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Authors: Olivia Judson
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“Eighty-five million years without meiosis. Without genetic exchange. Without men,” said Miss
Philodina
. “For eighty-five million years, we've done nothing but clone, and we're jolly proud of it. What's more, we think everyone should follow our example.”
Philodina
. “For eighty-five million years, we've done nothing but clone, and we're jolly proud of it. What's more, we think everyone should follow our example.”
She looked all set to go on advocating abstinence, but the belligerent ram in the front leapt to his hooves. He had fixed to his fleece a large badge exhorting everyone to “Save Our Sex.” He didn't believe for a moment that bdelloid rotifers are genuine ancient asexualsâor, indeed, that any ancient asexuals exist at all.
Punctuating his speech with vigorous nods of his woolly head, he bleated: “Baaaa. Miss
Philodina
, you claim to be the descendant of a militant virgin who got rid of men and abandoned sex millions of years ago. If what you say is true,” he paused to emphasize his skepticism, “it would be sensational. Baaaa. Perhaps you are not aware that other organisms have made this claimâand that it has never stood up to scrutiny.”
Philodina
, you claim to be the descendant of a militant virgin who got rid of men and abandoned sex millions of years ago. If what you say is true,” he paused to emphasize his skepticism, “it would be sensational. Baaaa. Perhaps you are not aware that other organisms have made this claimâand that it has never stood up to scrutiny.”
The ram clearly knew what was at stake. If ancient asexuals really walk the planet, the ramificationsâhere I bleated respectfullyâare dramatic. Again, to spell it out: if they can do without sex or men, maybe the rest of us can too. So you can see why he tried so hard to paint her as a fraud.
“Some of you will remember the case of the chaetonotid gastrotrichsâmicroscopic animals much like you, Miss
Philodina
, who also live in puddles and mosses,” he said. “Like you, they claimed to be ancient asexuals. But when scientists looked at them more closely, they were caught making sperm, an activity not exactly consistent with asexuality, I think you'll agree.
Philodina
, who also live in puddles and mosses,” he said. “Like you, they claimed to be ancient asexuals. But when scientists looked at them more closely, they were caught making sperm, an activity not exactly consistent with asexuality, I think you'll agree.
“And who could forget the aphids of the Tramini tribe? Baaaa. What liars! Those fat little insects, too, said they were ancient asexuals. Another hoax! Genetic tests showed they weren't as virtuous as they pretended. And sure enough, scientists found
that they kept their males hidden among the roots of the weeds where they live.
that they kept their males hidden among the roots of the weeds where they live.
“What can we learn from this?” He paused dramatically. “Besides the bdelloid rotifers, a handful of other groups still profess to be ancient asexuals. The darwinulid ostracodsâa score of species of small freshwater shellfishâclaim to have been without sex for a hundred million years. Baaaa. Certain families of oribatid mites also insist that they got rid of men aeons ago. And there are other self-styled supercelibates: shrimp that live in Old World salt flats, two species of North American fern, and a species of clam. But the evidence for these claims is flimsy, to say the least.
“I put it to you,” the ram thundered, “that all claims of ancient asexuality will turn out to be bogus, the supposed celibates unfrocked! Baaaaaaa. Miss
Philodina,
your chastity is a sham! Like the others before you, you are hiding males, and sooner or later you will be exposed!”
Philodina,
your chastity is a sham! Like the others before you, you are hiding males, and sooner or later you will be exposed!”
As the ram sat down, the audience burst into applause.
I had to say, he was on the mark. Over the years, various organisms
have
claimed to be ancient asexuals; and many of them have indeed been exposed as imposters. Until now, all claims of ancient asexuality have rested on negative evidenceâmainly that no one has ever seen a male of that species. Negative claims are weak and easy to dismiss. After all, biology is full of species where the males and females look so different from each other that for decades they were not recognized as being each other's other halves.
have
claimed to be ancient asexuals; and many of them have indeed been exposed as imposters. Until now, all claims of ancient asexuality have rested on negative evidenceâmainly that no one has ever seen a male of that species. Negative claims are weak and easy to dismiss. After all, biology is full of species where the males and females look so different from each other that for decades they were not recognized as being each other's other halves.
Well, I thought the audience was ready to tear up the seats. I must take my hat off to Miss
Philodina.
She kept her coolâand stunned the audience by producing strong evidence that the bdelloid rotifers are not charlatans but genuine ancient asexuals. To everyone's dismay, she made a credible case that she and her foremothers have indeed managed to live without men or meiosis for
millions of years. They are wholly virgin, purer than pure, the nun's nuns, ultimate maidens, poster children for abstinence.
Philodina.
She kept her coolâand stunned the audience by producing strong evidence that the bdelloid rotifers are not charlatans but genuine ancient asexuals. To everyone's dismay, she made a credible case that she and her foremothers have indeed managed to live without men or meiosis for
millions of years. They are wholly virgin, purer than pure, the nun's nuns, ultimate maidens, poster children for abstinence.
Her proof rests on the fact that cloning for millions of years has dramatic effects on the way that genes evolve. “Being asexual for generations leaves an unmistakable mark, a molecular tattoo on your genes,” she said smugly. “If you always clone, there's only one source of genetic novelty, only one thing that could cause my genes to differ from my mother's, grandmother's, or great-great-great grandmother's: mutation.”
I reminded everyone that mutations are nothing more than sporadic mistakes made by the cell's genetic copying mechanism.
Miss
Philodina
went on, “Let's go back to my ancestor of eighty-five million years ago, the last child of original sin in my family. Say she inherited two copies of a gene for, I don't know, wheel number. One copy came from her father, the other from her mother. And for the sake of argument, let's say they were identical. But now it's eighty-five million years later. Since a bdelloid lives about three weeks, that's about 1.5 billion bdelloid rotifer generations. So you would expect that my two copies of the wheel number gene would be extremely different from each other. Each will have accumulated different mutations.”
Philodina
went on, “Let's go back to my ancestor of eighty-five million years ago, the last child of original sin in my family. Say she inherited two copies of a gene for, I don't know, wheel number. One copy came from her father, the other from her mother. And for the sake of argument, let's say they were identical. But now it's eighty-five million years later. Since a bdelloid lives about three weeks, that's about 1.5 billion bdelloid rotifer generations. So you would expect that my two copies of the wheel number gene would be extremely different from each other. Each will have accumulated different mutations.”
The best way to understand this process is through an analogy. Imagine an ancient manuscript had been copied again and again by monks in two distant and lonely monasteries. If each new version of the manuscript is copied from the previous one, more and more mistakes will creep in. And unless the monks are telepathic from scriptorium to scriptorium, the mistakes they make will be different. As time goes by, the manuscripts owned by the two monasteries will diverge more and more. In contrast, if you were having sex, it would be as if the monks in the two monasteries were regularly copying from each other, as well as frequently replacing their versions with manuscripts from monasteries
elsewhere. The communication between all the monks would ensure that the manuscripts resembled each other closely.
elsewhere. The communication between all the monks would ensure that the manuscripts resembled each other closely.
Extensive divergence between the two copies of any given gene, Miss
Philodina
explained, is the molecular stamp of ancient asexuality.
Philodina
explained, is the molecular stamp of ancient asexuality.
The homing pigeon, his wings twitching with such excitement that he accidentally lifted off, shouted from the air above his perch, “But an ancient text copied 1.5 billion times by two independent groups would change beyond recognition! I don't believe these patterns are detectable!”
“Identifying the two copies of a gene can certainly be difficult. But luckily, in our case, they hadn't changed beyond recognition,” said Miss
Philodina.
And then she played her trump card. Triumphantly spinning her wheels in the ram's direction, she put an end to the accusations of imposture, revealing that genetic tests had shown that bdelloid rotifers have the predicted pattern of divergence. Brandishing a copy of
Science
magazine, Miss
Philodina
said with a bounce, “The evidence is conclusive. We bdelloids are celibate. Male bdelloid rotifers do not exist.”
Philodina.
And then she played her trump card. Triumphantly spinning her wheels in the ram's direction, she put an end to the accusations of imposture, revealing that genetic tests had shown that bdelloid rotifers have the predicted pattern of divergence. Brandishing a copy of
Science
magazine, Miss
Philodina
said with a bounce, “The evidence is conclusive. We bdelloids are celibate. Male bdelloid rotifers do not exist.”
The radical feminists at the back greeted this news with a rousing chorus of “That's all right, that's OK, nobody needs them anyway!”
The rest of the audience, however, didn't seem at all happy. As I looked around the studio, I saw long face after long face, and the room hummed with angry murmuring. No one could any longer dispute Miss
Philodina's
ancient asexual credentials, so instead the crowd became abusive, insinuating the bdelloid rotifers' success was a momentary aberration and that they were surely heading for extinction like other asexuals. A python coiled in a corner raised a large placard that read “The Bdelloids Are Bdoomed” and hissed menacingly, “You're going extinct, you spineless spinster, you're going extinct.”
Philodina's
ancient asexual credentials, so instead the crowd became abusive, insinuating the bdelloid rotifers' success was a momentary aberration and that they were surely heading for extinction like other asexuals. A python coiled in a corner raised a large placard that read “The Bdelloids Are Bdoomed” and hissed menacingly, “You're going extinct, you spineless spinster, you're going extinct.”
“In the long run we're all going extinct,” said Miss
Philodina
tartly. “Sex won't save you from extinction! The dinosaurs had rampant sex, and look what happened to them. You can have sex till you're blue in the face, but if your habitat vanishes, it's you and the dodo. Asexualsâ”
Philodina
tartly. “Sex won't save you from extinction! The dinosaurs had rampant sex, and look what happened to them. You can have sex till you're blue in the face, but if your habitat vanishes, it's you and the dodo. Asexualsâ”
The pocket mouse bravely interrupted: “But surely if you don't have sex, you can't adapt to the future? If you can't adapt to the future, you haven't got a future.”
“Who says asexuals can't adapt?” Miss
Philodina
spluttered. “I'll have you know that the bdelloid rotifers are one of the most versatile groups on earth. We make up a sisterhood of more than 360 species. We live in the moss, damp soil, funeral urns, gutters, and puddles of seven continents. You'll find us in the wastes of Antarctica and the jungles of Sumatra. We live in sulfurous hot springs and in the purest dew. Compare that with our distant cousins, the seisonid rotifers. They've always had sex, and a fat lot of good it's done them. There are only two species, and they both live on the bodies of one type of shrimp. Call that evolutionary success? Humph. I call it a miserable failure.”
Philodina
spluttered. “I'll have you know that the bdelloid rotifers are one of the most versatile groups on earth. We make up a sisterhood of more than 360 species. We live in the moss, damp soil, funeral urns, gutters, and puddles of seven continents. You'll find us in the wastes of Antarctica and the jungles of Sumatra. We live in sulfurous hot springs and in the purest dew. Compare that with our distant cousins, the seisonid rotifers. They've always had sex, and a fat lot of good it's done them. There are only two species, and they both live on the bodies of one type of shrimp. Call that evolutionary success? Humph. I call it a miserable failure.”
I stepped in to stop all the snarling. “The thing to focus on,” I said, “is how exceptional the bdelloid rotifers are. They are the only asexual group with lots of species. After eighty-five million years, nobody rational could suppose their extinction is looming. But most asexuals don't survive. Understanding why they don'tâand how the bdelloids haveâcan give us important clues as to why we need sex.”
Hallelujah! I'd managed to get the audience back to the matter at hand. I explained that there are more than twenty theories that purport to explain the role of sex, and I briefly summarized the three front-runners, popularly known as Muller's ratchet, Kondrashov's hatchet, and the Red Queen. According to both the ratchet and the hatchet, asexuals are driven extinct by the accumulation
of harmful mutationsâin other words, asexuals eventually die of genetic diseases. The Red Queen, in contrast, invokes a more traditional horseman of the apocalypse: pestilence, also known as infectious disease.
of harmful mutationsâin other words, asexuals eventually die of genetic diseases. The Red Queen, in contrast, invokes a more traditional horseman of the apocalypse: pestilence, also known as infectious disease.
Moby the puffer fish splashed straight into the subject of harmful mutations: “Miss
Philodina,
without sex, how can asexuals get rid of harmful mutations? And if you'll pardon my saying so, your wheels are looking a little wonkyâmy eyes aren't too good, so perhaps it's just the light, but the one on the left actually looks
square.
I bet it's those mutations you were saying you've accumulated.”
Philodina,
without sex, how can asexuals get rid of harmful mutations? And if you'll pardon my saying so, your wheels are looking a little wonkyâmy eyes aren't too good, so perhaps it's just the light, but the one on the left actually looks
square.
I bet it's those mutations you were saying you've accumulated.”
Miss
Philodina
hit right back. “I may be square, but my wheels are not.” She sounded confident enough, but I swear I saw her spinning them, just to check. âAnd if you'll pardon
my
saying so,
dear
puffer fish,”she went on, “mutations are greatly overrated as an evolutionary force. Geneticists think mutations are bad because their methods are so crude. All they can see are the bad mutations.
Obviously,
having no head is bad for you. And if you're a fly, having no wings isn't much good. For a start, you'd have to be called a âwalk'. But, in fact, most mutations are neutral. They have no effect. They change the DNA sequence of a gene, sure. But they don't affect the information. It's like switching from the English spelling of a word to the American spelling. “Pâlâoâuâgâh” and “pâlâoâw” look different on paper, but they mean the same thing and sound the same when said aloud.”
Philodina
hit right back. “I may be square, but my wheels are not.” She sounded confident enough, but I swear I saw her spinning them, just to check. âAnd if you'll pardon
my
saying so,
dear
puffer fish,”she went on, “mutations are greatly overrated as an evolutionary force. Geneticists think mutations are bad because their methods are so crude. All they can see are the bad mutations.
Obviously,
having no head is bad for you. And if you're a fly, having no wings isn't much good. For a start, you'd have to be called a âwalk'. But, in fact, most mutations are neutral. They have no effect. They change the DNA sequence of a gene, sure. But they don't affect the information. It's like switching from the English spelling of a word to the American spelling. “Pâlâoâuâgâh” and “pâlâoâw” look different on paper, but they mean the same thing and sound the same when said aloud.”
Well, well, well. Given all her mutations, I guess I should have known that Miss
Philodina
would be a neutralistâthat she would subscribe to a controversial, even radical school of thought that holds that most mutations are neither helpful nor harmful, just irrelevant. I couldn't let her get away with it, though. First of all, there's a vigorous debate going on over whether most mutations are neutral. And second of all, it's generally agreed that
when a mutation does have an effect, the effect is usually bad: small random changes are likely to harm, not help. Or to put it more starkly, many different mutations will kill you or make you sick, but none guarantees success in life. Which brings us to the ratchet and the hatchet.
Philodina
would be a neutralistâthat she would subscribe to a controversial, even radical school of thought that holds that most mutations are neither helpful nor harmful, just irrelevant. I couldn't let her get away with it, though. First of all, there's a vigorous debate going on over whether most mutations are neutral. And second of all, it's generally agreed that
when a mutation does have an effect, the effect is usually bad: small random changes are likely to harm, not help. Or to put it more starkly, many different mutations will kill you or make you sick, but none guarantees success in life. Which brings us to the ratchet and the hatchet.
According to Muller's ratchet (named for its inventor, the geneticist Hermann Muller, who won the Nobel prize for demonstrating that X rays cause mutations), asexuals are evolutionarily short-lived because, over time, the number of harmful mutations they carry will irrevocably and inevitably ratchet upward. Imagine a population that has just become asexual. For the sake of argument, imagine that all members of the population are free of harmful mutations. Over time, copying errors will lead to mutations among their descendants, and gradually the population will consist of individuals who carry several mutations. Then one day the last mutation-free individual will fail to leave children, and the ratchet will have clicked forward one notch. This process continues until eventually all the individuals are so sick that they die and the population goes extinct. Sexuals avoid this fate because the shuffling of genes in each generation produces individuals who carry few mutations.
Muller's ratchet is an elegant idea. But it works only if a number of assumptions are met. The most important of these is that the asexual population is small. In large populations, you see, there may always remain some individuals bearing few mutations. Kondrashov's hatchet (also named for its inventor, a Russian geneticist), however, is another story: it holds regardless of population size.
Suppose there's a threshold number of slightly harmful mutations that any individual can carry. Above that threshold, the hatchet fallsâand you're dead. In a population that has sex, the
shuffling of genes creates some lucky creatures with few harmful mutations. But it also creates some unlucky ones with many. The unlucky ones fall under the hatchet, taking their mutations to the grave. This quickly and efficiently purges the population of harmful mutations. Asexuals, however, have no such recourse. Far more asexual individuals will cross the threshold of having one bad mutation too many. According to the theory, if the harmful mutation rate is high enough, there is no way to survive without sex.
shuffling of genes creates some lucky creatures with few harmful mutations. But it also creates some unlucky ones with many. The unlucky ones fall under the hatchet, taking their mutations to the grave. This quickly and efficiently purges the population of harmful mutations. Asexuals, however, have no such recourse. Far more asexual individuals will cross the threshold of having one bad mutation too many. According to the theory, if the harmful mutation rate is high enough, there is no way to survive without sex.
BOOK: Dr. Tatiana's Sex Advice to All Creation
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