanations assume that genetic variability is enhanced mostly by mutations occurring in the genes of individuals in a population, or by immigrants from outside the population but from the same species. With horizontal gene transfer, a whole new source of genetic variation appears: injection of genes from entirely different species.

For several decades scientists have known that bacteria can exchange segments of DNA with one another. In a process called conjugation, one bacterium grows a tube that connects with another bacterium, and segments of DNA (usually small circles of genes called plasmids) travel through the tube. Bacteria can also absorb intact chunks of DNA from their fluid environments, although usually this requires some sort of temperature or chemical shock to make their cell membranes receptive to the transfer. Both conjugation and the absorption of DNA segments increase under conditions of environmental stress. This allows populations of bacteria to increase their variability of genetic combinations.

The surprise came when microbiologists realized that conjugation and other forms of gene transfer could occur between bacterial species. At first this was considered an interesting but unimportant aberration, but since antibiotic resistance genes are often on plasmids, interspecific conjugation turned out to be very important indeed: Antibiotic-resistant bacteria can make other bacteria, even of a different species, resistant to the antibiotic. This is how antibiotic resistance can spread so rapidly, even from one species of bacterium to another. Recent research suggests some pathogenic bacteria have obtained resistance genes from bacteria that live in the soil. Horizontal gene transfer has turned out to be one of the central facts in public health research. It is widespread enough that one prominent evolutionary biologist has claimed that there is really only one species of bacterium: According to the biological species concept, potentially interbreeding organisms can, if they produce . . .

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