Raup-Chapt 1 Almost All Species Are Extinct

Raup-Chapt 1 Almost All Species Are Extinct - E I CHAPTER I...

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Unformatted text preview: E I CHAPTER I ALMOST ALL SPECIES ARE EXTINCT Almost all professional football players are still alive. The same is probably true for nuclear physicists, city planners, and tax consultants. This survival record is due in part to the newness of professional football, nuclear physics, and so on and in part to population growth—there are lots more peo— ple now than ever before. Nathan Keyfitz, the great demog- rapher, estimated in 1966 that about 4 percent of all the people who had ever lived were alive then. Again, newness and population growth. Not so for species! There are millions of diflbrent species of animals and plants on earth—possibly as many as forty million. But somewhere between five and fifty [zillion spe- cies have existed at one time or another. Thus, only about one in a thousand species is still alive—a truly lousy survival EXTINCTION: BAD GENES OR BAD LUCK? record: 99.9 percent failure! This book examines two pri- mary questions: Why did so many species die out? How did they die out? Is EXTINCTION IMPORTANT? Yes, I think it is very important. All of us grow up with an acquired set of ideas and thoughts about the natural world around us, its history and its future. We get these ideas from a thousand sources—from comic books and classrooms and television sitcoms—and these ideas represent the collective attitudes of our culture. One idea I think most of us share is that earth is a pretty safe and benevolent place to live—not counting what humans can do to earth and to each other. Earthquakes, hurricanes, and disease epidemics may strike, but on the Whole our planet is stable. It is neither too warm nor too cold, the seasons are predictable, and the sun rises and sets on schedule. Much of our good feeling about planet earth stems from a certainty that life has existed Without interruption for three and a half billion years. We have been taught, as well, that most changes in the natural world are slow and gradual. Species evolve in tiny steps over eons; erosion and weather- ing change our landscape but at an almost immeasurably slow pace. Continents move, as in the present drift of North America away from Europe, but this movement is measured in centimeters per year and will have no practical efiect on our lives or those of our children. Is all this true or merely a fairy tale to comfort us? Is there more to it? I think there is. Almost all species in the past failed. If they died out gradually and quietly and if they 4 ,; §1 E ALMOST ALL SPECIES ARE EXTINCT deserved to die because of some inferiority, then our good feelings about earth can remain intact. But if they died violently and Without having done anything wrong, then our planet may not be such a safe place. BAD GENES OR BAD LUCK? I have taken the title of this book from a research article I published in Spain some years ago. I was concerned then with the failure of trilobites in the Paleozoic era. Starting about 570 million years ago, these complex, crab-like orga. nisms dominated life on ocean bottoms—~at least they domi— nate the fossil assemblages of that age. But through the 325 million years of the Paleozoic era, trilobites dwindled in numbers and variety, finally disappearing completely in the mass extinction that ended the era, about 245 million years ago. As far as we know, trilobites left no descendants. My question in Spain is the one I still ask: Why? Did the trilobites do something wrong? Were they fundamentally inferior organisms? Were they stupid? Or did they just have the bad luck to be in the wrong place at the wrong time? The first alternative, bad genes, could be manifested by things like susceptibility to disease, lack of good sensory perception, or poor reproductive capacity. The second, bad luck, could be a freak catastrophe that eliminated all life in areas where trilobites happened to be living. The question is basically one of nature versus nurture. Is proneness to ex— tinction an inherent property of a species—«a weakness—or does it depend on vagaries of chance in a risk-ridden world? Of course, the problem is more complex than I have presented it, just as the nature-nurture question in human EXTINCTION: BAD GENES OR BAD LUCK? behavior is complex. But in both situations, nature (genet— ics) and nurture (environment) operate to some degree, and the challenge is to find out which process dominates and whether the imbalance varies in time and space. THE NATURE OF EXTINCTION We could avoid the extinction problem——and therefore this book—by arguing it away. We could note that the average plant or animal species has a geologic life span of only four million years and that life goes back thousands of millions of years. On this basis, we could assert that it is nature’s way for species to have short life spans. Will Cuppy, in his delightful volume of essays entitled How to Become Extinct, wrote, “The Age of Reptiles ended because it had gone on long enough and it was all a mistake in the first place.” If we accept that turnover in species is merely nature’s way, just as nature has given humans a limited life span, then there is nothing in species extinction worthy of wonder. But there is absolutely no basis for equating the life spans of species with those of individual humans. There is no evi— dence of aging in species or any known reason why a species could not live forever. In fact, virtual immortality has been claimed for the so—called living fossils (cockroaches and sharks, for example). kg: We could also get rid of the problem by arguing that §Zspecies don’t go extinct, that they merely evolve into other species (presumably better ones) by natural selection. The essence of Darwin’s Origin (yr Species was that species change gradually into new species. When a new species is formed in 6 ALMOST ALL SPECIES ARE EXTINCT this way, the ancestral species does not die: it is merely transformed into another species. The ancestral species is then said to have undergone “pseudoextinction”—as op— posed to “true extinction.” Although pseudoextinction cer- tainly occurs in nature, we also know that true extinction has eliminated countless species. Many major groups of plants and animals that were once important parts of the global biota no longer exist and left no descendants. Much of the debate in evolutionary biology over the “punctuated equilibrium” theory (championed by Stephen Jay Gould) has centered on the question of proportions of true and pseudo extinctions in the history of life. Another kind of false extinction has been claimed. It has been argued that dinosaursdid not die out, but just evolved wings and flew away. At a certain level, this reasoning is sound. Birds evolved in the Jurassic period, about 150 mil— lion years ago, from dinosaurs of the time (Figure 1—1). The first fossil birds are hardly distinguishable from some smaller Jurassic dinosaurs. So birds, as a group, did descend from dinosaurs and have many anatomical similarities to show for it. All 8,600 species of birds living today carry some inheri— tance from their reptilian ancestors. But the bird lineage split off millions of years before the dinosaurs died out in the mass extinction that ended the Cretaceous period. Cretaceous dinosaurs died without issue! Their extinction was final. We cannot escape the fact that true extinction has claimed a large fraction of the evolution— ary progeny of life on earth——even though the size of that fraction is not known accurately. EXTINCTION: BAD GENES OR BAD LUCK? Evolution of Birds from Jurassic Dinosaurs present day ——l TE R TIAR Y PERIOD Dinosaurs 5-- CRETACEOUS' PERIOD JURAS‘S‘IC PERIOD FIGURE 1-1. Evolutionary tree (greatly simplified) showing the origin of birds from the dinosaur lineage in the Jurassic period. This pattern has led some to argue that dinosaurs did not die out at the end of the Cretaceous, that they merely evolved wings in the Jurassic and flew away. WHO STUDIES EXTINCTION? Strangely, extinction does not have a large body of schol— ars or scholarship. No scientific discipline carries the name. Nevertheless, we know a lot about the subject. Early in the nineteenth century, geologists discovered that short dura— tions of fossil species provide the best means of arranging 8 ALMOST ALL SPECIES ARE EXTINCT geologic events in a time sequence. Reversing the reasoning, geologists can locate a rock in time with great precision by identifying the fossils. Even today, much of the world’s exploration for oil and natural gas is built around a chronol— ogy based on a changing fossil record—in other words, origination and extinction of species. But the geologists, and their colleagues in paleontology who have done most of the hard work, have never devel— oped a strong interest in extinction itself. It may be that these scientists operate so close to the fossil record that they have lost any natural feelings of awe. Since virtually all species found in rocks are extinct, the question becomes not why but merely when. Surprisingly few geologists and pa— leontologists have taken an active part in today’s concern about endangered species and predictions of future extinc— tions. When I was in graduate school training to be a paleon- tologist, I did learn a few things about extinction. I learned that species compete constantly with each other for space and resources and struggle always with their physical envi— ronment. I learned that a steady, background level of extinction is an inevitable part of the history of life, punctuated occasionally by big events called mass extinc— tions. Conventional wisdom went little further. Although some attention in classes and textbooks was devoted to mass extinctions, these events were seen as too complex ever to be understood. Our mission in school was to learn to ident— ify the most important fossils and their ranges in geologic time. But if geologists and paleontologists don’t pay serious attention to extinction, surely the biologists care. Organic evolution is central to almost all aspects of biology. Whole 9 EXTINCTION: BAD GENES OR BAD LUCK? subdisciplines—molecular and population genetics, taxon- omy, and many areas of ecology and biogeography—seek to document evolutionary history or to investigate the pro- cess by which organisms evolve. Who begat whom, and when, why, and how? But to the typical biologist, extinc- tion plays a strangely minor role in evolution. An important topic in biology of the past several decades has been a phenomenon known as speciation. By common agreement, the term refers to the splitting or branching of an evolutionary line, producing two distinct species where there was one. Ironically, this is not the origin of species that Charles Darwin focused on. The main mode of Darwinian change is the gradual transformation of one species into another—with no increase in the number of coexisting spe- cies. In fact, the Darwinian kind of species origin is not even called speciation by most biologists, but rather, and some— what awkwardly, phyletz'c trangrormatz'on. In Figure 1-2, I have illustrated the difference between speciation and phyletic transformation. The two lines with symbols attached depict species lineages of some imaginary beast evolving through time. The anatomy of one species is caricatured by circles. With time, the circles become smaller (on average), suggesting evolution toward smaller body size. This change occurs by phyletic transformation. Partway through the sequence in Figure 1-2, a branching occurs: some of the circular organisms split off to start a lineage of square organisms. The squares then evolve by phyletic transformation; in this case, the anatomical change happens to be toward size increase. Note that I have made the ancestral species (round) go extinct but have let the descendant species (square) live on. If 99.9 percent of all species that have lived on earth are IO ALMOST ALL SPECIES ARE EXTINCT ‘—Z— SPE CIA TION (branching of lineages) PH YLE TIC TRAN S F ORMA TI 0N FIGURE 1-2. Hypothetical evolutionary tree showing the dis- tinction between phyletic transformation and speciation. Imagi- nary organisms (circles and squares) change gradually through time, becoming larger or smaller (PHYLETIC TRANSFOR- MATION). At the branch point (SPECIATION), the round organisms give rise to a lineage of square organisms. extinct, it follows that the total of species originations has been virtually the same as the total of species extinctions. Although present biodiversity—~the millions of living spe- cies—seems high to us, today’s biota results from a minor surplus of speciations over extinctions, accumulated over a long time. II ‘1? . Ziefi EXTINCTION: BAD GENES OR BAD LUCK? In View of these figures, it is puzzling that even evolu- tionary biologists have devoted almost no attention to extinction. Large monographs and textbooks have been written about speciation, and careers have been built around the subject. But extinction has barely been touched. It’s a little like a demographer trying to study population growth without considering death rates. Or an accountant interested in credits but not debits. Textbooks of evolutionary biology contain little about extinction beyond a few platitudes and tautologies like “Species go extinct when they are unable to cope with change” or “Extinction is likely when population size approaches zero.” The Encyclopaedia Britannica (1987) says, “Extinction occurs when a species can no longer repro- duce at replacement levels.” These statements are almost free of content. But interests in science change, and this is happening with extinction. Thanks to a rash proposal by a Nobel physicist, Luis Alvarez, and his colleagues at Berkeley, furious debate has broken out over whether a meteorite impact caused the dinosaur extinction. This has combined with concern over currently endangered species to encourage more people to probe the extinction phenomenon and its role in the history of life. The emerging field of extinction studies may one day even get a name ending in “ology.” My mission in this book is to share with you the attempts being made by many of us to understand more about extinction. I should emphasize that extinction is still a very small, cottage industry. It has none of the trappings of big sci— ence—nothing comparable to the Supercollider or the Human Genome Project or the Hubble Space Telescope. Yet the questions being asked of extinction are every bit as fundamental and interesting in our ongoing attempt to un- 12 ALMOST ALL SPECIES ARE EXTINCT derstand our place in the universe and to answer the ulti- mate question: Why are we here? A WORD ABOUT THE WORD Curiously, the word extinct is an adjective. We say that species (or volcanoes) become or go extinct. The word has a passive quality, implying a petering out. Although extinct once was used as an active verb, this usage passed out of the English language in the seventeenth century. Plants and ani- mals do all kinds of active things: they fight, eat, migrate, reproduce, and even speciate. But when species die, they become extinct. Perhaps extinction, as the death of species, is a little scary and we unconsciously avoid the active voice. Or perhaps the usage is meant to imply that the species becoming extinct is reacting to outside influences, beyond its control. This would be reasonable, I suppose, because there is no reason to suspect that any species is actively suicidal, even though some of its members may be. Digby McLaren, a noted Canadian paleontologist and student of extinction, argues for the use of the term mass killing in place of mass extinction. But he has done this to distinguish death of individual animals from death of spe- cies. McLaren is convinced that the most dramatic aspect of mass extinctions is sudden killing of a multitude of in- dividuals. Extinction of a species, to him, is merely a by— product in cases where the killing happens to be complete. Thus, McLaren is not suggesting a change in language— only a change of emphasis from species to individuals. In a couple of recent research articles, I have gone yet further by actually using kill and killing in place of extinct I3 EXTINCTION! BAD GENES OR BAD LUCK? and extinction. I am waiting, somewhat impishly, to see whether this usage is picked up by my colleagues. At the very least, I expect it will lead to some delightful conversa- tions with Digby McLaren. SPECIES DEFINED Before going much further, I should clarify what I mean by species. The species is the traditional unit of accounting in most extinction studies, McLaren’s views notwithstand- ing. A species is a species if a competent taxonomist says it is. Although a bit cynical, this is the operational definition most widely used in biology and paleontology. It works because the biological world is, in fact, divided into natural units. Professional taxonomists devote much time and en— ergy to classifying the organic world into its basic units—— kinds of organisms distinguishable from other kinds. Crite—i ria include anatomy, biochemistry, color, breeding systems, and sometimes behavior. The taxonomist’s experience is used to choose characteristics that make consistent classifi— cation possible. A more rigorous definition is possible: a species is a group (yr individual organisms that share a common pool (yr genetic material genome). All humans belong to a single species because they are all interfertile. Aside from gender itself, the only barriers to reproduction among members of our species are geographic and cultural. The biological world is an array of separate and independent genomes, each changing through time but not mixing with others. Because species I4 S l E i ALMOST ALL SPECIES ARE EXTINCT are reproductively isolated, differences in anatomy and be— havior evolve. The job of the taxonomist is to recognize and distinguish natural species. Unfortunately, breeding experiments to test for reproductive isolation are usually impractical;»such tests may even be impossible if the organisms live in different areas and do not behave naturally in captivity. Thus, the taxonomist usually relies on proxy information—physical appearance, behavior, breeding cycles, and so on. The taxonomist’s task is made difficult by the fact that there are differences within as well as between species. Populations of a species living in one area may be differ— ent—often strikingly so——from populations of the same species in another area. The differences may stem from minor adaptations to local conditions or simply from chance differences that develop between populations that normally do not interbreed. Geographic variants of a species are called subspecies, varieties, or races, indicating that they could inter— breed if they lived in the same area (and felt like it). Subspe— cies are incipient species; that is, the original species is in the process of speciation. Subspecies become fully independent species if the geographic separation is maintained long enough. There is occasional successful hybridization between spe‘ cies, especially in‘plants (oaks, for example), which tends to blur species boundaries. Hybrids are often intermediate in form. If hybridization were rampant in our world, the whole classification of organisms into species would break down. Fortunately for taxonomists, and probably for evo— lution as well, this has not happened. In the latter regard, it is the existence of independently evolving genomes that IS EXTINCTION: BAD GENES OR BAD LUCK? enables adaptations as different as flying and swimming to evolve and persist. Without the barriers, our world would be very different and we probably would not be here. Global biology would quite likely be dominated by gener— alized organisms that could do a little of everything—but nothing very well. Because it is rarely possible or practical actually to test the breeding capability of organisms, the taxonomist must make many educated guesses aboutspecies boundaries. One can verify that this approach ...
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