Evolution of Development

[ThaG]

I had to read this article in class and I thought it would be a good reading for those who are so behind with their reading to think that evolution is "just a theory"

http://www.nytimes.com/2007/06/26/science/26devo.html

From a Few Genes, Life’s Myriad Shapes

By CAROL KAESUK YOON
Published: June 26, 2007

Since its humble beginnings as a single cell, life has evolved into a spectacular array of shapes and sizes, from tiny fleas to towering Tyrannosaurus rex, from slow-soaring vultures to fast-swimming swordfish, and from modest ferns to alluring orchids. But just how such diversity of form could arise out of evolution’s mess of random genetic mutations — how a functional wing could sprout where none had grown before, or how flowers could blossom in what had been a flowerless world — has remained one of the most fascinating and intractable questions in evolutionary biology.

Now finally, after more than a century of puzzling, scientists are finding answers coming fast and furious and from a surprising quarter, the field known as evo-devo. Just coming into its own as a science, evo-devo is the combined study of evolution and development, the process by which a nubbin of a fertilized egg transforms into a full-fledged adult. And what these scientists are finding is that development, a process that has for more than half a century been largely ignored in the study of evolution, appears to have been one of the major forces shaping the history of life on earth.

For starters, evo-devo researchers are finding that the evolution of complex new forms, rather than requiring many new mutations or many new genes as had long been thought, can instead be accomplished by a much simpler process requiring no more than tweaks to already existing genes and developmental plans. Stranger still, researchers are finding that the genes that can be tweaked to create new shapes and body parts are surprisingly few. The same DNA sequences are turning out to be the spark inciting one evolutionary flowering after another. “Do these discoveries blow people’s minds? Yes,” said Dr. Sean B. Carroll, biologist at the Howard Hughes Medical Institute at the University of Wisconsin, Madison. “The first response is ‘Huh?’ and the second response is ‘Far out.’ ”

“This is the illumination of the utterly dark,” Dr. Carroll added.

The development of an organism — how one end gets designated as the head or the tail, how feet are enticed to grow at the end of a leg rather than at the wrist — is controlled by a hierarchy of genes, with master genes at the top controlling a next tier of genes, controlling a next and so on. But the real interest for evolutionary biologists is that these hierarchies not only favor the evolution of certain forms but also disallow the growth of others, determining what can and cannot arise not only in the course of the growth of an embryo, but also over the history of life itself.

“It’s been said that classical evolutionary theory looks at survival of the fittest,” said Dr. Scott F. Gilbert, a developmental biologist at Swarthmore College. By looking at what sorts of organisms are most likely or impossible to develop, he explained, “evo-devo looks at the arrival of the fittest.”

Charles Darwin saw it first. He pointed out well over a century ago that developing forms of life would be central to the study of evolution. Little came of it initially, for a variety of reasons. Not least of these was the discovery that perturbing the process of development often resulted in a freak show starring horrors like bipedal goats and insects with legs growing out of their mouths, monstrosities that seemed to shed little light on the wonders of evolution.

But the advent of molecular biology reinvigorated the study of development in the 1980s, and evo-devo quickly got scientists’ attention when early breakthroughs revealed that the same master genes were laying out fundamental body plans and parts across the animal kingdom. For example, researchers discovered that genes in the Pax6 family could switch on the development of eyes in animals as different as flies and people. More recent work has begun looking beyond the body’s basic building blocks to reveal how changes in development have resulted in some of the world’s most celebrated of evolutionary events.

In one of the most exciting of the new studies, a team of scientists led by Dr. Cliff Tabin, a developmental biologist at Harvard Medical School, investigated a classic example of evolution by natural selection, the evolution of Darwin’s finches on the Galápagos Islands.

Like the other organisms that made it to the remote archipelago off the coast of Ecuador, Darwin’s finches have flourished in their isolation, evolving into many and varied species. But, while the finches bear his name and while Darwin was indeed inspired to thoughts of evolution by animals on these islands, the finches left him flummoxed. Darwin did not realize for quite some time that these birds were all finches or even that they were related to one another.

He should be forgiven, however. For while the species are descendants of an original pioneering finch, they no longer bear its characteristic short, slender beak, which is excellent for hulling tiny seeds. In fact, the finches no longer look very finchlike at all. Adapting to the strange new foods of the islands, some have evolved taller, broader, more powerful nut-cracking beaks; the most impressive of the big-beaked finches is Geospiza magnirostris. Other finches have evolved longer bills that are ideal for drilling holes into cactus fruits to get at the seeds; Geospiza conirostris is one species with a particularly elongated beak.

But how could such bills evolve from a simple finch beak? Scientists had assumed that the dramatic alterations in beak shape, height, width and strength would require the accumulation of many chance mutations in many different genes. But evo-devo has revealed that getting a fancy new beak can be simpler than anyone had imagined.

Genes are stretches of DNA that can be switched on so that they will produce molecules known as proteins. Proteins can then do a number of jobs in the cell or outside it, working to make parts of organisms, switching other genes on and so on. When genes are switched on to produce proteins, they can do so at a low level in a limited area or they can crank out lots of protein in many cells.

What Dr. Tabin and colleagues found, when looking at the range of beak shapes and sizes across different finch species, was that the thicker and taller and more robust a beak, the more strongly it expressed a gene known as BMP4 early in development. The BMP4 gene (its abbreviation stands for bone morphogenetic protein, No. 4) produces the BMP4 protein, which can signal cells to begin producing bone. But BMP4 is multitalented and can also act to direct early development, laying out a variety of architectural plans including signaling which part of the embryo is to be the backside and which the belly side. To verify that the BMP4 gene itself could indeed trigger the growth of grander, bigger, nut-crushing beaks, researchers artificially cranked up the production of BMP4 in the developing beaks of chicken embryos. The chicks began growing wider, taller, more robust beaks similar to those of a nut-cracking finch.

In the finches with long, probing beaks, researchers found at work a different gene, known as calmodulin. As with BMP4, the more that calmodulin was expressed, the longer the beak became. When scientists artificially increased calmodulin in chicken embryos, the chicks began growing extended beaks, just like a cactus driller.

So, with just these two genes, not tens or hundreds, the scientists found the potential to recreate beaks, massive or stubby or elongated.

“So now one wants to go in a number of directions,” Dr. Tabin said. “What happens in a stork? What happens in a hummingbird? A parrot?” For the evolution of beaks, the main tool with which a bird handles its food and makes its living, is central not only to Darwin’s finches, but to birds as a whole.

BMP4’s reach does not stop at the birds, however.

In lakes in Africa, the fish known as cichlids have evolved so rapidly into such a huge diversity of species that they have become one of the best known evolutionary radiations. The cichlids have evolved in different shapes and sizes, and with a variety of jaw types specialized for eating certain kinds of food. Robust, thick jaws are excellent at crushing snails, while longer jaws work well for sucking up algae. As with the beaks of finches, a range of styles developed.

Now in a new study, Dr. R. Craig Albertson, an evolutionary biologist at Syracuse University, and Dr. Thomas D. Kocher, a geneticist at the University of New Hampshire, have shown that more robust-jawed cichlids express more BMP4 during development than those with more delicate jaws. To test whether BMP4 was indeed responsible for the difference, these scientists artificially increased the expression of BMP4 in the zebrafish, the lab rat of the fish world. And, reprising the beak experiments, researchers found that increased production of BMP4 in the jaws of embryonic zebrafish led to the development of more robust chewing and chomping parts.

And if being a major player in the evolution of African cichlids and Darwin’s finches — two of the most famous evolutionary radiations of species — were not enough for BMP4, Dr. Peter R. Grant, an evolutionary biologist at Princeton University, predicted that the gene would probably be found to play an important role in the evolution of still other animals. He noted that jaw changes were a crucial element in the evolution of lizards, rabbits and mice, among others, making them prime candidates for evolution via BMP4.

“This is just the beginning,” Dr. Grant said. “These are exciting times for us all.”

Used to lay out body plans, build beaks and alter fish jaws, BMP4 illustrates perfectly one of the major recurring themes of evo-devo. New forms can arise via new uses of existing genes, in particular the control genes or what are sometimes called toolkit genes that oversee development. It is a discovery that can explain much that has previously been mysterious, like the observation that without much obvious change to the genome over all, one can get fairly radical changes in form.

“There aren’t new genes arising every time a new species arises,” said Dr. Brian K. Hall, a developmental biologist at Dalhousie University in Nova Scotia. “Basically you take existing genes and processes and modify them, and that’s why humans and chimps can be 99 percent similar at the genome level.”

Evo-devo has also begun to shine a light on a phenomenon with which evolutionary biologists have long been familiar, the way in which different species will come up with sometimes jaw-droppingly similar solutions when confronted with the same challenges.

Among the placental mammals of the Americas and the marsupials of Australia, for example, have evolved the same sorts of animals independently: beasts that burrowed, loping critters that grazed, creatures that had long snouts for eating ants, and versions of wolf.

In the same way, the cichlids have evolved pairs of matching species, arising independently in separate lakes in Africa. In Lake Malawi, for example, there is a long and flat-headed species with a deep underbite that looks remarkably like an unrelated species that lives a similar lifestyle in Lake Tanganyika. There is another cichlid with a bulging brow and frowning lips in Lake Malawi with, again, an unrelated but otherwise extremely similar-looking cichlid in Lake Tanganyika. The same jaws, heads, and ways of living can be seen to evolve again and again.

The findings of evo-devo suggest that such parallels might in fact be expected. For cichlids are hardly coming up with new genetic solutions to eating tough snails as they each crank up the BMP4 or tinker with other toolkit genes. Instead, whether in Lake Malawi or Lake Tanganyika, they may be using the same genes to develop the same forms that provide the same solutions to the same ecological challenges. Why not, when even the beaked birds flying overhead are using the very same genes?

Evo-devo has even begun to give biologists new insight into one of the most beautiful examples of recurring forms: the evolution of mimicry.

It has long been a source of amazement how some species seem so able to evolve near-perfect mimicry of another. Poisonous species often evolve bright warning colors, which have been reproduced by nonpoisonous species or by other, similarly poisonous species, hoping to fend off curious predators.

Now in a new study of Heliconius butterflies, Dr. Mathieu Joron, an evolutionary biologist at the University of Edinburgh, and colleagues, found evidence that the mimics may be using some of the same genes to produce their copycat warning colors and patterns.

The researchers studied several species of tropical Heliconius butterflies, all of which are nasty-tasting to birds and which mimic one another’s color patterns. Dr. Joron and colleagues found that some of the main elements of the patterns — a yellow band in Heliconius melpomene and Heliconius erato and a complex tiger-stripe pattern in Heliconius numata — are controlled by a single region of DNA, a tightly linked set of genes known as a supergene.

Dr. Joron said he and colleagues were still mapping the details of color pattern control within the supergene. But if this turned out to function, as researchers suspected, like a toolkit gene turning the patterns on and off, it could explain both the prevalence of mimicry in Heliconius and the apparent ease with which these species have been shown to repeatedly evolve such superbly matching patterns.

One of evo-devo’s greatest strengths is its cross-disciplinary nature, bridging not only evolutionary and developmental studies but gaps as broad as those between fossil-hunting paleontologists and molecular biologists. One researcher whose approach epitomizes the power of such synthesis is Dr. Neil Shubin, an evolutionary biologist at the University of Chicago and the Field Museum.

Last year, Dr. Shubin and colleagues reported the discovery of a fossil fish on Ellesmere Island in northern Canada. They had found Tiktaalik, as they named the fish, after searching for six years. They persisted for so long because they were certain that they had found the right age and kind of rock where a fossil of a fish trying to make the transition to life on land was likely to be found. And Tiktaalik appeared to be just such a fish, but it also had a few surprises for the researchers.

“Tiktaalik is special,” Dr. Shubin said. “It has a flat head with eyes on top. It has gills and lungs. It’s an animal that’s exploring the interface between water and land.”

But Tiktaalik was a truly stunning discovery because this water-loving fish bore wrists, an attribute thought to have been an innovation confined strictly to animals that had already made the transition to land.

“This was telling us that a piece of the toolkit, to make arms, legs, hand and feet, could very well be present in fish limbs,” Dr. Shubin said. In other words, the genetic tools or toolkit genes for making limbs to walk on land might well have been present long before fish made that critical leap. But as fascinating as Tiktaalik was, it was also rock hard and provided no DNA that might shed light on the presence or absence of any particular gene.

So Dr. Shubin did what more and more evo-devo researchers are learning to do: take off one hat (paleontologist) and don another (molecular biologist). Dr. Shubin oversees one of what he says is a small but growing number of laboratories where old-fashioned rock-pounding takes place alongside high-tech molecular DNA studies.

He and colleagues began a study of the living but ancient fish known as the paddlefish. What they found, reported last month in the journal Nature, was that these thoroughly fishy fish were turning on control genes known as Hox genes, in a manner characteristic of the four-limbed, land-loving beasts known as tetrapods.

Tetrapods include cows, people, birds, rodents and so on. In other words, the potential for making fingers, hands and feet, crucial innovations used in emerging from the water to a life of walking and crawling on land, appears to have been present in fish, long before they began flip-flopping their way out of the muck. “The genetic tools to build fingers and toes were in place for a long time,” Dr. Shubin wrote in an e-mail message. “Lacking were the environmental conditions where these structures would be useful.” He added, “Fingers arose when the right environments arose.”

And here is another of the main themes to emerge from evo-devo. Major events in evolution like the transition from life in the water to life on land are not necessarily set off by the arising of the genetic mutations that will build the required body parts, or even the appearance of the body parts themselves, as had long been assumed. Instead, it is theorized that the right ecological situation, the right habitat in which such bold, new forms will prove to be particularly advantageous, may be what is required to set these major transitions in motion.

So far, most of the evo-devo work has been on animals, but researchers have begun to ask whether the same themes are being played out in plants.

Of particular interest to botanists is what Darwin described as an “abominable mystery”: the origin of flowering plants. A critical event in the evolution of plants, it happened, by paleontological standards, rather suddenly.

So what genes were involved in the origin of flowers? Botanists know that during development, the genes known as MADS box genes lay out the architecture of the blossom. They do so by turning on other genes, thereby determining what will develop where — petals here, reproductive parts there and so on, in much the same manner that Hox genes determine the general layout of parts in animals. Hox genes have had an important role in the evolution of animal form. But have MADS box genes had as central a role in the evolution of plants?

So far, said Dr. Vivian F. Irish, a developmental biologist at Yale University, the answer appears to be yes. There is a variety of circumstantial evidence, the most interesting of which is the fact that the MADS box genes exploded in number right around the time that flowering plants first appeared.

“It’s really analogous to what’s going on in Hox genes,” said Dr. Irish, though she noted that details of the role of the MADS box genes remained to be worked out. “It’s very cool that evolution has used a similar strategy in two very different kingdoms.”

Amid the enthusiast hubbub, cautionary notes have been sounded. Dr. Jerry Coyne, an evolutionary biologist at the University of Chicago, said that as dramatic as the changes in form caused by mutations in toolkit genes can be, it was premature to credit these genes with being the primary drivers of the evolution of novel forms and diversity. He said that too few studies had been done so far to support such broad claims, and that it could turn out that other, more mundane workaday genes, of the sort that were being studied long before evo-devo appeared on the scene, would play equally or even more important roles.

“I urge caution,” Dr. Coyne said. “We just don’t know.”

All of which goes to show that like all emerging fields, evo-devo’s significance and the uniqueness of its contributions will continue to be reassessed. It will remain to be seen just how separate or incorporated into the rest of evolutionary thinking its findings will end up being. Paradoxically, it was during just such a flurry of intellectual synthesis and research activity, the watershed known as the New or Modern Synthesis in which modern evolutionary biology was born in the last century, that developmental thinking was almost entirely ejected from the science of evolution.

But perhaps today synthesizers can do better, broadening their focus without constricting their view of evolution as they try to take in all of the great pageant that is the history of life.

“We’re still a very young field,” Dr. Gilbert said. “But I think this is a new evolutionary synthesis, an emerging evolutionary synthesis. I think we’re seeing it.”

 

[ThaG]

http://findarticles.com/p/articles/mi_m2843/is_6_29/ai_n15893076/print

Endless Forms Most Beautiful: a new revolution in biology: over the past two decades, a new revolution has unfolded in biology. Advances in embryology and evolutionary development biology —involving genetic switches and simple rules that shape animal form and evolution—have profoundly reshaped our picture of how evolution works
Sean B. Carroll

An excerpt from Endless Forms Most Beautiful: The New Science of Evo Devo.

The great variety in the size, shape, organization, and color of animal bodies raises deep questions about the origins of animal forms. How are individual forms generated? And, how have such diverse forms evolved? These are very old questions in biology that date back to Darwin's time and before, but only very recently have deep answers been discovered, many of them so surprising and profound that they have revolutionized our views on the making of the animal world and our place in it. We all share an attraction to animal form, but my aim is to expand that wonder and fascination to how form is created, that is, to our new understanding of the biological processes that generate pattern and diversity in animal design.

While Darwin, Huxley, and their allies knew that embryological development was key to evolution, for more than one hundred years after their chief works, virtually no progress was made in understanding the mysteries of development. The puzzle of how a simple egg gives rise to a complete individual stood as one of the most elusive questions in all of biology. Many thought that development was hopelessly complex, and would involve entirely different explanations for different types of animals. So frustrating was the enterprise, that the study of embryology, heredity, and evolution, once intertwined at the core of biological thought a century ago, fractured into separate fields as each sought to define its own principles.

Because embryology was stalled for so long, it played no part in the so-called "Modern Synthesis" of evolutionary thought that emerged in the 1930s and 1940s. In the decades after Darwin, biologists struggled to understand the mechanisms of evolution. At the time of The Origin of Species (1859), the mechanism for the inheritance of traits was not known. Mendel' s work was rediscovered decades later and genetics did not prosper until well into the 1900s. Different kinds of biologists were approaching evolution at dramatically different scales. Paleontology focused on the largest time scales, the fossil record, and the evolution of higher taxa. Systematists were concerned with the nature of species and the process of speciation. Geneticists generally studied variation in traits in just a few species. These disciplines were disconnected and sometimes hostile over which offered the most worthwhile insights into evolutionary biology. Harmony was gradually approached through an integration of evolutionary viewpoints at different levels. Julian Huxley's book Evolution: The Modern Synthesis (1942) signaled this union and the general acceptance of two main ideas. First, that gradual evolution can be explained by small genetic changes that produce variation which is acted upon by natural selection. And second, that evolution at higher taxonomic levels and of greater magnitude can be explained by these same gradual evolutionary processes sustained over larger periods.

The Modern Synthesis established much of the foundation for how evolutionary biology has been discussed and taught for the past sixty years. However, despite the monikers of "Modern" and "Synthesis," it was incomplete. At that time, we could say that forms do change, and that natural selection is a force, but we could say nothing about how forms change, about the visible drama of evolution as depicted, for example, in the fossil record. The Synthesis treated embryology as a "black box" that somehow transformed genetic information into three-dimensional, functional animals.

The stalemate continued for several decades. Embryology was preoccupied with phenomena that could be studied by manipulating the eggs and embryos of a few species, and the evolutionary framework faded from embryology's view. Evolutionary biology was studying genetic variation in populations, ignorant of the relationship between genes and form. Perhaps even worse, the perception of evolutionary biology in some circles was that it had become relegated to dusty museums.

Such was the setting in the 1970s when voices for the reunion of embryology and evolutionary biology made themselves heard. Most notable was Stephen Jay Gould, whose book Ontogeny and Phylogeny revived discussion of the ways in which the modification of development may influence evolution. Gould had also stirred up evolutionary biology when, with Niles Eldredge, he took a flesh look at the patterns of the fossil record and forwarded the idea of "punctuated equilibrium"--that evolution was marked by long periods of stasis (equilibrium) interrupted by brief intervals of rapid change (punctuation). Gould's book and his many subsequent writings reexamined the "Big Picture" in evolutionary biology and underscored the major questions that remained unsolved. He planted seeds in more than a few impressionable young scientists, myself included.

To me, and others who had been weaned on the emerging successes of molecular biology in explaining how genes work, the situations in embryology and in evolutionary biology were both unsatisfying states of affairs, and enormous potential opportunities. Our lack of embryological knowledge seemed to make much of the discussion in evolutionary biology about the evolution of form just futile exercises in speculation. How could we make progress on questions involving the evolution of form without a scientific understanding of how form is generated in the first place? Population genetics had succeeded in establishing the principle that evolution is due to changes in genes, but this was a principle without an example. No gene that affected the form and evolution of any animal had been characterized. New insights in evolution would require breakthroughs in embryology.

The Evo-Devo Revolution

Everyone knew that genes must be at the center of the mysteries of both development and evolution. Zebras look like zebras, butterflies look like butterflies, and we look like we do because of the genes we carry. The problem was that there were no, or very few, clues as to which genes mattered for the development of any animal.

The long drought in embryology was eventually broken by a few brilliant geneticists who, while working with the fruit fly, the workhorse of genetics for the past eighty years, devised schemes to find the genes that controlled fly development. The discovery of these genes and their study in the 1980s gave birth to an exciting new vista on development, and revealed a logic and order underlying the generation of animal form.

Almost immediately after the first sets of fruit fly genes were characterized came a bombshell that triggered a new revolution in evolutionary biology. For more than a century, biologists had assumed that different types of animals were genetically constructed in completely different ways. The greater the disparity in animal form, the less (if anything) the development of two animals would have in common at the level of their genes. One of the architects of the Modern Synthesis, Ernst Mayr, had written that "the search for homologous [the same] genes is quite futile except in very close relatives." But contrary to the expectations of any biologist, most of the genes first identified as governing major aspects of fruit fly body organization were found to have exact counterparts that did the same thing in most animals, including ourselves. This discovery was followed by the revelation that the development of various body parts such as eyes, limbs, and hearts, vastly different in structure among animals and long thought to have evolved in entirely different ways, were also governed by the same genes in different animals. The comparison of developmental genes between species became a new discipline at the interface of embryology and evolutionary biology--evolutionary developmental biology or "Evo-Devo" for short.

The first shots in the Evo-Devo revolution revealed that despite their great differences in appearance and physiology, all complex animals--flies and fly catchers, dinosaurs and trilobites, butterflies and zebras and humans--all share a common "toolkit" of "master" genes that govern the formation and patterning of their bodies and body parts. The important point to appreciate is that its discovery shattered our previous notions of animal relationships and of what made animals different, and opened up a whole new way of looking at evolution.

We now know from sequencing the entire DNA of species (their "genomes") that not only do flies and humans share a large cohort of developmental genes, but that mice and humans have virtually identical sets of about 25,000 genes, and that chimps and humans are nearly 99 percent identical at the DNA level. These facts and figures should be humbling to those who wish to hold humans above the animal world and not an evolved part of it. I wish the view I heard expressed by Lewis Black, the stand-up comedian, was more widely shared. He said he won't even debate evolution's detractors because "We've got the fossils. We win." Well put, Mr. Black, but there is far more to rely on than just fossils.

Indeed, the new facts and insights from embryology and Evo-Devo devastate lingering remnants of stale anti-evolution rhetoric about the utility of intermediate forms or the probability of evolving complex structures. We now understand how complexity is constructed from a single cell into a whole animal. And we can see, with an entirely new set of powerful methods, how modifications of development increase complexity and expand diversity. The discovery of the ancient genetic toolkit is irrefutable evidence of the descent and modification of animals, including humans, from a simple common ancestor. Evo-Devo can trace the modifications of structures through vast periods of evolutionary time-to see how fish fins were modified into limbs in terrestrial vertebrates, how successive rounds of innovation and modification crafted mouthparts, poison claws, swimming and feeding appendages, gills, and wings from a simple tube-like walking leg, and how many kinds of eyes have been constructed beginning with a collection of photosensitive cells. The wealth of new data from Evo-Devo paints a vivid picture of how animal forms are made and evolve.

The Toolkit Paradox and the Origins of Diversity

The stories of shared body-building genes and of the similarities of our genome to that of other animals have slowly been gaining in public awareness. What is generally neglected, however, is how the discovery of this common toolkit and of great similarities among different species' genomes presents an apparent paradox. If the sets of genes are so widely shared, how do differences arise? The resolution of this paradox and its implications are central to my story. The paradox of great genetic similarity among diverse species is resolved by two key ideas. These concepts are crucial for understanding how the species-specific instructions for building an animal are encoded in its DNA and how form is generated and evolves. The substance of these ideas has received scant, if any, attention in the general press, but they have profound implications for understanding great episodes in life's history such as the explosion of animal forms during the Cambrian period, the evolution of diversity within groups such as butterflies or beetles or finches, and our evolution from a common ancestor with chimps and gorillas.

The first idea is that diversity is not so much a matter of the complement of genes in an animal's toolkit, but in the words of Eric Clapton, "It's in the way that you use it." The development of form depends upon the turning on and off of genes at different times and places in the course of development. Differences in form arise from evolutionary changes in where and when genes are used, especially those genes that affect the number, shape, or size of a structure. There are many ways to change how genes are used and that this has created tremendous variety in body designs and the patterning of individual structures.

The second idea concerns where in the genome the "smoking guns" for the evolution in form are found. It turns out that it is not where we have been spending most of our time for the past forty years. It has long been understood that genes are made up of long stretches of DNA that are decoded by a universal process to produce proteins, which do the actual work in animal cells and bodies. The genetic code for proteins, a twenty-word vocabulary, has been known for forty years and it is easy for us to decode DNA sequences into protein sequences. What is much less appreciated is that only a tiny fraction of our DNA, just about 1.5 percent, codes for the roughly 25,000 proteins in our bodies. So what else is there in the vast amount of our DNA? Around 3 percent of it, made up of about one hundred million individual bits, is regulatory. This DNA determines when, where, and how much of a gene's product is made. Regulatory DNA is organized into fantastic little devices that integrate information about position in the embryo and the time of development. The output of these devices is ultimately transformed into pieces of anatomy that make up animal forms. This regulatory DNA contains the instructions for building anatomy, and evolutionary changes within this regulatory DNA lead to the diversity of form.

The Grandeur in a More Modern Synthesis: Act III

The continuing story of evolution may be thought of as a drama in at least three acts. In Act I, almost 150 years ago, Darwin closed the most important book in the history of biology by urging his readers to see the grandeur in his new vision of nature [see box]. In Act II, the architects of the Modern Synthesis unified at least three disciplines to forge a grand synthesis. Here in Act III, there is also a special grandeur in the view embryology and evolutionary developmental biology provide into the making of animal form and diversity. Part of it is visual, in that we can now see how the forms of different animals actually take shape.

[ILLUSTRATION OMITTED]

But beauty, in science, is much more than skin deep. The best science is an integrated product of our emotional and intellectual sides, a synthesis between what is often referred to as our "left" brain (reasoning) and "right" brain (emotional/artistic) hemispheres. The greatest "eurekas" in science combine both sensual aesthetics and conceptual insight. The physicist Victor Weisskopf noted, "What is beautiful in science is the same thing that is beautiful in Beethoven. There's a fog of events and suddenly you see a connection. It expresses a complex of human concerns that goes deeply to you, that connects things that were always in you that were never put together before."

Those connections are revealing some simple, elegant truths that deepen our understanding of all animal forms, including ourselves, and should transform how evolutionary science is taught and accepted.

Evo Devo and the Teaching of Evolution

The teaching of evolution faces two challenges. The first is that it is a vast and growing subject that encompasses many disciplines. The second is that it is actively opposed, particularly in the U.S., by some (but not all!) religious factions. I will address the new contribution Evo Devo can make to improving general public understanding first, the issue of opposition later.

In general, the public understanding of evolution in the United States is particularly abominable. In a survey of citizens in twenty-one countries regarding general environmental and scientific knowledge, the U.S. placed dead last on the question of human evolution. Looking at the bright side, the U.S.A. can only move up from here.

In another survey, by the National Science Board in 1996, 52 percent of Americans polled either agreed with (32 percent) or did not know (20 percent) whether the statement "The earliest humans lived at the same time as the dinosaurs" was true or not.

Score that fact as two points for The Flintstones, zero for Darwin, Huxley, and the educational system of the world's most wealthy, powerful, and technologically driven nation.

The scandal of this ignorance is on par, I would say, with not knowing how the U.S.A. was formed, the content of its Constitution, or the roots of Western civilization. This knowledge is considered basic literacy and taught and repeated at many grade levels. So, too, is biology and earth science for which evolution must provide the basic framework. Yet, the statistics are appalling.

The situation is bad enough, and reflected in other figures about science and math literacy, that the blame can probably be shared in many quarters. There are plenty of books written about and organizations studying the general problem of scientific illiteracy and its causes. I won't get into finger pointing here. The only way up is through education. I would rather focus on what biologists and their allies at all levels of the teaching profession can do to improve matters, particularly in regard to evolution.

First, we must insist that evolution is much more than just a topic in biology, it is the foundation of the entire discipline. Biology without evolution is like physics without gravity. Just as we cannot explain the structure of the universe, the orbits of the planets and moon, or tides from mere measurement, we cannot explain human biology or Earth's biodiversity via a compendium of thousands of little facts. All general survey courses and texts must have evolution as their central unifying theme.

With respect to the scientific content to be taught, Evo Devo has much to contribute that is new, tangible, and convincing. Since the Modern Synthesis, most expositions of the evolutionary process have focused on microevolutionary mechanisms. Millions of biology students have been taught the view (from population genetics) that "evolution is change in gene frequencies." Isn't that an inspiring theme? This view forces the explanation toward mathematics and abstract descriptions of genes, and away from butterflies and zebras, or Australopithecines and Neanderthals.

The evolution of form is the main drama of life's story, both as found in the fossil record and in the diversity of living species. So, let's teach that story. Instead of "change in gene frequencies," let's try "evolution of form is change in development." This is, of course, a throwback to the Darwin-Huxley era, when embryology played a central role in the development of all evolutionary thought. The advantages of an embryological approach to teaching evolution are several-fold.

First, it is a small leap to go from the building of complexity in one generation from egg to adult to appreciating how increments of change in the process, assimilated over greater time periods, produce increasingly diverse forms.

Second, we now have a very firm grasp of how development is controlled. We can explain how toolkit proteins shape form, that toolkit genes are shared by all animals, and that differences in form arise from changing the way they are used. The principle of descent by modification (of development) is crystal clear.

Third, an enormous practical advantage is the visual nature of the Evo Devo perspective. The Chinese proverb that "hearing about something a hundred times is not as good as seeing it once" is sound educational doctrine. We learn more by combining visuals with text. Let's show students embryos and how stripes, spots, and all the glorious features of animal forms are made. The evolutionary concepts follow naturally.

A fourth benefit of this approach is that it brings genetics much closer to the powerful evidence of paleontology. Dinosaurs and trilobites are the poster children of evolution that inspire the vast majority of those who touch them. By placing these wonders of the ancient past in a continuum from the Cambrian to the present life's, history is made much more tangible. It would indeed be a wonderful world if every student had guided, repeated classroom contact with some fossils.

Let me offer a couple more general suggestions. Natural selection is often at best described as a "just-so story" of adaptations. Finches beaks changed due to the type of food available, moths got darker because of pollution, etc. But I do not think that the power of small increments of selection, compounded over hundreds or thousands of generations, is widely taught or understood. The commonly repeated phrase "survival of the fittest" connotes more of a gladiator contest than the subtle power of selection to act on very small differences in overall survival and fecundity. The spread of favorable mutations in populations is easily simulated and illustrated, and it underscores the time dimension of evolution.

Finally, at the university level, the evolutionary view of life should be as fundamental to a college degree as Psychology 101 or Western Civilization. But rather than asking students to memorize and regurgitate mountains of testable facts, we should emphasize study of the history of the discovery of evolution, its major characters and ideas, and the basic lines of evidence. This would do far more to inform citizens and prepare teachers than forcing students to remember the Latin names of species. We are stoning our children to utter boredom with little pebbles and missing the big picture. The drama of the story of evolution will recapture student interest.

There is, especially in the U.S.A., another obstacle besides content and teaching methods to evolutionary literacy. I will address that next. But, even without the active opposition, we can do better, and we have to do better.

Evo Devo and the Evolution/Creation Struggle

In the short time between the first and second edition of The Origin of Species, Darwin inserted three more words into that famous closing paragraph, adding "by the Creator" to rewrite the phrase as "having been originally breathed by the Creator into a few forms or into one...." Darwin later expressed his regret for doing so in a letter to botanist J.D. Hooker:

But I have long regretted that I truckled to public opinion, and
used the Pentateuchal term of creation, by which I really meant
"appeared" by some wholly unknown process.

The insertion of these words was intended to appease critics and make Darwin's evolutionary ideas more palatable. It has certainly served to fuel much speculation about Darwin's actual religious views. For some, this olive branch and Darwin's reticence in disclosing his beliefs (which are only revealed to some degree in private correspondence and unpublished notebooks) were the foundation for reconciling and accommodating evolution and religion.

Plenty of scientists and a broad spectrum of religious denominations have found such an accommodation. For example, in 1996, Pope John Paul II reiterated the Catholic position that the human body has evolved according to natural processes. But while some denominations have explicitly accepted the reality of biological evolution, fundamentalists who insist upon a literal reading of the Bible (referred to as "Creationists") and proponents of "stealth" Creationist ideas such as Intelligent Design remain firmly opposed to evolutionary science and actively promote legislation aimed at crippling the teaching of evolution in public schools.

Goethe said that "Nothing is worse than active ignorance," and it is the agenda of these lost souls that the scientific and educational communities must thwart. I want to be very clear here in my position. I believe that the teaching of evolution and science are best served by promoting the scientific method and scientific knowledge and not by attacking religious views. The latter is a futile, counterproductive battle. However, I also believe, as many denominations have also concluded, that religion is better served by promoting and evolving their respective teachings and theologies, and not by attacking science, which is definitely a losing strategy.

Charles Harper, executive director of the John Templeton Foundation, an organization interested in the relationship between theology and science, wrote recently in the leading science journal Nature that:

As scientific knowledge grows, religious commitments predicated
on "gaps" in scientific understanding will invariably shrink as
those gaps are closed. Those Christians who are currently fighting
evolutionary science will eventually need to take it seriously.

Harper is right. In this time of unprecedented power in understanding embryos, genes, and genomes, and with the continual expansion of the fossil record, those gaps are fast disappearing.

One example of a mistaken faith in those gaps is that of biochemist Michael Behe who in 1996 published Darwin's Black Box: The Biochemical Challenge to Evolution. As a credentialed scientist, Behe's book was received as a godsend by creationists and is perhaps the best known treatment of Intelligent Design. But Behe's main claim, that the living cell is an entity of irreducible complexity, is empty. Behe was counting on biology to hit a wall in reducing complex phenomena to molecular processes. He joins a long line of prognosticators whose pessimistic forecasts have been obliterated in the continuing revolution in the life sciences.

Scott Gilbert, a biologist at Swarthmore College, author of the leading college text of developmental biology, and accomplished historian of embryology and evolutionary biology, has summarized the Behe position, and its failure as follows:

To creationists, the synthesis of evolution and genetics cannot
explain how some fish became amphibians, how some reptiles
became mammals, or how some apes became human.... Behe
named this inability to explain the creation of new taxa through
genetics "Darwin's black box." When the box is opened, he
expects evidence of the Deity to be found. However, inside
Darwin's black box resides merely another type of
genetics-developmental
genetics.

Developmental genetics has been shedding new light on the making of complexity and the evolution of diversity for twenty years. Creationists just plain refuse to see it. How is such overt evidence ignored or dismissed? I can't pretend to understand the psychological mechanisms that allow humans to deny reality. But I do understand the desperate political and rhetorical tactics of those who, holding a losing hand, refuse to accept it.

As exasperating as the continuous battle with creationists may seem, the scientific community is now better organized and more prepared to deal with the movement. But the battle against ignorance is not won.

Evo Devo: A Revolutionary Quartet

The impact and importance of Evo Devo emerges from four points:

First, Evo Devo constitutes a third major act in a continuing evolutionary synthesis. Evo Devo has not just provided a critical missing piece of the "Modern Synthesis"--embryology--and integrated it with molecular genetics and traditional elements such as paleontology. The wholly unexpected nature of some of its key discoveries and the unprecedented quality and depth of evidence it has provided towards settling previously unresolved questions bestow it with a revolutionary character.

Second, Evo Devo provides a new means of teaching evolutionary principles in a more effective framework. By focusing on the drama of the evolution of form, and illustrating how changes in development and genes are the basis of evolution, the deep principles underlying the unity and diversity of life emerge. Furthermore, the visible forms of gene expression patterns in embryos and the concrete inventories of toolkit gene sets in different species provide more effective ways of illustrating evolutionary concepts than previous, more abstract approaches.

Third, because Evo Devo reveals and illustrates the evolutionary process and principles in such tangible ways, it has a critical role to play in the forefront of the societal struggle over the teaching of evolutionary biology.

And finally, the importance of evolutionary biology is far more than mere philosophy. The fate of the endless forms of nature, including humans, depends on a broader understanding of humans' impacts on evolution.

A Grandeur ... and Endless Forms Most Beautiful

Darwin closed the first edition of The Origin of Species with what has become perhaps the most widely quoted passage in all of biology:

There is a grandeur in this view of life, with its several
powers, having been originally breathed into a
few forms or into one: and that whilst this planet
has gone cycling on according to the fixed law of
gravity, from so simple a beginning endless forms
most beautiful and most wonderful have been, and
are being, evolved.

I have chosen four words that remained completely untouched throughout all versions and editions, "endless forms most beautiful," as the inspiration for and theme of this article. This phrase resonates most perfectly with the theme of my book and captures the essence of the new science of Evo Devo.

Sean B. Carroll is an investigator at the Howard Hughes Medical Institute and a professor of genetics at the University of Wisconsin--Madison. He is one of the leading biologists of his generation, and his seminal discoveries have been featured in many popular publications. This article is excerpted by permission flora his new book Endless Forms Most Beautiful: The New Science of Evo Devo (W. W. Norton, New York, 2005). E-mail.. sbcarroll@ wisc.edu.

 

[Big E]

I read it all and it's still a theory, go to bed tweeker!!! LOL j/k

 

[ThaG]

did you understand any of it??

it is written with kindergarten kids as the target audience, you should have no problems

 

[ThaG]

read the first post, it is short and it is enough