Darwinian evolution [is] the transmission of genes and traits down the family line. DNA, it turns out, can also be passed laterally, between individuals, including those of different species. This discovery represented a tectonic shift in our understanding of nature, a story that David Quammen tells wonderfully in his exhaustively researched book, Tangled Tree: A Radical New History of Life.
Darwinian evolution, of course, can explain the rise of antibiotic-resistant bugs. It happens like this. A colony of bacteria gets doused in a deadly antibiotic. Amidst the die off, one bacterium has a lucky mutation that, say, lets it manufacture a molecule that can pump the antibiotic safely out of its cytoplasm into the surrounding slime. That lucky guy thrives and divides and replaces its massacred brethren.
But unfortunately for us (and unknown to Darwin), bacteria possess another means to acquire antibiotic resistance without having to sit around waiting for the next lucky mutation: They can swap genes the way we share recipes. When one bacterium rolls up close to another—not necessarily even of the same species—it can share a chromosome containing a slew of genes with, say, an enzyme that can smash penicillin into pieces.
Horizontal gene transfer is much more than a way for bacteria to share antibiotic resistance genes; it happens throughout nature and in the history of living things.
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