For several decades now, biologists have been puzzling over sex. In some ways, it seems like a huge waste of effort.

Sexual reproduction requires splitting a species into two sexes, only one of which will be able to produce offspring. There are some species of animals that do without males; the females simply trigger their eggs to develop into embryos without any need for sperm. All the offspring of an asexual animal can produce offspring of their own, instead of just half. So it would make sense that genes that gave rise to asexual reproduction would win out in the evolutionary race.

Clearly that hasn’t happened. The world is rife with sex. Animals do it. Plants do it. Even mushrooms do it. So evolutionary biologists have carried out a number of studies to get an answer to the question, “Why sex?”

In 2009, I wrote an essay for Science about this research. If I had been writing that essay today, I’d have focused some attention on an elegant experiment on the sex life of a humble worm. It gives a big boost to the long-floated idea that evolution favors sex because it lets hosts fight better against parasites.

Allow me to explain by self-plagiarizing:

In the 1970s, several researchers built mathematical models of how parasites influenced the evolution of their hosts and vice versa. Their research suggested that both partners go through cycles of boom and bust. Natural selection favors parasites that can infect the most common strain of host. But as they kill off those hosts, another host strain rises to dominate the population. Then a new parasite strain better adapted to the new host strain begins to thrive, leaving the old parasite strain in the dust.

This model of host-parasite coevolution came to be known as the Red Queen hypothesis, after the Red Queen in Lewis Carroll’s book Through the Looking Glass, who takes Alice on a run that never seems to go anywhere. “Now here, you see, it takes all the running you can do to keep in the same place,” the Red Queen explains.

The Red Queen conundrum, some researchers have argued, may give an evolutionary edge to sex. Asexual strains can never beat out sexual strains, because whenever they get too successful, parasites build up and devastate the strain. Sexual organisms, meanwhile, can avoid these dramatic booms and busts because they can shuffle their genes into new combinations that are harder for parasites to adapt to.

Red Queen models for sexual reproduction are very elegant and compelling. But testing them in nature is fiendishly hard, because biologists need asexual and sexual organisms that share the same environment and parasites.

Scientists have found some mixed populations in the wild where they’ve made some important discoveries. But it’s also possible to test the Red Queen in laboratories. It’s not easy, because scientists need to bring together a host that can reproduce sexually and asexually with a parasite, and then they both have to be able to evolve in response to each other. But that’s what a team of scientists at Indiana University managed to do recently.

As they describe in a paper published today in Science, they reared populations of a tiny worm called Caenorhabditis elegans. C. elegans are born either as males or hermaphrodites. A hermaphrodite worm can fertilize its eggs with its own sperm, or it can seek out a male. The worms typically don’t have a lot of sex, and the rate at which they do is partly programmed into their genes. The Indiana team of scientists were able to engineer the worms so that they could have no sex at all, or could only reproduce through sex.

For their parasite, they chose a species of soil bacteria called Serratia marcescens. Soil bacteria are the regular prey of C. elegans, but if they slurp up S. marcescens by accident, they get sick and can die in under 24 hours. Previous studies had shown that the worm can evolve stronger resistance to the germ, and the germ can evolve to be deadlier for the worm. So the Indiana researchers set about combining their evolution into one big experiment.

They mixed together worms and germs in several different arrangements and let them duke it out for 30 worm generations. In each trial, the worms were either sexual or asexual.  In some trials, the bacteria coexisted with the worms for the whole experiment, so that they could evolve along with the worms. In other trials, the worms were repeatedly presented with the same, fixed strain of S. marcescens. In other words, the bacteria could not evolve. And in control experiments, the worms enjoyed a Serratia-free life.

As this graph to the left shows, the asexual worms that faced co-evolving germs were annihilated in just 20 generations. (“Obligate selfing” means no sex.) If the germs couldn’t evolve, however, the asexual worms did fine. The scientists also tested the bacteria for deadliness after the experiments were over. They found that the bacteria that were allowed to co-evolve with the asexual became much deadlier. The co-evolving sexual worms, on the other hand, suffered far lower mortality rates from their germs.

In another experiment, the scientists started out with ordinary worms, which only had sex about 20 percent of the time they reproduced. Again, they exposed the worms to unchanging bacteria, or co-evolving ones, or no bacteria at all. The graph to the right says it all. The worms not exposed to the bacteria went on having infrequent sex. The worms that could evolve but faced fixed bacteria had more sex for a while, but eventually crashed back down to their original levels. The coevolving worms, on the other hand, became mostly sexual.

In each of these results, the Red Queen has left her mark. Far from being a waste of time, sex may save organisms from a swift oblivion.

[Images: turtles via Creative Commons from man of mud/Flickr. C. elegans via Creative Commons licence from AJ Cann/Flickr.]

(Update: paper link fixed)

I just got back yesterday from the annual meeting of the Society for the Study of Evolution. It took place in a big hotel on the outskirts of Norman, Oklahoma, during a windy heat wave that felt like the Hair Dryer of the Gods. It had been a few years since I had last been to an SSE meeting, and I was struck by how genomic everything has gotten. No matter how obscure the species scientists are studying, they seem to have outrageous heaps of DNA sequence to analyze. A few years ago, they would have been content with a few scraps. Fortunately, SSE hasn’t turned its back on good old natural history. There were lots of fascinating discoveries on offer, about species that I had assumed had been studied to death. My favorite was a talk about the rough-skinned newt, the most ridiculously poisonous animal in America.

The scientific tale of the rough-skinned newt begins five decades ago, with a story about three dead hunters in Oregon. Reportedly, the bodies of the hunters were discovered around a camp fire. They showed no signs of injury, and nothing had been stolen. The only strange thing about the scene was the coffee pot. Curled up inside was a newt.

In the 1960s, a biologist named Butch Brodie got curious about the story. The newt in the coffee pot–known as the rough-skinned newt–has a dull brown back, but when it is disturbed, it bends its head backward like a contortionist to reveal an orange belly as bright as candy corn. Bright colors are common among poisonous animals. It’s a signal that says, in effect, “If you know what’s good for you, you’ll leave me alone.” Brodie wondered if the newts were toxic, too.

Toxic, it turns out, doesn’t do the newts justice. They are little death machines. The newts produce a chemical in their skin called tetrodotoxin, or TTX for short, that’s made by other poisonous animals like pufferfish. Locking onto sodium channels on the surface of neurons, TTX blocks signals in the nervous system, leading to a quick death. In fact, TTX is 10,000 times deadlier than cyanide. While we may never know for sure what killed those three Oregon hunters, we do know that a single rough-skinned newt could have easily produced enough TTX to kill them, and have plenty of poison left over to kill dozens more.

Now, if the whole idea of evolution makes you uneasy, you might react by saying, “That couldn’t possibly have evolved.” Experience has shown that this is not a wise thing to say. Brodie said something different: the most plausible explanation for a ridiculously poisonous animal is that it is locked in a coevolutionary arms race with a ridiculously well-defended predator. Another biologist mentioned to him that he’d seen garter snakes dining on rough-skinned newts, and so Brodie investigated. He discovered that garter snakes in rough-skinned newt territory have evolved peculiar shape to the receptors on their neurons that TTX would normally grab.

The coevolution of newts and snakes became a family business. Brodie’s son, Edmund, grew up catching newts, and today he’s a biologist at the University of Virginia. Father and son and colleagues have discovered that snakes have independently evolved the same mutations to their receptors in some populations, while evolving other mutations with the same effect in other populations. They’ve also found that both newts and snakes pay a cost for their weaponry. The newts put in a lot of energy into making TTX that could be directed to growing and making baby newts. The evolved receptors in garter snakes don’t just protect them from TTX; they also leave the snakes slower than vulnerable snakes. They’ve studied newts and snakes up and down the west coast of North America and found a huge range of TTX potency and resistance. That’s what you’d expect from a coevolutionary process in which local populations are adapting to each other in different environments, with different costs and benefits to escalating the fight.

This story is so irresistible that I’ve written about it twice: first, ten years ago in Evolution: The Triumph of an Idea,, and then in updated form last year in The Tangled Bank. I figured that the Brodies et al had pretty much discovered all there was to know about these creatures. But in Oklahoma, I discovered that they had missed what is arguably the coolest part of the whole story.

Think about it: you’re a female newt, you’ve fended off attackers with a staggering amounts of poison in your skin, and now you want to pass on your genes to your descendants. You lay a heap of eggs in a pond, and what happens? A bunch of pond creatures come rushing in and have a feast of amphibian caviar.

What could you possibly do to ensure at least some of your offspring survived? Well, you have an awful lot of TTX in your system. You have enough of the stuff to give your eggs a parting gift to help them out there in the cruel, predator-infested world. Make your eggs poisonous.

That is exactly what female newts do. In fact, they load their eggs with TTX. To figure out if this poison provided a defense against predators, the Brodies and their students traveled to a group of ponds in central Oregon that are home to thousands of rough-skinned newts apiece. They collected dragonflies and other aquatic predators from the ponds and put them in buckets filled with newt eggs, along with muck from the pond bottoms. The scientists found that almost none of the predators would touch the newt eggs. Since these predators eat plenty of eggs of other species, this result shows that TTX does indeed help the newt eggs survive.

But there was one exception. Caddisfly larvae turned out to relish the newt eggs. In fact, the caddisflies actually grew bigger if they were supplied with newt eggs and pond muck than with pond muck alone. And yet the Brodies and their students estimate that there’s enough TTX in one newt egg to kill somewhere between 500 and 3700 caddisflies.

You know where this is going. At the evolution meeting, one of their students, Brian Gall, described feeding newt skin to caddisflies both from the central Oregon ponds and from ponds elsewhere without newts. The newt-free caddisflies would happily munch on newt skin from which all the TTX was removed. But if there was more than a trace TTX in the skin, they refused to eat. The caddisflies that fed on newt eggs, on the other hand, would eat the most toxic skin Gall could provide.

It appears that the caddisflies have evolved much like the garter snakes. In ponds where rough-skinned newts lived, the caddisflies have evolved defenses against TTX. In fact, Gall reported, the caddisflies appear to put the snakes to shame. Evolved snakes are 34 times more resistant to TTX than vulnerable ones. The caddisflies have increased their resistance 175 times.

It’s not clear whether the caddisflies and the newts are truly co-evolving, however. The Brodies will have to find out whether adding extra TTX to eggs increases their survival in the presence of caddisflies. Another intriguing possibility arises from their discovery that the caddisflies actually harbor some of the TTX they eat in their tissues for weeks after eating the eggs. Perhaps the caddisflies are stealing the poison to protect themselves, as happens in monarch butterflies eating toxic milkweed.

In other words, this wonderfully deadly story isn’t over yet.

[For more information, see this new paper in Can. J. Zool., and Understanding Evolution, an educational web site. Ed Brodie tells much of the story pre-caddisfly in a chapter of the new book, In The Light of Evolution (full disclosure: I wrote a chapter in it, too, which you can read as a pdf here)]

Image: California Herps

There comes a time in every science writer’s career when one must write about female sea monkeys having sex with male sea monkeys from the future, and the troubles that follow.

That time is now.

In many species of animals, males and females have a conflict of evolutionary interests. Males compete with each other for the opportunity to fertilize the eggs of females. Males use all sorts of strategies in these competitions. They fight with each other for territory, they scare off intruding males, they put scrapers into females to dump out the sperm from previous males, and they inject “anti-aphrodiasiacs” to make females unreceptive to other males.

A number of experiments suggest that females have to pay a steep price for these male shenanigans. Anti-aphrodisiacs are toxic to the females, shortening their lifetime. Why would males harm the females that carry their offspring? In many species, males can mate with many females. The long-term health of any one female doesn’t matter–in an evolutionary sense–to the male.

As natural selection favors increasingly deadly male mating strategies, this onslaught opens up the opportunity, in turn, for the evolution of counterstrategies in females. In some species, females may evolve antidotes to male poisons. The males, in turn, may evolve counterattacks to overcome these new defenses. Theoretically, this coevolution can become a never-ending cycle of sexual conflict, capable of producing some of nature’s greatest extravagances (like absurdly kinky ducks).

Up till now, the best evidence for this kind of sexual conflict came from experiments. Scientists manipulated Drosophila flies so that the males were free to evolve while the females couldn’t. The result: the lifespan of the females got shorter and shorter over the course of generations. In a flipped version of the experiment, scientists prevented males from mating with lots of females, as they normally do. Instead, the male flies were forced into monogamy. Now there was no evolutionary reward for competing with other males. Over time, the male fly toxins got less toxic, and the females lost their defenses.

Now Nicolas Rode of the the Center for Functional and Evolutionary Ecology in Montpellier, France, and his colleagues have found a new way testing this hypothesis: by having males travel through time to mate with females.

The time-traveling males in this case are brine shrimp (a k a sea monkeys). Brine shrimp produce tough eggs that can survive through droughts for years and then hatch into healthy young when water returns. In the Great Salt Lake in Utah, the brine shrimp egg cysts form layers on the lake bed going back decades. Rode and his colleagues gathered cysts from layers that formed in 1985, 1996, and 2007. They brought the cysts back to their lab and reared the sea monkeys. And then they orchestrated some sea monkey sex. They had females mate with males from their own time, as well as from the other years. For example, females from 1996 could mate with males from 2007 and 1985.

If sexual conflict is an ongoing evolutionary process, you’d expect females to fare differently with males from different time periods. They’d be best-adapted to the males of their own time, and worse adapted to males from other times. Evolutionary theorists have developed two different models for how this time-traveling sex would play out. It’s possible that males and females escalate their adaptations over time in an evolutionary arms race. It’s also possible that evolution moves more like a merry-go-round. For a while, one male strategy may dominate, and one female counterstrategy dominates as well. But then a new male strategy pops up, for which the females have no defense at all. That male strategy then rises to dominance, and a corresponding female counterstrategy eventually evolves as well.

Rode and his colleagues tracked the females, noting how  many eggs they had and how long they lived. And they discovered, as predicted, that having sex with males from another time is bad for a sea monkey’s health. The further away in time the sea monkeys were, the sooner the female sea monkey died. When the male traveled 22 years to mate with a female, her life was cut short on average by 12%.

There are lots of caveats to this study–which you’d expect for the first study of its kind. The results weren’t clear enough for the scientists to pick the arms race or merry-go-round model as the best explanation for the conflict between the sexes. And over the entire lifetime of the female sea monkeys, time-shifting didn’t have a measurable effect on their reproductive success. That’s because they females who were dying faster also produced eggs at a faster rate.

Another mystery is how the time-traveling males are harming the females. Rode and his colleagues note one unusual aspect of brine shrimp sex: males and females can stay clasped together for hours–even days. They’re not cuddling in some erotic afterglow. Studies on other species suggest that the males are holding on tight and the females would prefer to get on with their lives. Amplexus, as this embrace is known, may be yet another way for males to outcompete their rivals. By holding on tight to females, they can prevent their mates from finding other males.

Females pay a price for this guarding; it can make them easier targets for predators and prevent them from eating. Scientists have found that female water-striders have evolved lots of acrobatic moves to get clasping males off of them. It’s possible that sea monkeys engage in a similar sexual wrestling match, and that the male and female moves evolve over time.

Whatever the answer to these questions, one thing seems fairly clear. If you’re a female sea monkey, and you see a blinding flash of light, and a male sea monkey suddenly appears saying he’s got to protect you from an army of sea monkey robots from the future–take care. Your well-being is definitely at risk.

Reference: Nicolas O. Rode, Anne Charmantier, Thomas Lenormand. Male-female coevolution in the wild: evidence from a time series in Artemia franciscana” Evolution: in press. DOI: 10.1111/j.1558-5646.2011.01384.x

[Image: Fanpix. Backstory for those who've never heard of Kyle Reese]

[Updated to clarify the nature of sea monkey time travel and correct Marseilles to Montpellier]

During the World Science Festival, I met Baba Brinkman, who performs hip hop about, among other things, evolution. He let me know that his “Rap Guide to Evolution” will be opening this Friday at the SoHo Playhouse in New York. Here are details about the venue and getting tickets.

Here are a couple videos from Baba…

Here’s a one called, “Performance, Feedback, Revision”:

Here’s Baba at TEDxKids:

On Friday, as the E. coli outbreak gained horrific speed in Germany, Newsweek asked me to write about how this epidemic came to be. Scientists still have a lot to figure out about it, but some things are clear–in particular, that the bacteria have great scope for evolution into new deadly strains, thanks in part to the shuttling of viruses between them. (In my book Microcosm, I explain how this is true not just for E. coli, but for much of life.) My piece appears in the new issue of Newsweek, which you can read online here. (One late-breaking piece of news that didn’t make it in, by the way, is the finding yesterday that the new outbreak appears to have come from bean sprouts.)

While I was working on my Newsweek piece, a reporter for the BBC called me up for an article on the good side of E. coli. I explained how much of how we understand about life itself came out of research on this typically harmless bug, and that the biotechnology industry was build upon its biology. That piece came out over the weekend. Check it out.

[Image: glass microbe by Luke Jerram]

Long, long ago–actually, in 2006–I wrote an article for the New York Times about a very strange relative of today’s alligators and crocodiles. Effigia, which lived 210 million years ago, did not slouch around inTriassic swamps. Instead, it stood on two big hind legs, holding its front legs–arms, really–aloft. It looked an awful lot like a bipedal dinosaur, despite the fact that the ancestors of dinosaurs and crocodiles split off 250 million years ago.

As just one species standing upright, Effigia might have been an evolutionary fluke. But today at Science Now, Brian Switek writes that a contemporary relative of Effigia was a biped, too. So now it appears that there was a lineage of crocodile relatives running around upright at the same time as some dinosaurs were too. The dinosaurs went on to fame and glory–or, at any rate, a continued upright existence. The crocodile lineage ended up on all fours, where they remain today.

[Reconstruction by Carl Buell]

It’s time to pay another visit to Cordyceps, the fungus that turns its hosts into spore-sprouting zombies.

The fungus, which can be found in many parts of the tropics, penetrates an insect’s exoskeleton and then work its way into its host’s body. At first the ant seems normal to the human eye, but eventually it makes its way to a leaf, where it clamps down with its mandibles. Cordyceps then sprouts out of the ant’s body, lashing it to the leaf’s underside, and producing a long stalk tipped with spores. The spores can then shower down on unfortunate insects below.

David Hughes of Penn State University has been publishing a string of fascinating papers in recent years about this science-fiction-topping parasite. In 2009, I wrote about one study of his on the exquisite precision of the fungus’s manipulations. He and his colleagues found that one species that lives in Thailand almost always causes infected ants to clamp onto a leaf vein about 25 centimeters off the ground–a spot where the humidity and other conditions may be ideal for a fungus to grow. When Hughes and his colleagues moved infected ants higher up into the canopy, the fungus ended up deformed. On the other hand, when the scientists moved the ants to the ground, the ants simply disappeared–devoured most likely by other animals or washed away by rain.

Now Hughes is looking more closely at how the fungus pulls the strings on its insect marionette. First off, how does it drive the ant to its climatic sweet spot? The species of fungus that Hughes studies, Ophiocordyceps unilateralis, infects a species of ant, Camponotus leonardi, that usually stays 60 feet off the ground, living in the canopy of the Thai rain forest. It sometimes drops to the ground, but promptly walks a short distance along an ant trail to the nearest tree.

Infected ants, by contrast, were beset by convulsions that caused them to fall out of trees. Instead of following a trail, they wandered the forest floor in random directions for hours, and then climbed up small plants instead of trees. Healthy ants are active from dawn to dusk, but Hughes and his colleagues could only find infected ants during midday. And they all became synchronized in their leaf-biting, typically clamping onto a leaf around noon.

Hughes then took a close look at the death grip itself. In the hours before the zombie ants clamped onto a leaf and died, their jaws were in good working order. The ants could use their mandibles to clean off their antenna and legs without chomping them off, for example. Once the ants clamped onto leaves, Hughes dissected their heads and peered inside. They were full of fungal cells. He also observed that the muscles controlling the mandibles were atrophied.

It’s a bizarre finding, given that zombie ants typically have enough strength to pierce through a leaf vein. One possibility Hughes offers is that the atrophy sets in after the ants bite down, and it attacks the muscles that the ant might use to let go of the leaf.

All in all, Hughes’s research supports the idea that the ant becomes what Richard Dawkins has dubbed the extended phenotype of the fungus. The behavior of the host is not just a side-effect of having a bad fungal infection. It’s a manipulation that lets the parasite reproduce more successfully.

Still, Hughes has much left to figure out. Why, for example, do the ants pick leaves that point north northwest, for example? It may seem eerie that a fungus can work its zombification like clockwork, but other parasites can, too. A fluke that infects ants causes them to clamp grass blades at dusk. That timing works out nicely for the fluke, because it needs to get into grazing mammals like cows or sheep to complete its life cycle, and those animals like to graze when it’s cool out. The fluke-infected ants even climb down at dawn so they don’t bake in the sun, returning to the tops of the grass the next evening.

Hughes doesn’t know what’s so special about noon for this fungus. But I’d bet that there’s some eldritch horror in the answer.

[Image: PLOS One, via Creative Commons Licence]

I’ve got a request for the Loom’s hive mind. I’ve been asked to contribute a book chapter to a guide to evolutionary biology. The subject of my assignment is evolution and the media. I’ve already covered some of this territory in a 2010 review I wrote for the journal Evolution: Education and Outreach (pdf), but I’d like to flesh it out a bit. I’m familiar with the past thirty years of the subject from my own experience, and I’m familiar with Darwin’s reception in his own lifetime, thanks to all the scholarship that’s been produced about that period. But the century or so in between is a lot sketchier for me. The Scopes Monkey trial comes to mind, of course, but not a lot else. My search of the history-of-science literature has yielded little, probably because I’m not using the right search terms. If anybody has any pointers, I’d be most grateful (and will, of course, thank you in the acknowledgments). Thanks!

Charles Darwin was the original crowd-sourced scientist. He may have a reputation as a recluse who hid away on his country estate, but he actually turned Down House into the headquarters for a massive letter-writing campaign that lasted for decades. In her magisterial biography of Darwin, Janet Browne observes that he sometimes wrote over 1500 letters in a single year. Darwin was gathering biological intelligence, amassing the data he would eventually marshall in his arguments for evolution. In the letters he wrote to naturalists around the world, Darwin asked for details about all manner of natural history, from the color of horses in Jamaica to the blush that shame brought to people’s cheeks.

Given the skill with which Darwin used the nineteenth-century postal system, I always wonder what he would have done with the Internet. A new paper offers a clue: he might have enlisted thousands of citizen scientists to observe evolutionary change happening across an entire continent.

Darwin used his Victorian crowd-sourcing to collect evidence that was consistent with his evolutionary theory; he didn’t expect that he could actually document evolutionary change happening in his own lifetime. Ironically, he probably could have. Gregor Mendel worked out the basic rules of genetics around the time Darwin published The Origin of Species. At the time, pollution from England’s coal was turning trees dark, giving an evolutionary edge to dark moths over light ones. A naturalist even wrote directly to Darwin in 1878 to raise the possibility that natural selection was driving the shift in moth color. But it wasn’t until 14 years after Darwin’s death that a naturalist explicity put this idea into print.

In the decades after Darwin’s death, biologists translated Darwin’s ideas into the language of statistics. They figured out how to make measurements on animals and plants in the wild, and how to discover in those measurements the traits that led to the most reproductive success. Evolutionary biologists have now made thousands of measurements of natural selection in the wild. But each of those measurements has been hard won. To see natural selection, researchers must study dozens or hundreds of individuals. To get a sense of how tough this work is, read The Beak of the Finch: A Story of Evolution in Our Time, in which Jonathan Weiner chronicles the adventures of Peter and Rosemary Grant, who have traveled to an isolated island in the Galapagos Archipelago each summer for forty years in order to measure natural selection on Darwin’s finches.

But Jonathan Silvertown of The Open University in England and his colleagues have found a way to spread this kind of work far and wide. They set up a web site where volunteers could sign up to become amateur evolutionary biologists. They ende up with over 6,000 volunteers, who sent them measurements from across Europe.

Their measurements came from an animal that’s at once humble and iconic. The land snail Cepaea is common in gardens, ditches, forests, and meadows throughout Europe. The snails come in a beautiful variety of colors, as this photograph from Poland demonstrates. The patterns are encoded in genes, which the snails pass down to their offspring. In the early 1900s, many naturalists considered the patterns to be pretty but insignificant. They were just the result of random mutations that cropped up and then spread through the snail population thanks to chance.

Starting in the 1930s, a team of Oxford scientists took a close look at which snails lived and died. They could do so because the snails are a favorite meal for thrushes, which like to pick up their prey and carry the snails into the air, whereupon they drop the snails onto rocks below to crack the snails. The Oxford researchers were able to catalog these smashed shells, noting their colors and stripes.

The researchers found that some colors and stripes were more common among the shell debris than you’d expect from chance alone. It turns out that the birds are more likely to pick out the snails that stand out against the background. So snails that are better camouflaged are more likely to survive. Which pattern works best depends on where a snail lives; what hides a snail crawling over a dark forest floor doesn’t work so well in a field grazed short by cattle. The researchers found that the most common patterns were, indeed, well-matched to where the snails lived.

This research on Cepaea snails helped establish natural selection as a powerful force in evolution–although bird-driven natural selection turns out to be http://www.ncbi.nlm.nih.gov/pubmed/10983823“>not the sole force at work. Studies across Europe revealed, for example, that southern European snails are more likely to have yellow shells than their darker northern cousins. The difference is probably due to the climate: yellow shells bounce sunlight away and keep the southern snails cool.

A few years ago, Silvertown and his colleagues set out to gather a new batch of observations on Cepaea to compare with these historical records. They wondered, for example, if the warming that Europe has experienced might have made northern snails more yellow. Through the web site Evolution Megalab, they enlisted people from 15 different countries, who set out into their own neighborhoods to find the snails and note their colors. Combining the new observations with the old, the scientists ended up with half a million snails, organized in a geographical database along with information such as the habitat where the snails lived, as well as the temperature and rainfall at each location. Nothing quite like it has ever been achieved by evolutionary biologists–professional or otherwise.

The collection chronicles fifty years of snail evolution, over the course of about twenty generations. In that time, the researchers didn’t detect a continent-wide change in the frequency of yellow shells. Only in populations that lived on beach dunes did yellow shells become more common. Silvertown and his colleagues suspect that most snails have been coping with the warming temperatures in Europe by spending more time in the shade. On the treeless dunes, however, that’s not an option. As a result, natural selection has favored the yellow snails, which can stay cooler without the help of foliage.

But Silvertown and his colleagues did find other shifts that took place across all of Europe. Snails without a stripe on their shell declined by about 10%, while snails with a mid-line band increased by 5%. The scientists doubt that the rise of striped snails has anything to do with a shifting climate. Indeed, the striped snails have become more common in southern Europe than in northern Europe–the opposite of what you’d expect if stripes were a defense against heat.

The scientists don’t know for sure what’s behind this evolution, but they have an idea. The song thrushes that eat the snails have been declining in some places for the past thirty years. It’s possible that the change in the predatory pressure of birds is shifting the force of natural selection.

It’s the sort of idea you could imagine Darwin coming up with as he sat in his study, paging through letters from his farflung correspondents. But now Silvertown and his colleagues are going to test it, taking advantage of the original citizen scientists: birders. And if you’re reading this in Europe, you can be part of the investigation.

PS: In case you don’t know how to hunt for a snail, here is a charming video from the Megalab:

In tomorrow’s New York Times, I take a look into nature’s crystal ball. Scientists have long been warning that we may be headed into Earth’s sixth mass extinction. But most projections just carry forward the causes of recent extinctions and population plunges (overfishing, hunting, and the like). Global warming is already starting to have an effect on many species–but it’s a minor one compared with the full brunt that we may experience in the next century.

I’ve written in the past about studies scientists have carried out to project what that impact will be like. I decided to revisit the subject after reading a spate of provocative papers and books recently. While the scientists I talked to all agree that global warming could wreak serious havoc on biodiversity in coming decades, they’re debating the best way to measure that potential harm, and the best way to work against it. We all crave precision in our forecasts, but biology is so complex that in this case we may well have to live without it. Check it out.

[Image: Photo by DJ-Dwayne/Flickr]

Brian Malow and I talked yesterday about some of my favorite things on the latest episode of Dr. Kiki’s Science Hour–including the evolution odometer. You can watch it on Youtube, or you can head over to Dr. Kiki’s Science Hour site to download the video or audio. (The Skype goes berserk briefly, but we get back on track.)

I’m a big fan of Xtranormal movies. And I write, among other things, about big debates in evolutionary biology. So this creation, from Jon Wilkins, a biologist at the Santa Fe Institute, is the essence of awesomesauce.