Why didn't humans evolve to reproduce identical twins all the time?

Why didn't humans evolve to reproduce identical twins all the time?

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According to the selfish gene theory, it seems like because identical twins sometimes get produced, a mutation to a gene that says, "if you have an identical twin, be fully altruistic towards them" would get selected for when ever it happens to arise in an identical twin. After humans evolve to be fully altruistic towards identical twins, wouldn't a mutation to a gene that makes a woman more likely to produce identical twins also be selected for because if it happens to appear in a a child who has a non-identical sibling, that gene might be discontinued from reproduction by the partial selfishness of the sibling but if it appears in an identical twin, the other identical twin will preserve that gene with their altruism.

What you describe could have happened under the right conditions. However, there are a few things you haven't considered.

  1. Because humans are especially altricial, always having twins would double the cost of children on parents.
  2. The benefit of sexual reproduction is immune diversity. So a population like this could be far more vulnerable to disease.

So as far as this thought experiment goes, genes can't be too selfish or they dramatically reduce their own fitness. It's something worth simulating computationally to see what happens.


From jzx's answer, I thought of a possible answer to my own question. Maybe not producing identical twins all the time is a paradoxical evolutionary stable strategy for the following reason:

Until recently in evolutionary history, the population remained constant when parents only had enough food to feed 2 children. Siblings didn't evolve to be fully altruistic towards each other so if a parent had 3 children in case of the small chance of having enough food to feed 3, none of them would willing let themself starve to make sure the others survive, risking making only one survive, so the parents would have had an evolutionary advantage in only producing 2 children in the first place. Although definitely producing identical twins is an evolutionary advantage, a mutation that makes the chance of producing identical twins slightly higher than it already is is an evolutionary disadvantage because a woman with an increased chance of producing identical twins might have produced 1 child on her first pregnancy and identical twins on her second pregnancy, giving a total of 3 children and increasing the risk of only rearing 1.

If we produced identical twins all the time then we'd completely lose genetic diversity which would, I assume, overwhelmingly compromise our adaptive capabilities. Don't forget, those identical twins then need to reproduce with other identical twins of others families. Moreover, and this is the more obvious explanation, as a consequence deleterious recessive disorders would rapidly increase in rate.

For more, see

You might also like to study frequency dependent selection for why it is sometimes advantageous to have identical twins, albeit at a low rate.

Why haven't all primates evolved into humans?

While we were migrating around the globe, inventing agriculture and visiting the moon, chimpanzees — our closest living relatives — stayed in the trees, where they ate fruit and hunted monkeys.

Modern chimps have been around for longer than modern humans have (less than 1 million years compared to 300,000 for Homo sapiens, according to the most recent estimates), but we've been on separate evolutionary paths for 6 million or 7 million years. If we think of chimps as our cousins, our last common ancestor is like a great, great grandmother with only two living descendants.

But why did one of her evolutionary offspring go on to accomplish so much more than the other? [Chimps vs. Humans: How Are We Different?]

"The reason other primates aren't evolving into humans is that they're doing just fine," Briana Pobiner, a paleoanthropologist at the Smithsonian Institute in Washington, D.C., told Live Science. All primates alive today, including mountain gorillas in Uganda, howler monkeys in the Americas, and lemurs in Madagascar, have proven that they can thrive in their natural habitats.

"Evolution isn't a progression," said Lynne Isbell, a professor of anthropology at the University of California, Davis. "It's about how well organisms fit into their current environments." In the eyes of scientists who study evolution, humans aren't "more evolved" than other primates, and we certainly haven't won the so-called evolutionary game. While extreme adaptability lets humans manipulate very different environments to meet our needs, that ability isn't enough to put humans at the top of the evolutionary ladder.

Take, for instance, ants. "Ants are as or more successful than we are," Isbell told Live Science. "There are so many more ants in the world than humans, and they're well-adapted to where they're living."

While ants haven't developed writing (though they did invent agriculture long before we existed), they're enormously successful insects. They just aren't obviously excellent at all of the things humans tend to care about, which happens to be the things humans excel at.

"We have this idea of the fittest being the strongest or the fastest, but all you really have to do to win the evolutionary game is survive and reproduce," Pobiner said.

Our ancestors' divergence from ancestral chimps is a good example. While we don't have a complete fossil record for humans or chimps, scientists have combined fossil evidence with genetic and behavioral clues gleaned from living primates to learn about the now-extinct species whose descendants would become humans and chimps.

"We don't have its remains, and I'm not sure if we'd be able to place it with certainty in the human lineage it if we did," Isbell said. Scientists think this creature looked more like a chimpanzee than a human, and it probably spent most of its time in the canopy of forests dense enough that it could travel from tree to tree without touching the ground, Isbell said.

Scientists think ancestral humans began distinguishing themselves from ancestral chimps when they started spending more time on the ground. Perhaps our ancestors were looking for food as they explored new habitats, Isbell said.

"Our earliest ancestors that diverged from our common ancestor with chimpanzees would have been adept at both climbing in trees and walking on the ground," Isbell said. It was more recently — maybe 3 million years ago — that these ancestors' legs began to grow longer and their big toes turned forward, allowing them to become mostly full-time walkers.

"Some difference in habitat selection probably would've been the the first notable behavioral change," Isbell said. "To get bipedalism going, our ancestors would have gone into habitats that didn't have closed canopies. They would have had to travel more on the ground in places where trees were more spread out."

The rest is human evolutionary history. As for the chimps, just because they stayed in the trees doesn't mean they stopped evolving. A genetic analysis published in 2010 suggests that their ancestors split from ancestral bonobos 930,000 years ago, and that the ancestors of three living subspecies diverged 460,000 years ago. Central and eastern chimps became distinct only 93,000 years ago.

"They're clearly doing a good job at being chimps," Pobiner said. "They're still around, and as long as we don't destroy their habitat, they probably will be" for many years to come.

Twins Separated at Birth Reveal Staggering Influence of Genetics

WASHINGTON — Jim Lewis and Jim Springer were identical twins raised apart from the age of 4 weeks. When the twins were finally reunited at the age of 39 in 1979, they discovered they both suffered from tension headaches, were prone to nail biting, smoked Salem cigarettes, drove the same type of car and even vacationed at the same beach in Florida.

The culprit for the odd similarities? Genes.

Genes can help explain why someone is gay or straight, religious or not, brainy or not, and even whether they're likely to develop gum disease, one psychologist explains.

Such broad-ranging genetic effects first came to light in a landmark study — Minnesota Twin Family Study — conducted from 1979 to 1999, which followed identical and fraternal twins who were separated at an early age. [Seeing Double: 8 Fascinating Facts About Twins]

"We were surprised by certain behaviors that showed a genetic influence, such as religiosity [and] social attitudes," said Nancy Segal, an evolutionary psychologist at California State University, Fullerton, who was part of the study for nine years. "Those surprised us, because we thought those certainly must come from the family [environment]," Segal told Live Science. Segal described the groundbreaking research on Aug. 7 here at a meeting of the American Psychological Association.

Born together, raised apart

Researchers at the University of Minnesota, led by Thomas Bouchard, launched the landmark study in 1979. Over the course of 20 years, they studied 137 pairs of twins — 81 pairs of identical twins (twins who developed from one egg that split in two), and 56 pairs of fraternal twins (twins who developed from two eggs fertilized by two different sperm).

The Jim twins were probably the most famous set of twins involved in the study, but other pairs were equally fascinating. One pair of female twins in the study were separated from each other at 5 months old, and weren't reunited until age 78, making them the world's longest separated pair in Guinness World Records.

The Minnesota study resulted in more than 170 individual studies focusing on different medical and psychological characteristics.

In one study, the researchers took photographs of the twins, and found that identical twins would stand the same way, while fraternal twins had different postures.

Another study of four pairs of twins found that genetics had a stronger influence on sexual orientation in male twins than in female twins. A recent study in Sweden of 4,000 pairs of twins has replicated these findings, Segal said. [5 Myths About Gay People Debunked]

Nature vs. nurture

A 1986 study that was part of the larger Minnesota study found that genetics plays a larger role on personality than previously thought. Environment affected personality when twins were raised apart, but not when they were raised together, the study suggested.

Reporter Daniel Goleman wrote in The New York Times at the time that genetic makeup was more influential on personality than child rearing — a finding he said would launch "fierce debate."

"We never said [family environment] didn't matter," Segal said at the APA meeting. "We just made the point that environment works in ways we hadn't expected."

Another study, commissioned by the editor of the journal Science, looked at genetics and IQ. The Minnesota researchers found that about 70 percent of IQ variation across the twin population was due to genetic differences among people, and 30 percent was due to environmental differences. The finding received both praise and criticism, but an updated study in 2009 containing new sets of twins found a similar correlation between genetics and IQ.

Moreover, a study in 1990 found that genetics account for 50 percent of the religiosity among the population — in other words, both identical twins raised apart were more likely to be religious or to be not religious, compared with unrelated individuals.

Other studies found a strong genetic influence on dental or gum health. That research helped to show that gum disease isn't just caused by bacteria, it also has a genetic component, Segal said.

Another study found that happiness and well-being had a 50 percent genetic influence.

In another study, researchers surveyed the separated twins about how close they felt to their newfound sibling. Among identical twins, 80 percent of those surveyed reported feeling closer and more familiar with their twin than they did to their best friends, suggesting a strong genetic component in the bond between identical twins.

The Minnesota study gave scientists a new understanding of the role of genes and environment on human development, Segal said. In the future, twin studies will aim to link specific genes to specific behaviors, as well as investigate epigenetics — what turns genes on or off, she said.

Segal, who wrote a book about the study called "Born Together — Reared Apart: The Landmark Minnesota Twins Study" (Harvard University Press, 2012), is now doing a prospective study of Chinese twins raised apart, often in different countries, by adoptive families.

What increases the odds of having identical twins?

  • Luck needs to be on your side. Identical twins do not run in families and can happen for any couple with any pregnancy. Non-identical twins are influenced by genetics women whose mothers or grandmothers had non-identical twins do have more chance of having non-identical twins themselves. But identical twins are just random.
  • Some families do seem to have a higher than average number of identical twins. Though this can only be contributed to luck, coincidence and chance rather than any familial genetic tendency.

Twin peak: Global twin rates at historic highs, and here’s why

Researchers analysed records from more than 100 countries and found a substantial rise in twin birthrates since the 1980s, with one in 42 people now born a twin, equivalent to 1.6 million children a year. According to the study, the global twin birthrate has risen by one-third, on average, over the past 40 years.

Rather than seeing the trend continue, the world may have reached “peak twin”, the authors say, as the most recent data suggest some countries have begun to see twinning rates plateau or even fall from historic highs.

While the birthrate for identical twins has barely changed over time, [sociologist Christiaan] Monden and his colleagues found that naturally conceived, non-identical twins and twins born as a result of medically assisted reproduction – an umbrella term for a range of fertility treatments – had risen globally.

The main drivers are increased access to hormone treatment, IVF and other fertility services but also the postponement of parenthood – the chances of having natural, non-identical twins increases with age and peaks at 35 to 39 years old.

A strategy for prolonging fertility

The reason a switch is beneficial is fetal survival – the chance that a fertilised egg will result in a liveborn child – decreases rapidly as women age

So switching to releasing two eggs increases the chance at least one will result in a successful birth.

But what about twinning? Is it a side effect of selection favouring fertility in older women? To answer this question, we ran the simulations again, except now when women double ovulated the simulation removed one offspring before birth.

In these simulations, women who double ovulated throughout their lives, but never gave birth to twins, had more children survive than those who did have twins and switched from single to double ovulating. 1200w, 1800w, 754w, 1508w, 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">

Author provided

This suggests the ideal strategy would be to always double ovulate but never produce twins, so fraternal twins are an accidental side effect of a beneficial strategy of double ovulating.

Joseph L Tomkins receives funding from The Australian Research Council.

Rebecca Sear and Wade Hazel do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.

Why do we have sex?

The birds and the bees and, of course, the fleas. Plants, fungi and amoebas too. It sometimes seems like sex is everywhere. But in biological terms, it is a minority pursuit. For the first 2 billion years of life on Earth, it didn’t exist. Even now, the organisms that dominate the planet – bacteria and archaea – don’t bother.

The origin of sex, then, is a bit of a mystery. And if its origins are hard to understand, its function is just as baffling. At first sight that seems ludicrous. Surely sex has an obvious function: it generates variation, the raw material for evolution. The reshuffling and recombining of genetic information helps species adapt. It can also help spread beneficial genes throughout a population and eliminate harmful ones. But there are big problems with this common sense argument.

The first is that sex is grossly inefficient. It makes much more sense to clone yourself. Cloning produces many more offspring than sex, which means that asexual species should rapidly drive sexual ones to extinction by dint of producing far more offspring competing for the same resources. What’s more, each clone has a combination of genes that has already been shown to be fit for purpose. Sex, by contrast, creates new, untested and possibly inferior combinations. In fact, sexual recombination disrupts favourable gene combinations more often than it generates them.

Sure, sex should be an advantage in the long term, over thousands and millions of years. Asexual species eventually accumulate mutations that they can’t get rid of, and which drive them to extinction. But evolution doesn’t work like that. It doesn’t plan ahead. All it cares about is the here and now.


And the trials and tribulations don’t end there. Sexual species have to find a mate, fight off rivals, and risk catching sexually transmitted diseases.

Finally, if sex is so beneficial, why is it that bacteria and archaea never evolved it, even though they do exchange bits of DNA from time to time? Conversely, if asexual reproduction is so great, why do almost all eukaryotes reproduce sexually at least some of the time? All this makes sex one of the biggest head-scratchers in biology.

For many years the best answer was the Red Queen hypothesis, a subtle variant on the “sex means variety” explanation. This imagines an arms race between parasites and their hosts. The parasites’ generation time is so short that they can out-evolve their hosts. By throwing up new mixtures of genes with each generation, sex enables at least a few individuals to survive. It is named after the Red Queen because, like Alice in Through the Looking Glass, we have to run fast just to stay in the same place.

Unfortunately, it does not solve the problem. Parasites give sex a decisive advantage only when parasite transmission is very high and their effects are very serious. Under normal circumstances, clones still win.

In recent years a new explanation has started to take hold. This is based on the discovery that all eukaryotes are, or at least were, sexual (there are plenty of species that multiply by cloning, but they evolved celibacy only very recently). The logical conclusion is that sex evolved very early on in the eukaryote lineage, in a common ancestor of all living eukaryotes around 2 billion years ago.

Aside from sex, the other thing that unites all eukaryotes is the possession of mitochondria, the cell’s power supply. The new explanation claims that this is no coincidence: mitochondria made the evolution of sex inevitable. How so? The key point is that mitochondria have their own genomes. This is a remnant of the complete genome of the free-living bacterium that was engulfed at the dawn of eukaryote evolution. We know that as the two co-evolved, most of the genes were transferred to the host’s genome. The symbiont also bombarded the host with parasitic jumping genes.

Love conquers all

In other words, the acquisition of mitochondria unleashed a bout of turbulent genetic disruption. Under such high mutation pressure, the balance was tilted and sex became more advantageous than asexual reproduction. Any early eukaryote that evolved it would have outcompeted its asexual rivals, which were succumbing to unsurvivable levels of mutation.

Mitochondria also explain why sex remains advantageous today. The mitochondrial genome encodes vital genes, but cannot do anything on its own. It relies on the nuclear genome to make proteins and replicate its DNA, for example. Close cooperation between the cells’ two genomes is therefore vital to the functioning of the cell, especially in the crucial task of energy generation.

That cooperation is what sex ensures. Because the mitochondrial genome accumulates mutations at a higher rate than the nuclear genome – about ten times faster in mammals – the accord between the two genomes gradually breaks down. We and our mitochondria are drifting apart, and though it is the mitochondria’s fault, we are the ones who suffer. Sex resolves this disharmony by throwing out new combinations of nuclear genes that are more compatible with the mitochondria’s needs.

That is the why of sex. The how, however, remains very unclear. The simplest eukaryotes – amoebas – have sex by splitting their genome in half and then cleaving themselves in two, with half a genome in each portion these half-amoebas then merge with others to create new individuals. That may be how the first sex was done. In broad brush terms, it is still how it is done. Sex just means ripping a genome in half and uniting it with another half-genome from someone else to create a new whole genome. Humans and most other animals achieve that by having two sexes, one of which dumps their half-genomes into the other through the act of copulation. Who said romance was dead?

Your book’s subtitle is “An Intimate History.” Can you talk about the personal inspiration behind the story of the gene?

I’ve been thinking about this book for a very long time. Even before I had written my book on cancer, I had thought about the perennial question of why we are like and unlike each other. In my particular case, the question revolved around mental illness. Why were two uncles and one cousin of my family affected while the rest of us didn’t seem to be? That question was very much part of the background of my childhood and adolescence. My uncle Jagu, who lived with us, was provisionally diagnosed with a mental disease, which was called schizophrenia, but died before a lot of the terminology became clear.

A host of studies were being published in the 2000s linking schizophrenia and bipolar disorder, suggesting that there are strong familial and potentially genetic links between these mental disorders. Added to that was the question why, in crisscrossing family histories, some members are affected and others not. In other words, how did genes intercept with environment and chance to create powerful influences on human form and fate? That’s the central question in the book—and it’s also encapsulated in the history of my family.

The Sex Paradox

Megan Scudellari
Jul 1, 2014


S arah &ldquoSally&rdquo Otto was sitting in a lab meeting of evolutionary biologist Marcus Feldman&rsquos group at Stanford University in 1988 when she overheard a graduate student describe sex as &ldquosuch a big puzzle.&rdquo Otto, an undergraduate at the time, didn&rsquot agree. Sexual reproduction&mdashgenetic recombination between individuals&mdash&ldquoobviously&rdquo promotes variation, she thought, allowing species to adapt to changing environments.

WHY SEX?: See full infographic: JPG | PDF Otto&rsquos reaction makes sense, and echoes one of the oldest formal explanations for why sex evolved to be ubiquitous in the animal kingdom. In 1886, German evolutionary biologist August Weismann proposed that sexual reproduction reshuffles genes to create &ldquoindividual differences&rdquo upon which natural selection acts. Additional ideas have emerged since Weismann&rsquos hypothesis: sex rids the genome of deleterious mutations sex rapidly introduces beneficial mutations sex helps organisms dodge parasitic infections. Yet these evolutionary justifications for sex have remained hypotheses because there is.

“Sexual reproduction permeates biology at every level, and yet it’s not intuitively obvious why it should be that way, because the costs are so high,” says Aneil Agrawal, an evolutionary geneticist at the University of Toronto.

More than 99 percent of multicellular eukaryotes reproduce sexually and have evolved elaborate ways to do so, including behavioral, physiological, and biochemical adaptations. So there must be some enduring benefit. But despite years of observing, theorizing, and experimenting, researchers have been unable to pin down exactly what that might be. “Why sex evolved is very hard to answer,” says Timothy James, who studies sex in fungi at the University of Michigan in Ann Arbor. “[Many] evolutionary biologists are trying to understand why it’s so rarely lost, even though it’s so costly.”

One reason for the continued mystery may be scientists’ choice of study organisms—typically familiar animals with unremarkable sex lives. “We know a lot about sex in humans and mice and Drosophila,” says Otto, now a theoretical biologist at the University of British Columbia who studies how sex evolved. But to better understand the pros and cons of sexual reproduction, the best place to look may be the outliers, such as eukaryotic organisms that have weird sex, or those that have no sex at all.

Today, research on such outliers is happening in force. A new batch of species is pushing researchers closer to resolving the paradox of sex, providing some of the first experimental evidence for and against ideas such as Weismann’s hypothesis. Recent studies using freshwater rotifers and genetically engineered yeast, for example, suggest sex is critical in times when environmental stress requires adaptation, while parasite-battling snails make the case that sex evolved as a powerful defense against infection.

“We are at a stage where we are pushing the boundaries, both in computer simulations and in experiments,” says Otto. “It’s not hopeless. There is movement in the field.”

Finding a partner

Most model organisms, such as mice and flies, are not useful for experiments related to the evolution of sex because they don’t reproduce both sexually and asexually. “If you want to test the effect of sex, you need to hold everything constant apart from the presence or absence of sex,” says Matthew Goddard of the University of Auckland in New Zealand. “Most higher organisms have to have sex to reproduce, so they’re out. Bacteria only reproduce asexually, so they’re out.” But there are a handful of species that fit the bill.

CHANGING WITH THE TIMES: In the lab, yeast that reproduce sexually adapt more rapidly to harsh environmental conditions and survive better than asexual strains, supporting Weismann’s hypothesis. Asexual yeast also appear to be stunted by accumulating deleterious mutations, suggesting that Muller’s ratchet also impacts yeast evolution.
See full infographic: JPG | PDF © KIMBERLY BATTISTA Yeast reproduce asexually—via either budding or fission—in the presence of plentiful food. When food is scarce, however, they undergo meiosis and form haploid spores that eventually fuse with other spores to form a diploid zygote. (See illustration.) But the starvation process is mutagenic in yeast, making it impossible to isolate the effects of sex. In 2005, when Goddard was a postdoc at Imperial College London, he and his colleagues found a way to sidestep the problem: genetically engineer yeast that continue to reproduce asexually when starved. This allowed the researchers to compare starved populations of asexual yeast with starved populations of a sexually reproducing wild-type strain.

The team plopped both strains into benign and harsh conditions, measured the fitness of the populations, and got a straightforward result: sex offered no benefit in the benign environment—there was no difference in fitness between the two populations—but in the harsh environment, sexual populations adapted more rapidly and survived better than the asexuals. 1 “Lo and behold, our data were in line with Weismann’s central idea,” says Goddard. “It showed that sex increased the rate of adaptation compared to asexual populations.”

Goddard’s team then went a step further to test two other theoretical predictions about sex, made by American geneticist and Nobel laureate Hermann Muller and English statistician and geneticist Ronald Fisher. The Fisher-Muller hypothesis, developed by the two thinkers independently, proposes that sex is a means of rapidly introducing beneficial mutations into a population, while Muller’s “ratchet” hypothesis suggests that sex is necessary for purging the genome of deleterious mutations. By deleting a gene involved in DNA repair, Goddard created strains of both asexual and sexual yeast with increased mutation rates, then compared those strains to normally mutating yeast under benign and stressful conditions.

This time, the data weren’t so clear-cut. But in general, “we came to the conclusion that it was both,” says Goddard: the sexual species appeared to gain beneficial mutations and adapt better than asexuals, supporting the Fisher-Muller hypothesis, and the fitness of asexuals seemed stunted by increased mutation loads, succumbing to Muller’s ratchet. 2

BECOMING MORE SEXUAL: Researchers have found that a higher rate of sexual reproduction is favored in Brachionus calyciflorus rotifers when the animals are in spatially heterogeneous environments or adapting to new conditions. While sex generates genotypes that are less successful than their asexual counterparts in the short term, sexual populations fare better during periods of environmental change. © KIMBERLY BATTISTA A second organism that has proven to be a fruitful laboratory species for testing theories about sex is a freshwater species of rotifer called Brachionus calyciflorus. The transparent, microscopic animal can be found in ponds and other freshwater communities, and, importantly for the study of sex, it reproduces asexually at low densities but sexually in crowded environments. (See illustration.) When bunched together, the rotifers release a chemical cue that stimulates some females to produce haploid eggs that either develop into males or are fertilized to become diploid females. The propensity for the animals to switch from asexual to sexual reproduction is genetically controlled, allowing populations to evolve to be more sexual, and allowing the University of Toronto’s Agrawal to test some key evolutionary ideas.

He and his postdoc Lutz Becks first wanted to test the spatial heterogeneity hypothesis, which suggests sex is favored in a heterogeneous environment. Within a diverse environment, gene combinations may be beneficial in one area but detrimental in another. If individuals migrate between different regions, sex could be valuable because it breaks apart maladaptive gene combinations while creating new, potentially beneficial ones. Becks designed an experiment to see if a heterogeneous environment would promote sex in rotifers. “Frankly, I didn’t think it would work,” says Agrawal. “But it turned out really well.” In homogeneous environments, with all good- or all bad-quality food, rotifer clones became overwhelmingly asexual, while an environment with mixed food sources resulted in populations that were more likely to make the switch to sex. 3

Next, they tested Weismann’s hypothesis that sex promotes adaptation, by looking to see if sex is favored as rotifers adapt to new environments. After switching rotifers from one environment to another, differing in food type and sodium chloride concentration, Agrawal and Becks counted the number of offspring and assessed the genotype diversity of both sexually derived and asexually derived populations. They found that sex does generate genetic variation, but in the short term, the genotypes created by sex were on average less successful than their asexual counterparts. Some genotypes, however, were much more successful, with the result that in the long term, the rotifer lineages that were more likely to make the switch to sex fared better, blossoming during periods of environmental change. 4

But Agrawal’s and Goddard’s experiments, both researchers admit, have a major limitation: they were done in a lab using a limited set of environments. “At the end of the day, we’d like to know why sex is maintained in nature,” says Agrawal. “That’s a much harder nut to crack.”

Chasing the queen

RUNNING AWAY FROM PARASITES: The evolutionary dynamics of New Zealand snails (Potamopyrgus antipodarum) lend the strongest support for the idea that escaping parasitic infection could drive the ubiquity of sexual reproduction—the Red Queen hypothesis. Threatened with infection by more than a dozen species of trematode worms, sexually reproducing P. antipodarum snails are the most successful in generating resistant offspring. Asexual clones may start to rise in frequency, but the parasites quickly evolve to infect these increasingly common genotypes, thereby driving them down in frequency once again. © KIMBERLY BATTISTA As a postdoc at the University of Canterbury in New Zealand in the 1980s, ecologist Curt Lively picked up Graham Bell’s The Masterpiece of Nature: The Evolution and Genetics of Sexuality. In it, Lively read about a type of unusual mud snail (Potamopyrgus antipodarum) that exhibited both sexual and asexual reproduction and was studied by Mike Winterbourn, a Canterbury freshwater ecologist who happened to work just down the hall. Lively thought he’d stumbled upon the perfect system to study how organisms switch from asexual to sexual reproduction, and he hurried over to Winterbourn’s office to see about getting started. But Winterbourn explained to him that the New Zealand snails are either one or the other—they don’t change during their lifetimes. So the snails weren’t a good system to study sexual switching, but they were ideal for testing hypotheses about the maintenance of sex.

Observing the snails in New Zealand lakes and in the lab, Lively looked for evidence that sex prepares offspring for complex and competitive environments (the tangled bank hypothesis) that sex yields a range of offspring, which can better adapt to environments that change over time than asexual clones (the lottery model) and that sex is favored for unknown reasons, but abandoned when mates are hard to find (the reproductive assurance hypothesis). “I found no support for these ideas,” Lively says.

Then he looked for evidence to evaluate the Red Queen hypothesis, which posits that interactions with parasites can drive selection for sexual reproduction. The assumption is that parasites evolve to infect the most common host genotypes, and that sexual reproduction has the advantage of being more likely to produce rare resistant genotypes.

Initially, Lively thought the Red Queen hypothesis wasn’t viable because he didn’t think parasites were virulent enough to outweigh the costs of sex for the host. Also, he adds, he had no experience studying parasites, and he didn’t want to start. “It’s embarrassing,” he says. “I had a bias against that hypothesis.” Today it’s the only hypothesis left standing for the tiny New Zealand snails.

P. antipodarum snail populations are infected in nature by more than a dozen species of trematode worms. (See illustration.) Upon infection and reproduction in their hosts, the worms sterilize the snails, putting the snails under strong selection pressure to evade the worms. When Lively and his team began sampling wild snail populations in one lake in 1994, there were several asexual clones with common genotypes, but they weren’t infected. This ran contrary to the Red Queen, which predicts that parasites would go after the most common genotypes, whether they are produced sexually or asexually. But as his team continued to genotype the snails, the pattern changed: those common asexual clones became infected. “Clones common in 1994 were resistant to infection, which is probably why they were so common, but by 2001 they were highly susceptible, and had been driven down in frequency,” says Lively. “That’s very fast”—just seven generations for the annually reproducing mollusks. The researchers replicated the experiment in the lab and saw the same results: the trematode parasites evolved to infect the most common genotypes, whose prevalence subsequently dropped.

We are at a stage where we are pushing the boundaries, both in computer simulations and in experiments. It’s not hopeless. There is movement in the field. —­Sarah “Sally” Otto,
University of British Columbia

Now at Indiana University in Bloomington, Lively has continued to test the Red Queen hypothesis—looking to see, for example, if clonal genotypes common in the recent past are more susceptible to infection by local populations of parasites. In work that is currently in press, his team sampled four sites in a lake over five years and determined that asexual individuals averaged across all four sites were more infected than sexual snails, usually by a large amount, in four of those years. Only in the fifth year were sexuals more infected, which Lively attributes to the parasites reducing the prevalence of the asexual clones. This April, his team showed that exposure to parasites increases both the rate of mating and the number of different mating partners for both males and females in the sexual snail populations. 5

“So, over time, I’ve begun to think the Red Queen theory is probably a real driver” of the evolution of sex, says Lively. Parasites target common genotypes, encouraging host genetic recombination. Without sex, P. antipodarum snails, at least, would almost surely be extinct.

Perks of abstinence

With so many hypotheses extolling the benefits of sex, one might think asexual creatures are doomed to extinction—unless, that is, there are other ways to achieve those same benefits.

Bdelloid rotifers used to be just a quick footnote in textbooks as “animals that lost the ability to have sex.” Today, they’re a whole chapter unto themselves. Though other animal species, especially insects, occasionally experiment with total asexuality, these attempts are rarely successful. Bdelloids, on the other hand, have been successfully reproducing for more than 80 million years and have diversified into 450 different species, despite molecular evidence that they lost the ability to have sex tens of millions of years ago.

“These organisms have apparently avoided the extinction that is the inevitable consequence of giving up sex,” says David Mark Welch, an evolutionary biologist at the Marine Biological Laboratory in Woods Hole, Massachusetts. “So what are they doing that all the other asexuals tried and failed? If we can identify that, then that will give us a strong hint as to why everything else needs to have sex.”

BLOWING IN THE WIND: Bdelloid rotifers are the most successful animals that are completely asexual. But that doesn’t mean they can’t outwit their pathogenic enemies and introduce genetic novelty into the population. By ridding themselves of all their water, for example, desiccated rotifers can escape parasites simply by being blown away in the wind. Moreover, researchers believe the extreme desiccation compromises the rotifers’ cellular integrity, permitting DNA from nearby organisms to integrate into the genome. © KIMBERLY BATTISTA Researchers have failed to find evidence of Muller’s ratchet in bdelloids the animals have no observable mechanism for avoiding the accumulation of deleterious mutations. But there is evidence supporting the Red Queen hypothesis in the unique way that the rotifers evade parasitic infections. Ridding themselves of all their water, the rotifers become as light as flecks of sand and blow away in the wind, leaving their parasites behind. (See illustration.) Christopher Wilson and colleagues at Cornell University in New York isolated three bdelloid species from moss, grew them in petri dishes with rainwater, and exposed them to a fungal parasite. When the rotifers were exposed to air, they desiccated within 24 hours, and then a light breeze from several fans blew the dried-up organisms around a wind chamber and onto fresh Petri dishes, where new populations grew—all of them infection free. The bdelloids rid themselves of six different parasites this way. 6

Last year, researchers found evidence that desiccation also provides a mechanism by which the bdelloids introduce genetic variation. Desiccation is thought to compromise cellular integrity, which allows the absorption of DNA from other sources—bacteria, fungi, and other nearby organisms. “In bdelloids, something like 7 to 10 percent of the genes are of nonmetazoan origin,” says Mark Welch. That novel DNA is then put to use, it seems. Two years ago, a team of UK researchers found that most of the rotifers’ foreign genes are expressed, and some of their proteins are even used in critical processes such as breaking down toxins and resisting antibiotics. 7 And if the bdelloids are picking up DNA from other organisms, they are likely also picking up DNA from each other. “That transfer may be a surrogate for the exchange that happens during meiosis,” says Mark Welch. Thus, while they may not have sex, bdelloid rotifers are diversifying nonetheless, a process that appears critical to survival on this planet.

At the end of the day, we’d like to know why sex is maintained in nature. That’s a much harder nut to crack. —­Jef­frey Albaugh,
NorthShore University HealthSystem

In a static world, sex is likely unnecessary. The ever-changing environments of Earth call for a different scenario, however. Sex allows species to adapt to the loss of food sources, the arrival of parasites, rising temperatures, and more.

There is some doubt, however, whether the environment fluctuates fast enough to warrant the prevalence and persistence of sex in the eukaryotic kingdom. “Is the force favoring sex large enough in the face of the costs?” asks Otto. “The niggling doubt in the back of my head is that it is not.”

And so the paradox of sex lives on. “We still really don’t know the answer to this very most basic question,” says Mark Welch. “We don’t know why sex exists.”

Table for One

CDC/DR. LEANOR HALEY Duke University microbiologist Joseph Heitman trained as a Saccharomyces geneticist, and when he began to work on another fungal species, the pathogen Cryptococcus neoformans, he expected its sexual reproduction to be pretty much the same. “But the more I learned about [Cryptococcus], the more I realized it was completely different,” says Heitman.

Like Saccharomyces, Cryptococcus have two separate haploid mating types, a and α, but unlike the even distributions observed in other fungi, Cryptococcus populations are 99.9 percent α and 0.1 percent a, a ratio that seemingly precludes normal sex because only one of the two partners is readily available. Because of this, most researchers had believed Cryptococcus to be primarily asexual. Then Heitman observed the fungi doing something pretty strange.

Clones of the same mating type, with exactly the same genome, were having sex, both in the lab1 and in the field. 2 Haploid α clones fuse with other α haploids to form diploid offspring—a cell with two copies of the exact same haploid genome. It is an extreme form of inbreeding—mating with one’s identical twin—that Heitman christened “unisexual reproduction.” But why would an individual invest the time and energy to undergo meiosis, find a partner, then recombine identical genomes, instead of simply budding off an exact copy of itself through asexual reproduction?

In the last year, Heitman’s team has shown that this unique sexual process has two benefits. First, it supports the switch from a yeast stage—a rounded cellular structure, good for growing in liquid—to a filamentous fungal stage, in which a pin-like structure projects upwards to release spores. This allows Cryptococcus to disperse throughout an environment, gaining access to nutrients and mating partners. 3

Second, Heitman and colleagues found that meiosis generates novel diversity. Unisexual reproduction recombines two identical genomes, but the process itself is mutagenic, resulting in genetic diversity. Comparing 96 progeny generated by mitosis and 96 progeny generated by unisexual reproduction, Heitman’s team found that the former looked exactly like their parents while the latter showed variable genotypes and phenotypes. 4 The progeny of unisexual reproduction had many new traits, including ones important for survival, such as temperature sensitivity and antifungal resistance. “It illustrates that sex not only mixes preexisting genetic diversity, but it can create some level of genetic diversity all by itself,” says Heitman.

What’s more, unisexual reproduction also suggests that the origins of sex may not have required the existence of two sexual forms, like a male and a female or an a and an α. “Given how sex works in most animals and plants, we’re fixated on the idea that sex must have involved two sexes and must have evolved to mediate genetic exchange,” says Heitman. “I’m not convinced.”

Half-Identical Twins

A set of twins who look and act for all the world like they're identical, except for the miiiinor detail that one's male and the other's female.

It's common &mdash especially in drawn or animated media, where the creator has complete control over the appearance of the characters &mdash to use brother-sister twins as being each other's Distaff Counterpart and Spear Counterpart. They often display identical twin tropes, such as Twin Telepathy or Synchronization &mdash especially if they are Creepy Twins. It's a way to make sure that twins, even if they're not identical, are immediately identifiable as such if it's relevant to their characters, and there are no other visual cues to show that siblings are fraternal-twins instead of just regular brothers and sisters. If the two siblings drift apart at any point during the story, a Twin Desynch is likely, and often occurs at puberty when the secondary sexual characteristics start becoming more apparent. In fact, if they don't desynch to some degree during puberty, they're likely to come across as a little too close.

Also note that due to the androgynous features of this trope, such twins will traditionally be depicted as Ambiguous Gender, playing Dude Looks Like a Lady, Bishōnen and Wholesome Crossdresser for the brother, and Genki Girl, Tomboy-type Tsundere for the sister. For this reason, this trope is also very common in stories with Transgender Fetishization and Otokonoko Genre or Sweet on Polly Oliver, where it serves as a standard explanation for the protagonist's femininity or the reason for his forced dressing up. On a less fetishistic note, one twin being trans can be used as a plausible explanation for this trope. In this case, the twins look the same because they're genetically identical, but their gender identity is different, and one twin has transitioned to better reflect that identity.

In Real Life, this would fall under Strong Family Resemblance, as any set of fraternal twins, opposite sex or otherwise, are no more or less likely to resemble each other than normal brothers and sisters, other than being the same age.

Compare to the rare real-life phenomenon of semi-identical twins, where the twins have identical genes from their mother and different genes from their father, and thus can be the same sex or opposite sexes. This could lead to an Uncanny Family Resemblance if the sperm are similar enough.

This trope is perhaps the most glaring example of Always Identical Twins.

How science takes the Bible to bits

THE Bible has been called “the greatest story ever told”. Steve Jones begs to differ. In The Serpent’s Promise, Jones, a British geneticist and outspoken anti-religionist, sets out to retell the Bible from the point of view of science.

Well, not exactly. Instead of a point-by-point fact-checking of the Christian holy book, Jones has opted to pick some of its main themes. From big topics such as the origins of the world and of humans, Noah’s flood and other epic disasters, and the ultimate fate of Earth, he sketches out our scientific knowledge for each.


Sometimes this works well. The chapter on origins, for example, takes a quick tour through the big bang, the formation of Earth, the history of the continents, the origin of life, its evolutionary history, plus human evolution – and all in less than 40 pages.

Needless to say, Jones is aiming to hit the high points, not provide a comprehensive lesson. But it all hangs together, and it gives a fair overview of science’s alternative to the first chapters of Genesis.

At other times, though, this approach seems to be little more than an excuse for rambling. For instance, the Bible pays a huge amount of attention to matters of reproduction&colon think of all the “begats”, not to mention the virgin birth of Jesus. Jones takes this as a pretext to launch into a discussion of reproductive biology that wanders from sea urchin embryology and why there are two sexes, to sperm donation and genetic imprinting. By the time we get to the end of the chapter, we have strayed a long way from any remotely biblical topic.

Much of the book is like this – a collection of random walks from biblical starting points – and it leaves the reader feeling rather adrift. That is a shame, because, paragraph by paragraph, Jones is always lively and often wickedly funny. He notes, for example, that vicars and insurance sellers are in the same game – of convincing people to forgo immediate pleasures for long-term security.

“Much of the book is a collection of random walks from biblical starting points”

To those who believe that humans are endowed with a soul from the moment of conception, he points out that his mother was an identical twin formed when a fertilised egg accidentally split into two separate embryos. What happened to the single soul when it found itself with two bodies? “Were my mother and her sister, my Aunt Pegi, blessed with just half a copy each,” he asks, “or does God have a stock of spares ready to insert when needed?” Whatever the book’s other faults, Jones is never boring.

But he can be hasty and careless. At one point he says that the pre-Columbian New World was sparsely populated by small, scattered bands five pages on he says that large parts of South America were heavily settled and “buzzed with activity”. Elsewhere, he writes that HIV is an exclusively human virus, but four pages later that it also infects chimps. Then there are the many unclear pronouns that sometimes leave us unsure as to the precise meaning of sentences. For example, when Jones writes about the first multi-drug-resistant plasmid – a transmissible ring of DNA – emerging in a strain of plague in Madagascar, he muddies the water with unclear uses of “it”. If this book passed through an editor’s hands, he or she left few prints. In a perfect world, authors would be perfect, but in the real world, we need editors to pick up the errors.

A bigger problem, though, is that Jones seems unclear who his audience is. He oversimplifies some concepts and goes into dizzying detail on others. The book skims too lightly over the surface to interest most science enthusiasts, and religious readers are likely to be put off by the barbed comments.

Perhaps the best target would be “sci-curious” people starting to explore scientific alternatives to the literal truth of the Bible. Such readers may well be receptive to Jones’s efforts to show that science tells the more convincing story. But he provides none of the endnotes or extra references those people will need to dig deeper.

Readers – all of them – surely deserve better.

This article appeared in print under the headline “In the beginning was science”