خرید و دانلود نسخه کامل کتاب Replacing Darwin: The New Origin of Species – + PDF

In technical terms, this view led to very specific conclusions in two arenas. Since species fit their locales so well, it would seem that they were made for their individual habitats — that is, that they were created separate and distinct from other species. is implied that species do not become other species — a view known as species’ fixity. Conversely, since species appear to have been made for their individual habitats, it would seem that they have always been in their current locations. us, in the arena of geography, the design argument implied the fixity of species’ locations. In jigsaw puzzle terms, it was as if each species formed its own isolated puzzle. Environmental clues might decorate the outer edges of each species’ puzzle. But, rather than connect different species’ puzzles to one another, these clues seemed to separate the puzzles. With few pieces in hand, this view was easy to maintain. e absence of obvious connecting pieces between puzzles produced a convincing set of isolated images. Nevertheless, due to the fact that many pieces were still waiting to be discovered, the potential for massive overhaul lingered. By 1859, Darwin and his contemporaries had discovered many new pieces to these puzzles. Drawing on the growing knowledge in the fields of biogeography (i.e., the geographic distribution of species around the globe), anatomy, physiology, embryology, geology, and paleontology, Darwin began to see connections where prior investigators saw only empty space.6 Eventually, Darwin proposed that all species evolved from one or a few common ancestors — a massive paradigm shift. Because of the unusual nature of the puzzle of the origin of species, paradigm shifts are inevitable. Like the 18th century, the scope of species diversity in Darwin’s day was a fraction of today’s variety. In 1859, the scientific community had no knowledge of the majority of species we have now documented. With over 1.6 million7 plant, animal, fungal, and bacterial species currently known, hundreds of thousands of pieces were missing from Darwin’s puzzle.8 With Darwin barely 100 years removed from Linnaeus’ foundational work, this fact shouldn’t be surprising. Darwin didn’t compensate for this ignorance of species diversity with any special abilities. His lifespan wasn’t any longer than the average lifespan today. He observed the world for 73 years. And then he died. Furthermore, in those 73 years, he was subject to the technology of the 1800s. He couldn’t travel the globe as easily as we do. Without the information exchange facilitated by the Internet, he couldn’t benefit as easily from the travels and discoveries of others. Yet Darwin tried to tackle one of the biggest questions in biology. Since 1859, we’ve had time to reevaluate his picture — much more time than he had to propose and appraise it. We’ve also had more space. Today, travel is virtually unrestricted. Few corners of the world have remained recalcitrant to scientific exploration. Furthermore, the Internet makes information sharing faster than ever before. A global community of millions9 of scientists can pool their resources and build on one another’s work. ough lifespans have changed little, the cumulative observations of these scientists have built an unprecedented body of knowledge on the diversity and operation of life. Consequently, the puzzle image has changed. ree developments have led the way. First, after Darwin wrote On the Origin of Species, an entire field of science was born — and then matured. Unlike any other field of science, this field directly constrains and guides the answer to the origin of species. Consequently, it’s the most relevant field to our question. You could say that the edge pieces of the puzzle have finally been found. Second, premature conclusions were corrected. Anyone who has gotten stuck trying to put together a puzzle without a box cover and without edge pieces would do what Darwin did — they would test piece after piece until they found a plausible connection. However, without the constraints of a box cover image and of edge pieces, it’s easy to link pieces where no link exists. Connections that initially appear plausible eventually give way to the correct links, once additional pieces are found. Darwin made many such premature links. Since 1859, we’ve been able to unlock some of the connections that he erroneously made, while cementing correct ones. ird, in the last few years, the critical corner pieces were found. Several remarkable scientific discoveries were made — ones entirely unanticipated by the trajectory of discoveries prior. With these pieces in hand, the framework of the puzzle and the existing connections among pieces have been reoriented. Individually, each of these developments carried minor significance. By analogy, if you were trying to put a puzzle together, the discovery of a few edge pieces would be helpful. But it wouldn’t be earth-shattering. Conversely, if you found a corner piece, this discovery would be fantastic. But if the remaining pieces have been forced together in clumsy and incorrect ways, the corner would do little good. Finally, if all you did was unlock a few misconnected pieces, you would rejoice in the removal of barriers to progress. But the happiness of this success would soon be outweighed by the intimidating scope of the remaining task. In isolation, these discoveries would do little to reveal the final image. Together, they produced a compelling picture of how species came to be. To be sure, large chunks of the puzzle still need to be filled in. Having the corner pieces, edge pieces, and a couple of correctly connected center pieces is a huge step forward. But significant holes in the puzzle remain.10 Explaining in detail the origin of every species that ever lived is a monstrous undertaking. Much work remains to be done. Nevertheless, the puzzle picture that we possess today is far different from the one that Darwin created. And it is far superior. It puts the far reaches of the globe — and the species that they contain — into an image that is as captivating as it is surprising. is book tells the story of how this picture came to be. * Unless otherwise noted in this book, when I use the term species, I am using it in the biological sense — in other words, as a formal unit of classification. Part One A Field Is Born S Chapter 2 The Secret of Life weating over a large, unassembled jigsaw puzzle, I take deliberate steps to simplify the vexing challenge. First, I search for the edge pieces. Once I’ve found them all, it’s a fairly simple task of trial and error to connect them. Far fewer possible connections exist among these pieces than among the center pieces. Furthermore, once connected, they provide an enormously helpful framework in which to connect the rest of the puzzle. If my puzzle came without a box cover, the edges would be doubly useful. Edges limit the amount of possible horizontal and vertical connections among the center pieces. is saves me the enormous frustration that follows endless trial and error of unassembled center pieces. Edges also naturally suggest how the final image will look. Even though each piece contains a tiny part of the whole image, I can sense the final subject matter just by looking at the edge pieces. In the assembly of a jigsaw puzzle, the identification of edge pieces is a major step forward. Since species are not literal jigsaw puzzles, the biological analogy for edge pieces might not be obvious. e parallel becomes clearer upon brief reflection. Consider species with which we are familiar. We recognize zebras by their stripes, elephants by their trunks, giraffes by their long necks, bald eagles by the color of the feathers on their heads, and monarch butterflies by the patterns on their gossamer wings. Species are defined by their traits. is is true across all life. Mammals, reptiles, birds, amphibians, fish, starfish, sea urchins, crustaceans, arachnids, insects, worms of all sorts, shellfish, octopi, snails, corals, jellyfish, sponges, mosses, ferns, conifers, grasses, orchids, fruit trees, fungi, algae, bacteria, and all the other life forms on earth possess unique combinations of traits. ough some features require a microscope to visualize, traits define species. erefore, the question of the origin of species is a question of the origin of traits. If you want to know the origin of zebras, you need to discover the origin of stripes. e origin of elephants is bound up in the origin of trunks. Giraffe origins are inextricable from the origin of long necks. e origin of eagles goes hand in hand with the origin of white feathers. Butterfly history is read off the history of butterfly wing patterns. e origin of the rest of the species is found in the origin of the traits that define them. e solution to the origin of traits represents the hard constraints on the origin of species — the edge pieces of the puzzle. In 1859, zero edge pieces were known. With careful reflection, the reason for this is easy to grasp. For example, if you wanted to discover the origin of these traits, you could begin by watching how they behave each generation. However, unlike humans, species don’t keep written records of their family trees. us, as a first step, you might start by investigating human family trees. e simplest place to start is your own family tree. Perhaps you recognize your father’s chin in your jaw. You might investigate how far back on your family tree you can trace this chin shape. However, if you’re like me, your family tree is probably small. Going back further than a few generations, I don’t know who my relatives are. If your tree is like mine, your ability to examine trait behavior is severely limited by your ignorance of your ancestors. Your attempts to track traits might encounter a second hurdle. If your family tree is small, you might compensate by tracing additional family trees. In doing so, you’d probably have to follow more traits than chin shape. For example, you might document the behavior of red hair and freckles. If you did, you might observe that, on occasion, the trait disappears from a family tree. A red-headed parent might have no red-headed offspring. Or a great-grandparent might have red hair, but several generations of descendants might not. As the scope of your investigation expands, you might find several traits that behave in odd and inexplicable ways. ese idiosyncrasies apply to both living and extinct species. In fact, when fossils are part of the equation, the problems multiply. Unlike recorded family trees, fossils have no explicit genealogical connection to anything alive today. Ancestral relationships have to first be inferred from indirect data before trait behavior can be tracked. Even more troubling, the placement of fossils on family trees requires implicit assumptions about how traits behave. Assuming a mode of inheritance to prove a mode of inheritance is circular reasoning. In other words, fossils cannot inform how traits behave. If you had access to a microscope, you’d discover the most inexplicable behavior of all. All traits are erased each generation — and then rebuilt. When sperm meets egg, the visible features that define multicellular species are not present. Instead, these characteristics arise via the process of development. In summary, if you rigorously tracked the behavior of visible traits, you’d discover an intimidating number of paradoxes. ese paradoxes would raise a host of perplexing questions. Do traits form spontaneously? Can they be destroyed? Can they be changed? If so, how much can they be changed? Are traits blended? Particulate? Inherited as a whole? Separated into units? Independent? Interdependent?1 Consider the ramifications of this uncertainty. If traits can appear and disappear, how could you trace species’ ancestry? What markers would you use to fill in the family tree? Furthermore, if all traits are rebuilt every generation, can any species become any other species? Do any constraints on change exist? Might a fish spontaneously spawn a spider? Without an answer to the mystery of heredity, the origin of species would be an enigma. When Darwin wrote On the Origin of Species, paradoxes — not edge pieces of the puzzle — were all that the scientific community possessed. e first steps toward resolving these paradoxes were taken in 1865 — six years after Darwin’s seminal publication. An Austrian monk, Gregor Mendel (Figure 2.1), solved the paradoxes of family trees. Like nearly every other species, his subject of study — pea plants — did not keep written records of inheritance. So Mendel did it for them.