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Book: Arrival of the Fittest: Solving Evolution's Greatest Puzzle by Andreas Wagner
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Authors: Andreas Wagner
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planet: bacteria.
This isn’t just because bacteria produce new generations in minutes rather than years, and so can improve their genetic toolkits much faster than we. The innovation advantage of bacteria goes much deeper than that. To grasp how big it is, imagine a teenage boy trying to make his high school basketball team, even though he’s only a shade over five feet tall. Hard work and exercise can only take him so far. He just doesn’t have the right genes—not like his best friend, who can practically touch the rim on his tiptoes.
A bacterium wanting the bacterial equivalent of a forty-inch vertical leap isn’t limited by the genes bequeathed it by previous generations. If, in some science-fiction movie, perhaps, the two basketball-playing friends had the same innovation equipment as bacteria, the process would look something like this. Our two characters are dining in their favorite restaurant when a slender hollow tube begins to grow out of the taller boy’s body, blindly groping toward the shorter. As soon as it connects, this tube injects a random fragment of the taller guy’s DNA text into the other body, and if this DNA contains the right genes, the high school basketball team gets a new power forward .
This is an example of horizontal gene transfer, a phenomenon tragically unavailable to disadvantaged humans but rampant in microbes. Sometimes when two bacteria are in proximity, one of them will extrude a slender stalklike hollow tube in the direction of the other. When the tube docks, it shrinks in length, draws the two cells closer, and through the tunnel thus created, one cell transfers DNA to its neighbor.
This transfer resembles sex as we know it, because a penislike tube transfers genetic material from one organism to another. But bacterial and human sex are quite different. Their sex, unlike ours, does not serve reproduction. And it doesn’t even shuffle a whole genome, but usually just transfers a few genes.
Bacteria can acquire new genes in other ways too. Some absorb DNA from other cells after they die, rupture, and spill their molecular innards. As boneheaded as a person who would rather eat books—surely a great source of fiber—than read them, they use some of this DNA as food. Occasionally, though, the eaten DNA becomes hitched to their genome and helps make new proteins. 17
Gene transfer can also take advantage of viruses, those tiny lifeless particles whose DNA can enslave cells many times their size. 18 While viruses reprogram a cell into a helpless factory that clones its viral masters, small pieces of the cell’s DNA can fuse with the viral genome. These pieces piggyback on newly minted viruses that leave the cell, and get injected along with the viral genome into the next hapless victim. In this scenario, one of our basketball-playing friends might simply have to sneeze on the other, whereupon his talent could be transferred to his teammate’s newly improved genome.
If all this horizontal gene transfer went on unchecked, the size of a genome would constantly increase over time and become grotesquely bloated. But excessively long DNA strings break more easily, and copying them wastes energy and materials—a mortal sin that nature won’t tolerate. 19 Fortunately, such bloating does not occur, because gene transfer is balanced by gene deletions. These are the by-products of errors that happen when cells cut or splice DNA molecules as they repair and copy their DNA. Unlike DNA mutations that alter one letter at a time, deletions can strike out thousands of letters and many genes. As long as a deletion affects no essential genes, the cell can live with it. Such survivable deletions occur all the time. They ensure that only useful genes stay around in the long run, and they keep genomes lean.
In another difference from sex as we know it, gene transfer occurs not just between similar organisms but also between baker’s yeast and fruit flies, between microbes and plants, and
This isn’t just because bacteria produce new generations in minutes rather than years, and so can improve their genetic toolkits much faster than we. The innovation advantage of bacteria goes much deeper than that. To grasp how big it is, imagine a teenage boy trying to make his high school basketball team, even though he’s only a shade over five feet tall. Hard work and exercise can only take him so far. He just doesn’t have the right genes—not like his best friend, who can practically touch the rim on his tiptoes.
A bacterium wanting the bacterial equivalent of a forty-inch vertical leap isn’t limited by the genes bequeathed it by previous generations. If, in some science-fiction movie, perhaps, the two basketball-playing friends had the same innovation equipment as bacteria, the process would look something like this. Our two characters are dining in their favorite restaurant when a slender hollow tube begins to grow out of the taller boy’s body, blindly groping toward the shorter. As soon as it connects, this tube injects a random fragment of the taller guy’s DNA text into the other body, and if this DNA contains the right genes, the high school basketball team gets a new power forward .
This is an example of horizontal gene transfer, a phenomenon tragically unavailable to disadvantaged humans but rampant in microbes. Sometimes when two bacteria are in proximity, one of them will extrude a slender stalklike hollow tube in the direction of the other. When the tube docks, it shrinks in length, draws the two cells closer, and through the tunnel thus created, one cell transfers DNA to its neighbor.
This transfer resembles sex as we know it, because a penislike tube transfers genetic material from one organism to another. But bacterial and human sex are quite different. Their sex, unlike ours, does not serve reproduction. And it doesn’t even shuffle a whole genome, but usually just transfers a few genes.
Bacteria can acquire new genes in other ways too. Some absorb DNA from other cells after they die, rupture, and spill their molecular innards. As boneheaded as a person who would rather eat books—surely a great source of fiber—than read them, they use some of this DNA as food. Occasionally, though, the eaten DNA becomes hitched to their genome and helps make new proteins. 17
Gene transfer can also take advantage of viruses, those tiny lifeless particles whose DNA can enslave cells many times their size. 18 While viruses reprogram a cell into a helpless factory that clones its viral masters, small pieces of the cell’s DNA can fuse with the viral genome. These pieces piggyback on newly minted viruses that leave the cell, and get injected along with the viral genome into the next hapless victim. In this scenario, one of our basketball-playing friends might simply have to sneeze on the other, whereupon his talent could be transferred to his teammate’s newly improved genome.
If all this horizontal gene transfer went on unchecked, the size of a genome would constantly increase over time and become grotesquely bloated. But excessively long DNA strings break more easily, and copying them wastes energy and materials—a mortal sin that nature won’t tolerate. 19 Fortunately, such bloating does not occur, because gene transfer is balanced by gene deletions. These are the by-products of errors that happen when cells cut or splice DNA molecules as they repair and copy their DNA. Unlike DNA mutations that alter one letter at a time, deletions can strike out thousands of letters and many genes. As long as a deletion affects no essential genes, the cell can live with it. Such survivable deletions occur all the time. They ensure that only useful genes stay around in the long run, and they keep genomes lean.
In another difference from sex as we know it, gene transfer occurs not just between similar organisms but also between baker’s yeast and fruit flies, between microbes and plants, and
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