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One of the most interesting large-scale patterns in evolution is parallelism. For example, flight has evolved many times, in parallel, from numerous non-flying organisms; many species of vertebrates that are not fish have evolved swimming, in parallel. One study discovered parallel evolution in body armor among freshwater stickleback fish from numerous saltwater ancestors.

Another interesting thing about evolution, which has only been appreciated in recent decades, is the fact that there is not a simple correspondence between genes and traits. Rarely does one gene determine one trait, and rarely does one trait vary because of one gene. There are dozens of examples of simple gene-trait relationships, many of which were discovered years ago. Because these relationships were relatively easy to find and describe, our textbooks are full of them and our thinking about genetics was for a long time based on them. But this is a little like basing our conception of how all vehicles work by deeply understanding the workings of a toy wagon. The mechanics and engineering of a little red wagon will not help us understand escalators, submarines, or Apollo lunar launch systems. We now think that most genes affect multiple traits and most traits are affected by multiple genes, and that it is all very complex.

A recent study looking at stickleback behavior seems to be an example one gene affecting multiple traits.

Sticklebacks are members of the Gasterosteidae family of fish, with species that live in salt and fresh water. The freshwater sticklebacks evolved from saltwater ancestors who were landlocked less than about 17,000 years ago at many locations across the Northern Hemisphere. For this reason, differences among freshwater and saltwater sticklebacks represent recent and rapid evolution among a well-known group of species and are thus especially interesting to scientists.

Saltwater sticklebacks have up to 36 bony plates associated with a smaller number of sharp spines. These plates and spines protect the fish from predators, but they are costly to produce and maintain. The bony plates require extra calcium, which is rare in some environments, and they restrict the body movements of the fish.

Freshwater sticklebacks tend to have fewer spines and bony plates. Some have a gap in the row of plates (this is called a “partial morph”) while others have only a few plates at the back end of the fish (“low morph”). Fresh water has less calcium than salt water, so this may be an adaptation to a limiting resource. Also, freshwater environments tend to have fewer predators than saltwater environments, so the protective features of the bony plates may be less important in fresh water; perhaps there was relaxed natural selection on this armor, and over time it was lost in many different populations in parallel.

Top: The ninespine stickleback, Pungitus pungitus, is typical of the saltwater form. Bottom: A freshwater form of stickleback with fewer bony plates and fewer spines. Image based on drawings from the Queensland Government Fisheries

In a 2005 study, scientists looked at a gene (Eda) that determines the growth of the bony plate and found that freshwater sticklebacks had a variant of the gene that caused fewer plates to form in those populations. The gene Eda probably serves a regulatory function, so it could determine one of a range of phenotypes from the fully armored saltwater version to the two lesser armored versions found in fresh water. A combination of genetic and population analysis led the researchers to discover that most freshwater sticklebacks in the Northern Hemisphere which exhibit a loss of bony plates do so because they all inherited a variant of Eda that is rare in the original saltwater populations. So the trait evolved in parallel in many lineages, all of which came from different saltwater populations, but it also evolved from a single pre-existing form of the gene. However, it was also found that one or more of the Northern Hemisphere sticklebacks with reduced bony plates got this trait from an entirely different genetic change.

This trait is thus an example of a feature determined by more than one gene, and an example of parallel evolution occurring by more than one means.

A second study just reported at a scientific meeting looks at what seems to be an entirely different question about stickleback evolution. Most sticklebacks form schools, which is a common adaptation among fish, following the principle that there is safety in numbers. But there is one population of freshwater sticklebacks that does not form schools. The sticklebacks of Paxton Lake, in British Columbia, Canada swim around alone most of the time. Rather than forming schools, they hide out in thick vegetation on the bottom of Lake Paxton.

The research team led by Anna Greenwood of the Fred Hutchinson Cancer Research Center in Seattle devised a machine to test for and measure schooling behavior in sticklebacks. This consists of a mobile-like cluster of fake fish which move together as a robotic school in a circle around a large aquarium. When fish from a schooling population of sticklebacks were placed in the water with this machine, they joined the fake fish and swam around with them. When fish from the non-schooling population were placed in the water with this machine, they did not school. These two populations are so closely related that they can interbreed. The researchers tested offspring of the schooling and non-schooling fish to see which behavior each fish would exhibit. As expected, some schooled, and some did not. Once the hybrid fish were sorted out, their genes were examined to see if there was a particular signature that went with schooling versus solitary swimming.

It turns out that the gene that seems to control schooling behavior in these fish is none other than Eda, the same gene that controls the number of bony plates.

So the sticklebacks not only give us a great example of how parallel evolution can arise, but also a great example of a gene affecting more than one trait. But how does that work? The fish that do not develop bony plates also do not develop a fully functioning lateral line. A lateral line is a sense organ many fish have that allows fish to detect movement elsewhere the water. Some predatory fish use the lateral line to find their prey, other fish use the lateral line to detect predators and thus avoid becoming prey, and schooling fish use the lateral line to keep track of the other fish in the school. Apparently, the sticklebacks with the poorly developed lateral lines can’t school because they can’t properly sense the other fish with whom they would need to coordinate their movements.

Sources:

Colosimo, Pamela F., Kim E. Hosemann, Sarita Balabhadra, Guadalupe Villarreal, Jr., Mark Dickson, Jane Grimwood, Jeremy Schmutz, Richard M. Myers, Dolph Schluter, and David M. Kingsley. 2005. Widespread Parallel Evolution in Sticklebacks by Repeated Fixation of Ectodysplasin Alleles Science 25 March 2005: 307 (5717), 1928-1933. [DOI:10.1126/science.1107239]

Pennisi, Elizabeth. 2012. Robotic Fish Point to Schooling Gene. News and Analysis. Science 335(6066):276-277. DOI: 10.1126/science.335.6066.276-b

How do boa constrictors know when to stop constricting? Image courtesy of Flickr user wwarby

Ed. note: We welcome back guest blogger Greg Laden for a two-week blogging tour on Surprising Science.

This is a story of snakes, islands and students. Let’s start with the snakes.

Among the many different kinds of snakes are the constrictors: boas and pythons. They are close relatives that diverged millions of years ago. Pythons are found in the Old World (Africa and Asia) as well as Australia. Boas (family Boidae) are found in the New World (North, Central and South America including some Caribbean islands). All of them kill their prey by wrapping around it and squeezing it to death.

Among the boas there is an island-dwelling form in Belize that is the subject of interest to conservationists, ecologists and, lately, behavioral biologists. This is the miniature boa of Snake Cayes, a group of islands off the coast of southern Belize. When I say “miniature” I mean that they range in length from 30 cm to about 2 meters (1 to 6 feet). This is small compared to the mainland boas of the same species, which can reach 4 meters (13 feet) in length.

It is common for animal populations that live on islands to exhibit differences in size from those on the mainland. Medium and larger mammals like deer tend to be smaller on islands, small mammals like rodents tend to be larger. Something like this may happen with snakes as well.

Allison Hall (left) says “It’s a normal thing to be a little afraid of snakes, but you really get into the project and come to love the animals.” Amanda Hayes is on the right. Image provided by Dickinson News and Events.

Scott Boback is an expert on these animals, and from the time he was a graduate student at Auburn University, he’s been trying to answer the question “how and why are these snakes small?”

The most likely explanations for size differences would seem to be either diet or other features of the environment, or genetics. Perhaps there is a limited food supply on the islands, so snakes grow slowly, and thus there are few or no large ones. It would take them so long reach a large size that somewhere along the line they would have met their demise. Alternatively, it could be that snakes that grow slowly or nearly stop growing as they approach a certain size survive longer or reproduce more effectively (probably owing to food supply being limited). If so, the genes involved in growth would be shaped by natural selection and over time the island snakes would be small because they are genetically different. You can easily imagine how the two processes would work together, perhaps with environmental effects working initially but genetic changes accruing over time.

Boback did eventually come to a conclusion about the small size of the island boas. He recently told me, “we determined that there is some genetic component to dwarfism on islands. However, we believe that it is actually a combination of genetic and environmental effects that ultimately determine island boa size. That is, growth rates are different between island and mainland boas and this appears to be determined partly by genetics.” (See below for the reference to his paper on this research.)

More recently, Boback and his students at Dickinson College have been addressing a different question about boas: How do they know when to stop squeezing their prey? This is an interesting question because, as you might imagine, contracting the majority of muscles in one’s body for an extended period of time is energetically costly, but letting go of prey before it is fully dead may cause the loss of a meal. As an informal experiment, I asked five different people this question over the past two days, after reading of Boback’s research, and everyone gave approximately the same answer: The snakes let go when the prey is dead and stops struggling.

Well, it turns out that we do science to prove ourselves wrong, because that is not the answer. Suspecting a particular mechanism, Boback his students, who maintain a colony of these boas in their lab at Dickinson, devised a brilliant experiment. They took a number of dead rats that would normally be fed to the snakes, and installed robotic “hearts” in them. When the snakes constricted the rats, the hearts were allowed to beat for a while, then they were turned off. Soon after, the snakes loosened their grip, then let go.

It turns out that boas have the ability to detect a heartbeat in the prey, and they use this information to determine how much pressure to apply. Snakes that had never killed or eaten live prey acted the same as snakes with experience with live prey, suggesting that this behavior is innate and not learned.

“Many of us think of snakes as audacious killers, incapable of the complex functions we typically reserve for higher vertebrates,” says Boback. “We found otherwise and suggest that this remarkable sensitivity was a key advancement that forged the success of the entire snake group.”

One of the neat things about this project is that it involved the efforts of undergraduate researchers. The undergraduates not only participated in the research, but they helped produce the peer reviewed paper and are listed as authors. Katelyn McCann, who was a student on this project and now works as a clinical-research coordinator at Children’s Hospital in Boston, notes, “I got to experience the true collaborative nature of research as well as the hours of independent work that go into the final product. Now, working in research I feel like I truly understand the scientific method and what goes into any study.” Boback adds, “student-faculty research at Dickinson is an opportunity for students to experience science in action. It is the most fundamental level of learning in science as the student actively participates in the process of discovery.”

Source:
Boback, S., Hall, A., McCann, K., Hayes, A., Forrester, J., & Zwemer, C. (2012). Snake modulates constriction in response to prey’s heartbeat Biology Letters DOI: 10.1098/rsbl.2011.1105

Boback, S. M. and D. M. Carpenter. 2007. Body size and head shape in island boas (Boa constrictor) in Belize: Environmental versus genetic contributions. Pages 102-116 in R. W. Henderson and R. Powell, editors. Biology of the boas, pythons, and related taxa. Eagle Mountain Publishing, Eagle Mountain, UT.

Additional information for this story came from Dr. Scott Boback, and a press release from Dickinson College.

The aurora borealis in northern Norway. Photo by AP Photo / Scanpix Norway, Rune Stoltz Bertinussen

Our photo gallery of the most stunning images from the recent northern lights show.

Precious few people around the world have ever had the chance to witness the remarkable phenomenon known as the aurora borealis, or northern lights. The collision of magnetically charged solar particles with the earth’s magnetosphere produces dancing waves of florescent green and deep blue that appear to wave across the sky, but under normal conditions, the lights can been seen only in far northern latitudes. Even then, the aurora borealis is unpredictable in occurrence and can be difficult to spot.

Recent storms on the surface of the sun, though, have produced levels of solar particles headed towards the earth not seen for a decade—and dazzling northern lights. Skygazers report that, over the past week, remarkably intense displays have appeared in skies in Scandinavia and Northern England. Scientists predict that recent surges are just a small taste of what’s to come over the next year or so, as the cycle of solar activity is expected to peak in 2013 and 2014.

A dragon statue in Ljubljana, Slovenia. Photo courtesy Wikimedia Commons.

Around the world, people are celebrating the Chinese New Year and the start to the Year of the Dragon. This got us wondering: Where did the myth of the dragon come from in the first place? Scholars say that belief in dragons probably evolved independently in both Europe and China, and perhaps in the Americas and Australia as well. How could this happen? Many have speculated about which real-life animals inspired the first legends. Here’s our run-down of the likeliest suspects.

Dinosaurs. Ancient people may have discovered dinosaur fossils and understandably misinterpreted them as the remains of dragons. Chang Qu, a Chinese historian from the 4th century B.C., mislabeled such a fossil in what is now Sichuan Province. Take a look at a fossilized stegosaurus, for example, and you might see why: The giant beasts averaged 30 feet in length, were typically 14 feet tall and were covered in armored plates and spikes for defense.

The Nile Crocodile. Native to sub-Saharan Africa, Nile crocodiles may have had a more extensive range in ancient times, perhaps inspiring European dragon legends by swimming across the Mediterranean to Italy or Greece. They are among the largest of all crocodile species, with mature individuals reaching up to 18 feet in length—and unlike most others, they are capable of a movement called the “high walk,” in which the trunk is elevated off the ground. A giant, lumbering croc? Might be easy to mistake for a dragon.

The Goanna. Australia is home to a number of species of monitor lizards, also referred to as Goannas. The large, predatory animals have razor-sharp teeth and claws, and they are important figures in traditional Aboriginal folklore. Recent studies even indicate that Goannas may produce venom that causes bite victims’ wounds to develop infections after an attack. At least in Australia, these creatures may be responsible for the dragon myth.

Whales. Others argue that the discovery of megafauna such as whales prompted stories of dragons. Ancient humans encountering whale bones would have no way of knowing that the animals were sea-based, and the idea of such gargantuan creatures might well have led people to assume that whales were predatory. Because live whales spend up to 90 percent of their time underwater, they were poorly understood for most of human history.

The Human Brain. The most fascinating explanation involves an unexpected animal: the human. In his book An Instinct for Dragons, anthropologist David E. Jones argues that belief in dragons is so widespread among ancient cultures because evolution embedded an innate fear of predators in the human mind. Just as monkeys have been shown to exhibit a fear of snakes and large cats, Jones hypothesizes that the trait of fearing large predators—such as pythons, birds of prey and elephants—has been selected for in hominids. In more recent times, he argues, these universal fears have been frequently combined in folklore and created the myth of the dragon.

Last summer, on July 6, solar scientist Karel Schrijver spotted something unusual. Looking at a coronagraph—an image created by blocking out the center of the sun, revealing only the corona, the area near its surface—he saw a bright comet, identified as C/2011 N3, descending into the solar atmoshpere. When he searched for the comet on images produced by the Solar Dynamic Observatory (SDO), a solar observation satellite that orbits the earth, he realized he was seeing something unprecedented. For the very first time, the death of a comet crashing into the sun had been caught on camera.

A new paper, published by Schrivjer and a team of scientists today in Science, details the find and what it means for astronomy. Comets dive into the sun frequently, but previous ones had been too small and dim to be seen against the glaring backdrop of the sun. But this comet, an ultra-bright one from a group known as the Kreutz comets, was caught by SDO imaging equipment plunging to its death. Over the course of 20 minutes, it clearly appears descending across the sun before disappearing into its surface. Space.com notes:

“It was very surprising to see this comet at all,” Karel Schrijver, an astrophysicist at Lockheed Martin Advanced Technology Center in Palo Alto, Calif., told SPACE.com. “We may think that an object of some 60,000 metric tons and some 50 meters [164 feet] across is large and heavy, but if you compare it to the sun, which can easily hold a million Earths, it is astonishing that such a small object glows brightly enough to be seen.”

The find, it turns out, is more than merely interesting: It has helped the scientists develop a new method for calculating the size of comets from afar. Using two figures—the amount of time it took the comet to evaporate and the distance over the sun it traveled while doing so—the team figured out its size and speed.

The C/2011 N3 comet is caught on a coronagraph, an image that blocks out the sun to reveal its corona. Image courtesy of NASA Solar and Heliospheric Observatory

“It was moving along at almost 400 miles per second through the intense heat of the sun—and was literally being evaporated away,” said Schrijver, the lead author of the paper. As the Bad Astronomy blog points out, that speed means it would have crossed the width of the United States in about 8 seconds.

The researchers also estimate that the comet came within 62,000 miles of the sun’s surface before evaporating, and was 70,000 tons in size (about the weight of an aircraft carrier), trailed by a tail 10,000 miles in length.

Some aspects of the discovery, though, are still confusing for the scientists. Most surprising is the fact that we could see the comet at all. Because objects passing in front of the sun absorb light, the comet should have appeared as a dim spot rather than a bright one. Solving this mystery, along with others, might help reveal information about the composition of comets, the sun’s corona and perhaps even the origins of the solar system. Scientists will continue to look to the sun—and scrutinize the data—for answers.