Research Rundown
Science Journal -- Spring 1996 -- Vol 13, No. 2
What is twice as big as a fruit fly, acts like a sailboat, and has given biologists some flying lessons that were published in a recent issue of the journal Nature? It is central Pennsylvania's winter stonefly, an important piece in one of the great puzzles of evolutionary biology-how insects developed the ability to fly-according to Penn State researchers James H. Marden, assistant professor of biology, and undergraduate biology student Melissa G. Kramer. Marden says their research is the first to use living insects to demonstrate an ancient form of locomotion that could have fostered the evolution of muscles and large wings for full airborne flight. "These tiny stoneflies emerge from streams in winter at the end of their nymph stage, use their legs like skis to support their weight on the surface of the water, and simply hold their stubby little wings up in the air to let the wind blow them toward land," says Marden, who earlier studied a more advanced species of flightless stonefly that skims across the water in early spring by flapping its much larger wings. Marden collected both species in Centre County Pennsylvania's Bald Eagle Creek.
His research with the smaller-winged winter stonefly, Allocapnia vivipara, upsets a theory of insect-flight evolution that has guided biologists for over half a century. According to the theory, ancient insects with rudimentary wings used them for gliding through the air after leaping from vegetation, giving them a survival advantage over wingless insects and driving the evolution of progressively larger wings. But "nobody had ever tested that theory with a real insect," Marden says.
Marden and Kramer's research with the winter stonefly revealed the insects get much more benefit from their small wings for sailing on water than for gliding in air. Contrary to the prevailing evolutionary theory, Marden and Kramer conclude that "sailing has a greater potential for driving the evolution of insect wings than does aerial gliding."
When winter stoneflies sail on the water they position their wings to catch a lot of wind, like a sailboat's big, billowy spiniker sail-but their wings are not nearly as effective when the insect is dropping through the air, the researchers discovered. "You can maneuver a sailboat to always have good wind, but if we dropped you out of an airplane in your sailboat you would tumble uncontrollably and your sail would flap uselessly, only randomly catching the prevailing wind," Marden explains. "With their small wings, these stoneflies can't control their body orientation during free-fall," he says.
To measure the insects' flight performance in air, the researchers developed an innovative tracking set-up that they say is unique for studying insect flight. The key to the device is a modified motion-measurement system originally designed for studying biomechanical movements such as a golf swing. Two calibrated cameras emit strobe flashes of infrared light that bounce back from infrared-reflective tape typically placed on a subject's major joints. The cameras record the reflected light on infrared-light-sensitive grids and a computer translates the signals into movement data in three dimensions.
Because winter stoneflies are so tiny they can't even bear the effect of being painted with infrared-reflective paint, Marden asked the manufacturer to customize the device for inverse video-instead of recording bright spots on a dark background, it records dark spots on a bright background.
"We just drop the insects in front of a bright background and the cameras can track them with precision that is orders of magnitude beyond what you can get with normal video," Marden explains. "Nobody before ever tracked things this small in three dimensions without applying any kind of markers, at such high sample sizes, at such high speeds, and with such high resolution-way beyond submillimeter."
According to Marden, the winter stonefly demonstrates a very early stage in the evolution of insect flight-even before insects could flap their wings. "Flapping on the water requires a pretty complicated combination of anatomy, behavior, and physiology," Marden says. The winter stonefly, which never flaps its small wings, just raises them up in response to wind. "Its sailing performance keeps improving as its wings get bigger and some even have wings theoretically big enough in relation to their body size for flapping flight," Marden says. "This paper fills in a big gap in our knowledge of how fossil insects with moveable gill plates-the precursors for wings-could eventually have evolved into today's flying insects."
Another 50-year-old evolutionary doctrine called into question by the Marden/Kramer research is that flight evolved only once in the history of insect evolution because wings appear to have evolved only once, eons before the insect family branched off into its different lineages. "The paradigm has been that an aerodynamic wing equals flight, but we have shown that sailing on the water also is an aerodynamic function," Marden says. "We have behavioral evidence from a living insect that the whole basis for that paradigm is flawed."
He concludes that the Eve of all flying insects could have been a sailor rather than a flyer and that flight could have evolved independently at different times within many different insect lineages.
The old paradigm also says that flightless insects with wings must have evolved from flying ancestors then later lost their ability to fly. Instead, Marden and Kramer argue it is possible that the winter stonefly species never used its wings to fly. "Because we throw out the paradigm that equates wings with flight, we are now allowed to explore all kinds of new possibilities for the evolution of flight in insects," Marden says.
To clinch the Penn State biologists' new theory, scientists need to see fossils of the very first winged insect, but none have yet been found. "We are providing a picture of function from some very primitive modern-day insects that suggests new ways to interpret fossils and the evolutionary history of winged insects," Marden says. "I think our research will provide a very strong basis for the interpretation of those fossils if and when they become available."
Barbara K. Kennedy
Birds whose genes are lean might be the best flyers, according to research published in a recent issue of the journal, Nature, by Penn State's husband-wife researchers Austin L. Hughes, assistant professor of biology, and Marianne K. Hughes, research associate. The biologists discovered that bird genes have lost significant amounts of "junk DNA" during their evolution, providing the latest clue in the evolution of flight in birds.
Scientists have known that the nuclear DNA content in bird cells is much less than in the cells of mammals and other animals, but they have not known why. One theory is that modern birds lost a large amount of genetic material due to a chance mutation in a small ancestral breeding population millions of years ago. Another theory is that modern birds got some unknown benefit from developing smaller genes throughout their evolution.
"If there is some adaptive reason why birds have a reduced genome size then it should be reflected in just about every gene," Austin Hughes reasoned. "We would expect to find that each gene sequence is smaller?not that a huge single chunk of DNA is missing."
The Hughes team compared gene sequences from humans and chickens. "The largest database of mammal genes is for humans and the largest database of bird genes is for chickens," Hughes says.
They focused on sections of DNA called introns, which Hughes calls "junk DNA." Introns serve primarily as boundary markers on the DNA chain between gene segments called exons, which have important functions in the protein-making process. The biologists compared 31 genes that had the same function in chickens as in humans.
They found that chicken introns were smaller, especially for gene sequences that are very big in humans. "Small intron segments were missing in every gene, which indicates to me that there is some sort of overall pressure in the direction of reducing the genome size in birds," Hughes explains. "Introns basically just sit there, so if you were looking for a way to make the genome smaller that would be a good place to cut," he says.
Hughes notes that animals with smaller genes are known to have smaller cells. He speculates, "a good metabolism for flight requires each cell to exchange carbon dioxide for oxygen really fast, which is easier to do in a small cell because it has a relatively larger surface area." Another possible reason that reduced DNA might be an advantage for flight is increased speed of gene replication.
The Hughes team also ranked 40 families of birds according to their flying ability and found that the best flyers consistently had the smallest genome size. Hughes says his research shows the reduction of the genome is adaptive because it has been continuing over a long period of time. "We have reason to suspect that it could be adaptive for flight," he adds.
"We next want to compare the introns of other birds?like a penguin, which can't fly at all and a humming bird, which is a great flyer?to see if this hypothesis holds up," Hughes says. "It also would be great to do the same thing with bats, which fly and also have less DNA than the average mammal," he adds.
This research was supported by a National Institutes of Health Research Career Development Award.
Barbara K. Kennedy
Research on bird genes has settled a controversy as old as the United States about one of the most unusual birds in the worldóthe only species with a digestive system like a cow's. "How the South American hoatzin is related to other birds has been a mystery since this bizarre species was first described in 1776," says S. Blair Hedges, assistant professor of biology at Penn State, whose research with other investigators was published in a recent issue of the Proceedings of the National Academy of Sciences.
Since 1776, most experts in bird anatomy have argued that the hoatzin (pronounced Watson) should be grouped with pheasants, chickens, and other heavy-bodied land birds that biologists classify in the order Galliformes because of its similar anatomy. Some experts, however, argued that the plump, blue-faced, South American bird should be placed on the bird family tree next to cuckoos, which its plumage and markings resemble. But the hoatzin is like no other bird in the world in the way it digests food.
"There is no other bird with a digestive system like the hoatzin's," Hedges says. "It is the only bird that has a foregut for fermentation like a cow'sóhas bacteria like a cow's to help it digest celluloseóand has an enzyme like a cow's to extract nutrients from the bacteria."
The Hedges research, which compares the similarities between the hoatzin's genes and those of other birds, does not explain the hoatzin's strange digestive system, but it is the first to show conclusively that the hoatzin is "not even closely related to the Galliformes but is definitely related to the cuckoos, although it may not itself actually be a cuckoo," according to Hedges.
Charles G. Sibley, a coauthor of the paper and the author of the well-known reference book, "Birds of the World," was in 1973 the first researcher to use molecular-genetic rather than anatomical techniques in an attempt to resolve the controversy over the hoatzin's family tree. The Hedges research confirms the results of the subsequent two decades of genetic research, which had been inconclusive, but contradicts most of the classifications based on physical characteristics that have been made during the past 120 years.
Hedges says physical characteristics, which are the result of evolutionary adaptations to the environment, can fool researchers about the relationships between species. Gene studies, he says, are more reliable because the genes reflect the ancestry of an animal more than they reflect the animal's environment.
Hedges says his research has finally resolved the hoatzin's heritage because his team sequenced twice as many base pairs of DNA as previous researchers and they used two different mitochondrial genes, which he says are a more reliable indicator of genetic relationships. The Hedges group also sequenced DNA material from the nucleus of the birds' cells, resulting in more than 1850 base pairs of those two different types of DNA. "It is nice to have both the mitochondrial and the nuclear sequences giving us the same results," Hedges says. "I believe this study makes a good case that molecular genetic techniques reflect the true relationships between species better than morphological studies," he says.
The authors of the paper, titled "Phylogenetic Relationships of the Hoatzin, an Enigmatic South American Bird" (Proc. Natl. Acad. Sci. Vol. 92, pp 11662-11665), are Hedges, Melitta D. Simmons of Penn State, Marjon A. M. van Dijk of the University of Nijmegen (The Netherlands), Gert-Jan Caspers of the University of Nijmegen (The Netherlands), Wilfried W. deJong of the University of Nijmegen and the University of Amsterdam (The Netherlands), and Sibley.
Barbara K. Kennedy
Astronomers using a new method for probing rapidly spinning neutron stars known as pulsars, made possible by the Hubble Space Telescope, have revealed unexpected nonthermal radiation coming from a middle-aged pulsar and, for the first time, have measured the surface temperatures of two old pulsars, according to France Cordova, chief scientist at the National Aeronautics and Space Administration and professor of astronomy and astrophysics at Penn State. The method, which allows astronomers to detect emissions from these stars in optical and ultraviolet wavelengths, also revealed that an unexplained heating mechanism is operating deep inside the oldest neutron stars in the study.
Cordova revealed the novel ways in which her research team used the Hubble Space Telescope and the results and implications of their work in a lecture titled "All Eyes on the Universe: Multiwavelength Astrophysics" at the 1996 Annual Meeting of the American Association for the Advancement of Science in Baltimore, Maryland. She and her colleagues at Penn State, George Pavlov, visiting professor, and Guy Stringfellow, postdoctoral fellow, have submitted a paper detailing their discovery to The Astrophysical Journal.
Astronomers have used the unusual properties of pulsars to test Einstein's theory of relativity and to discover the first planets ever found outside our solar system. But previous research on pulsar emissions have studied mainly radio, X-ray, or gamma-ray wavelengths. "We needed the crystal vision of the Hubble Space Telescope to detect the optical and ultraviolet counterparts of radio pulsars because emissions at these wavelengths from these objects are predicted to be fainter than could be resolved using ground-based telescopes," Cordova explains.
Cordova's team was attempting to confirm a hypothesis about the unknown interior of neutron stars, where matter is pushed to extremes of gravitational forces and magnetic fields. "Thirty years after the discovery of neutron stars, we still do not know the properties of the superdense matter in their cores and cannot fully explain how they radiate in all wavelengths," she says.
When Cordova's team combined their results for the neutron stars in all wavelengths, they found the emission from the youngest one was "non-thermal in character," whereas the oldest were much hotter than predicted. "The relatively high temperature means some kind of heating or reheating mechanism has to be operating inside old neutron stars?otherwise they would have cooled down?but we do not yet adequately understand the source of this heat," Cordova says. "What we need now are further observations of neutron stars with the Hubble Space Telescope to help drive the development of a theory that will adequately explain the properties of these puzzling objects," she says.
Barbara K. Kennedy
Hubble Space Telescope Peers Deep into
the Heart of the
Densest Known Star Cluster
By pinpointing individual suns in the glare of the most tightly packed cluster of stars in our galaxy, the Hubble Space Telescope has unveiled hints of either a massive black hole or another remarkable phenomenon: a "core collapse" driven by the intense gravitational pull of so many stars in such a small volume of space.
A team of astronomers used the telescope's sharp images to count an extraordinary number of stars in the ancient globular cluster M15, about 37,000 light-years away. Hubble spied hundreds of stars in a tiny area at the center of M15, whereas earthbound telescopes see a single blur of light. Careful analysis of the distribution of these and thousands of neighboring stars suggest that at some point in the distant past, the stars converged on M15's core, like bees swarming to their hive. This runaway collapse, long theorized by researchers but never seen in such detail, may have lasted a few million years?a flash in the 12-billion-year life of the cluster.
Thanks to the laws of physics, the core probably stopped collapsing before many of the stars collided. Rather, stars near the center would have settled into an uneasy cosmic waltz, both attracted to each other by gravity and repelled by close encounters that slingshot them through space.
An alternate scenario also could explain the pileup of stars at M15's core: a black hole that may have formed early in the cluster's history. The black hole would have gradually gained mass as more stars spiraled inward. If it exists, it would now be several thousand times more massive than our sun.
The study, which appeared in a recent issue of the Astronomical Journal, was led by Puragra Guhathakurta of UCO/Lick Observatory, UC Santa Cruz. Coauthors are Brian Yanny of the Fermi National Accelerator Laboratory, Donald Schneider of Pennsylvania State University, and John Bahcall of the Institute for Advanced Study in Princeton. All of the astronomers were associated with the Institute for Advanced Study when the research began.
A precise reading of the speeds at which stars move near M15's core would reveal whether the stars are packed so tightly because of the influence of a single massive object, or simply by their own mutual attraction. Stars would orbit more quickly in the grip of a black hole's gravitational field. Such measurements are time consuming but possible with the Space Telescope.
"It is very likely that M15's stars have concentrated because of their mutual gravity," Guhathakurta says. "The stars could be under the influence of one giant central object, although a black hole is not necessarily the best explanation for what we see. But if any globular cluster has a black hole at its center, M15 is the most likely candidate."
The team began using Hubble to observe the centers of globular clusters in 1991 and now has data on about twenty clusters, but the images of M15 are by far the most stunning. Hubble's Wide Field Planetary Camera 2 (WFPC2) probed M15 in April 1994, four months after astronauts installed corrective optics to sharpen the telescope's blurry focus.
"I first started thinking about this observation in 1970," says Bahcall. "I never expected that Hubble would see things as clearly as it does. The results are so exciting that they are a dream come true." Bahcall and astrophysicist Jeremiah Ostriker of Princeton University first proposed in 1975 that M15 might harbor a black hole. While distinguished by its extreme density of stars, M15 is in other respects similar to the rest of the dozens of globular clusters that freckle space in and around our Milky Way.
Each cluster is like a miniature galaxy, with 100,000 to one million stars in a compact spherical blob. The largest and closest?including M15, in the constellation Pegasus?are visible to the naked eye on dark nights as faint hazy patches.
Globular clusters contain almost no gas or dust and show few signs of recent star formation. Astronomers believe they are primordial remnants, left over from the birth of the Milky Way. As such, they are ideal laboratories for studying how stars evolve. Cluster stars also provide a limit on the age of the universe, independent of the expansion of the universe itself.
Stars at the core of M15 may be crowded closer together than anywhere in the Milky Way except in the galaxy's hidden heart. Attempted studies of this exotic locale with ground-based telescopes proved frustrating. Atmospheric blurring washed out the interesting details at the core. Astronomers used Hubble before its repair mission to examine M15, but even after correcting the distorted images they could not discern the true distribution of the innermost stars. In contrast, the latest WFPC2 photos of the inner 22 light-years of the cluster revealed about 30,000 distinct stars. That's a fraction of M15's population, but far more stars than scientists had ever imaged in such a small region of a globular cluster.
The astronomers used the Planetary Camera (the highest- resolution part of WFPC2) to study M15's core. The closer they looked toward the core, the more stars they found. This increase in stellar density continued all the way to within 0.06 light-years of the center?about 100 times the distance between the sun and Pluto.
"Detecting separate stars that close to the core was at the limit of Hubble's powers," Yanny says. Beyond that point, even Hubble's eagle eye could not reliably resolve individual stars or locate the exact position of the core. However, the researchers suspect that stars jam together ever more tightly inside that radius.
The team plotted the distribution of the stars as a function of distance from the core. Computer simulations helped them include stars they may have missed when bright stars drowned out faint ones in the Hubble images. The resulting pattern matches the predictions of Bahcall and others for what would happen under the influence of a central black hole. But the pattern also is consistent with a core collapse, known as a "gravothermal catastrophe." Astronomers think the cores of about 20 percent of all globular clusters may have collapsed in this way.
For a gravothermal catastrophe to occur, globular clusters must transfer energy from the inner parts of the cluster to outer regions. As this happens, stars near the core lose some of the energy of their random ("thermal") motions. Several billion years might pass before the stars become too lethargic to resist the gravitational pull of their neighbors. At that point, they begin to collapse inward as a group.
"It's a catastrophe in the sense that once it starts, this process can run away very quickly," Guhathakurta says. "But other processes could cause the core to bounce back before it collapses all the way." The major such process, researchers believe, is the powerful jolt of new motion that binary-star systems can impart to a third star that wanders too close?effectively spreading the stars out again.
Robert Irion (University of California) and Barbara K. Kennedy
Astronomy Rocket Experiment Launched from Australian Outback
A Penn State astronomy experiment took a 15-minute ride in space on a rocket launched from the Australian Outback late this fall. The experiment took an X-ray snapshot of a huge space object that covers an eighth of the sky but is invisible at optical wavelengths, according to David N. Burrows, associate professor of astronomy and astrophysics and leader of the Penn State team that designed the experiment.
"We are going to measure the temperature, chemical composition, and density of a superbubble of hot gas called Loop-1 in an area that can be seen only from the southern hemisphere," Burrows says. A superbubble forms when groups of "O" and "B" stars?the largest and hottest types of stars in the universe?explode. "These stars tend to be produced in clumps and to explode as supernovae at about the same time, blowing a huge hole filled with gas?as hot as several million degrees?into the interstellar medium," Burrows explains.
The Earth is inside a bubble like Loop-1 that stretches roughly 300 light years in most directions, but it is insulated by a smaller cloud of cool gas, according to Burrows. Although the gas particles in a superbubble are super-hot, Burrows says they are so far apart?about 1 particle in a litre of space?that an astronaut sitting in the gas would not get burned. "The Earth is surrounded by its own magnetosphere, which sits in the solar wind, which protects the solar system out to an unknown distance," Burrows explains.
One goal of the Penn State research is to understand how the edge of a superbubble interacts with the interstellar medium. "It also is going to help our understanding of the physical processes that occur when you heat up a gas to such extremes," Burrows says, "and it will help us understand more about the structure of the interstellar medium and the evolution of the gas in our galaxy." He plans to launch a similar experiment from the northern hemisphere to compare gases in different regions of Loop-1 to determine whether the huge object is actually a single structure or an optical illusion. "It is hard to tell because Loop-1 is so enormous," Burrows says.
The Penn State experiment will be the first to use a super-sensitive Charge Coupled Device (CCD) X-ray detector to study this part of the Loop-1 bubble, the closest superbubble in the universe to the one that surrounds Earth. The brief sounding-rocket experiment will serve as a dress rehearsal for this type of detector, which astronomers plan to use on a series of more extensive experiments designed to orbit the Earth on satellites. Burrows plans to use the CCD detectors on a multi-year satellite version of the sounding-rocket experiment next year.
According to Burrows, the CCD detector is the same type of device used in home cam-corders except it has been specifically designed and optimized for the high-resolution detection of X-ray wavelengths. "The CCD detector that we are using has about 10 times better energy resolution than proportional counters?the instruments used on previous studies of X-rays in our galaxy," Burrows says.
The research of his group has pushed the development of higher-quality hardware and instrumentation by companies in the United States and England, Burrows says. "We have to push the boundaries of technological research to build a good X-ray CCD detector like ours. We have to have extremely clean materials, high purity all through the entire production line, and an extremely accurate lithography processes to fabricate these very complex semiconductor devices," Burrows explains. "We are giving certain segments of the semiconductor industry a push toward very high-quality work that they might not have gotten from other kinds of applications," he says.
Because sounding-rocket experiments are easier, faster, and cheaper to build and launch than satellite experiments they are a good experience for graduate students, says Burrows, whose team for this project includes two graduate students, Jeff Mendenhall and Laura Cawley. "When you give a graduate student a sounding-rocket experiment, they can follow the whole project through, from the time you write the proposal through the building and calibration of the detector and the payload system, to the launch and the analysis of the data." The researchers also can try things out with a sounding rocket that might be a little too risky or novel to get into a satellite program. "It is much easier to get a sounding rocket launched than to get an experiment on the Space Shuttle," Burrows adds.
This research was supported by the U.S. National Aeronautics and Space Administration.
Barbara K. Kennedy
Basic research is supposed to be like caviar, you get what you pay for. But that doesn't mean there aren't bargains. George Andrews, Evan Pugh Professor of Mathematics and head of the Penn State Department of Mathematics, thinks that basic research in his field, number theory, and especially partitions, is just such a gilt-edged bargain.
A partition is an elementary idea in number theory. Partitioning a number merely means breaking it up into a sum. For instance, 5 can be partitioned in seven ways: 5, 4+1, 3+2, 3+1+1, 2+2+1, 2+1+1+1, and 1+1+1+1+1. This simple concept has developed into a sophisticated and deep branch of number theory.
At the 1996 annual meeting of the American Association for the Advancement of Science, Andrews outlined how Rodney Baxter, an Australian physicist, coupled number theory and physics to produce an exact mathematical description of the behavior of liquid helium on a sheet of graphite. This union is even more amazing when one considers that number theory is sometimes viewed as application-less, pure mathematics, while liquid helium is one of the most studied substances in the world, Andrews said.
Baxter's work initiated extensive research in statistical mechanics (the statistical study of atomic phenomena) where the theory of partitions plays a central role. Such theoretical studies are immensely helpful in understanding chemical and physical interactions, said Andrews, who collaborated with Baxter on follow-up research. As a direct consequence of their effort, mathematicians learned new and important facts about the theory of partitions, increasing the odds the theory would be used in other scientific disciplines.
Andrews recently worked with scientists in Switzerland applying number theory to programming computer software. This work centered on a study of the average running time of computer programs and how to predict when a program is likely to be hopelessly inefficient.
"I'm surprised each time that questions in other fields end up related to partitions," Andrews said. "As a number theorist, it's stimulating when partitions can be used as a tool of statistical or mathematical analysis to help another scientific field."
Andrews said pure math research is often applied to other disciplines. He points to Non-Euclidean geometry, thought to have no use until Einstein applied it to his theory of relativity. "Basic research in mathematics is important and not terribly expensive," he said. "Often, all you need is pencil and paper or at most a small computer."
The mathematics used by Baxter in his breakthrough research was pioneered
by the turn-of-the-century
mathematical genius Srinivasa
Ramanujan. In 1976, Andrews discovered misplaced papers of Ramanujan
among old hotel bills and letters in a box in the Trinity
College Library, Cambridge University, England. Ramanujan, who died
in 1920 at age 32, developed formulas that helped revolutionize the theory
of numbers, particularly partitions. Andrews is considered one of the foremost
authorities on Ramanujan's work, and thus it was natural that Baxter would
enlist Andrews in a collaboration.
Scott Turner
And you thought you had trouble getting organized . . .
Physicists have been wondering whether complex dynamical phenomena like earthquakes and evolution are examples of the idea known as self-organized criticality, which has achieved growing popularity among scientists during the past five years or so. Research results published in a recent issue of Physical Review Letters by Maxim Vergeles, a physics graduate student at Penn State, indicate that the answer could be "maybe not."
"Hundreds of papers have been written on this intriguing subject, first put forward by Per Bak, Chao Tang, and Kurt Wiesenfeld in 1987," explains Jayanth R. Banavar, Penn State professor of physics and faculty advisor to Vergeles. "Even though the idea is far-reaching, it is very simple and beautiful and can even be realized in 'toy' computer models."
The most famous model of self-organized criticality in a dynamical system is a growing sandpile, in which avalanches of various magnitudes and frequencies occur when the slope of the sand reaches a critical value.
"The system is said to be self organized because it reaches the critical state on its own, without any fine-tuning of parameters, and it is critical because of the absence of a single well-defined length or time scale," Vergeles explains.
Data from models of sandpile growth and numerous natural phenomena fall into a pattern that has become the "fingerprint" of a self-organized system. Simply stated, events of all sizes take place, with small events occurring frequently while big events occur infrequently. Scientists have observed this pattern in phenomena such as earthquakes, tides in the river Nile, and punctuated equilibrium in evolution.
Previous models of self-organized criticality are based on rules that govern the evolution of the system and, in these previous models, the system obeys the rules perfectly. "For example, in models of evolution, it is assumed that the species with the lowest barrier to mutation is always the one that mutates, accompanied by changes in the local environment," Banavar explains.
Vergeles relaxed the perfection requirement by giving the species with the lowest barrier to mutation a very high probability of being the one to mutate, but he allowed any of the other species to mutate a very small percentage of the time. "Strikingly, when I introduced even an infinitesimal amount of imperfection in the system, the fingerprint of self-organized criticality disappeared," Vergeles says.
"Either Nature is perfect, which would be exceptionally interesting, or maybe the kinds of models that have been used may need to be modified," Banavar comments.
"A certain kind of criticality seems to be ubiquitous throughout nature," Banavar says. "It would be a very valuable contribution to understand what is really going on."
Barbara K. Kennedy