Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [81]
THE FLAGELLUM AFTER DOVER
It was a delicious coincidence that the Dover trial, which brought E. coli’s flagellum to the world’s attention, took place right around the time scientists were starting to get a good look at the flagellum’s evolution. They began to trace the history of its genes by finding related genes both in E. coli and in other microbes. Together those genealogies are beginning to add up to a history of the flagellum—and an illustration of how life can produce a complex trait.
The most important lesson of this new research is that it’s absurd for creationists to talk of the flagellum. From species to species there’s a huge amount of variation in flagella. Even within a single species different populations of microbes may make different kinds of flagella.
Flagella vary at all levels, from their finest features to their biggest. Take flagellin, the protein that E. coli uses to build the tail of its flagella. Scientists have identified forty kinds of flagellin in various strains of E. coli, and they expect to find many more as they expand their survey. And from species to species, flagellins vary even more. In 2003, a ship of microbiologists and geneticists trawled microbes in the Sargasso Sea and analyzed their genes. They discovered 300 genes for flagellins.
These patterns make eminent sense in light of evolution. A single ancestral flagellin gave rise to many new flagellins through gene duplication and mutations. As different species adapted to different environments—from feeding inside the human gut to swimming the Sargasso Sea—their flagellins evolved as well. After E. coli emerged tens of millions of years ago, its flagellins continued to evolve. The variation in its flagellins was probably driven by the need to evade the immune system of its host, which recognizes intruders by the proteins on their surface, such as flagellin. If a mutation makes the outer surface of flagellin harder for an immune system to recognize, it may be favored by natural selection. And just as you’d expect, the most variation found in flagellins in E. coli lies in the parts that face outward. The parts that face inward—and have to lock neatly into the other flagellins—are much more similar to one another. Natural selection does not look kindly on mutations that disturb their tight fit.
Flagella also vary in other ways. E. coli drives its motors with protons, but some species use sodium ions. E. coli spins its flagella through a fluid. Other species make flagella for slithering across surfaces. Scientists have observed some species of bacteria that can make either kind, depending on what sort of swimming they have to do.
In 2005, Mark Pallen of the University of Birmingham in England and his colleagues discovered a set of genes for building slithering flagella in an unexpected place: the genome of E. coli. E. coli cannot actually build these slithering flagella, because the switch that turns on the genes was disabled by a mutation. In some strains, scientists have found all forty-four genes necessary for building all the parts of the slithering flagellum—its hooks, its rings, its filament. In other strains, some of the genes have disappeared entirely. In K-12 only two badly degraded genes remain.
Pallen’s discovery makes ample sense if flagella are the product of evolution, and it makes no sense at all if they are the result of intelligent design. A complex feature evolves and is passed down from ancestors to descendants. In some lineages it falls apart. Darwin described many rudimentary organs, from the flesh-covered eyes of a cave fish to the stubby wings of ostriches. If natural selection no longer favored their use, Darwin argued, individuals would be able to survive well enough even if the organs no longer served their original function. E. coli carries vestiges as well, like ancient passages hidden in a palimpsest.
E. coli also carries clues to how its flagellum evolved in the first place. As Kenneth