The Dominant Animal: Human Evolution and the Environmentby Paul R. Ehrlich, Anne H. Ehrlich
In humanity’s more than 100,000 year history, we have evolved from vulnerable creatures clawing sustenance from Earth to a sophisticated global society manipulating every inch of it. In short, we have become the dominant animal. Why, then, are we creating a world that threatens our own species? What can we do to change the current trajectory toward more
In humanity’s more than 100,000 year history, we have evolved from vulnerable creatures clawing sustenance from Earth to a sophisticated global society manipulating every inch of it. In short, we have become the dominant animal. Why, then, are we creating a world that threatens our own species? What can we do to change the current trajectory toward more climate change, increased famine, and epidemic disease?
Renowned Stanford scientists Paul R. Ehrlich and Anne H. Ehrlich believe that intelligently addressing those questions depends on a clear understanding of how we evolved and how and why we’re changing the planet in ways that darken our descendants’ future. The Dominant Animal arms readers with that knowledge, tracing the interplay between environmental change and genetic and cultural evolution since the dawn of humanity. In lucid and engaging prose, they describe how Homo sapiens adapted to their surroundings, eventually developing the vibrant cultures, vast scientific knowledge, and technological wizardry we know today.
But the Ehrlichs also explore the flip side of this triumphant story of innovation and conquest. As we clear forests to raise crops and build cities, lace the continents with highways, and create chemicals never before seen in nature, we may be undermining our own supremacy. The threats of environmental damage are clear from the daily headlines, but the outcome is far from destined. Humanity can again adaptif we learn from our evolutionary past.
Those lessons are crystallized in The Dominant Animal. Tackling the fundamental challenge of the human predicament, Paul and Anne Ehrlich offer a vivid and unique exploration of our origins, our evolution, and our future.
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The Dominant Animal
Human Evolution and The Environment
By Paul R. Ehrlich, Anne H. Ehrlich
ISLAND PRESSCopyright © 2008 Paul R. Ehrlich and Anne H. Ehrlich
All rights reserved.
Darwin's Legacy and Mendel's Mechanism
"Nothing in biology makes sense except in the light of evolution."
THEODOSIUS DOBZHANSKY, 1973
HURRICANE KATRINA was a new kind of experience for most Americans—a huge natural disaster that demonstrated how poorly prepared the United States was, in 2005, for natural disasters. It also underlined that something strange is going on with the weather, even if Katrina's destructiveness itself might just have been a rare event in normal variability in the size and paths of hurricanes. If you regularly watch TV, read newspapers, or go to the movies, you can hardly have missed news about unusual weather. Global temperatures are going up, glaciers are melting, storms seem more frequently intense and so do droughts, and sea level is gradually rising.
Other animals are noticing too, at some level. Polar bears are finding it more difficult to make a living as the sea ice from which they hunt seals disappears. Coral animals in some places are dying as seawater warms, threatening the existence of coral reefs. If you are a bird-watcher, you might have noticed that some migratory birds from Latin America are arriving earlier each spring on their North American breeding grounds. In the Yukon of Canada, red squirrels are having their young earlier because of the great quantities of spruce seeds made available by a warming climate. In Europe, flowers are blooming about a week earlier in spring than they did in 1975—and in 2006 they were still blooming in Moscow in November.
That populations of organisms do not remain static in the face of environmental change is a recurring theme in the study of the natural world. Today, numerous animals and plants are changing their lives in response to the human-caused alteration of climatic regimes—the annual sequence of changes in temperature and rainfall—in their environments. While the average temperature increase of Earth has been less than a degree Celsius (?°C is 1.8°F) over the past fifty years, in the same period at the high-altitude laboratory in Colorado where we've worked since 1960, magpies have arrived from the lowlands for the first time and many flowers are blooming earlier, all apparently in response to earlier spring melting of the alpine snow.
Pitcher-plant mosquitoes (Wyeomyia smithii) are a good example of such climate-related changes. They inhabit eastern North America, and their early stages (eggs, larvae, and pupae) all live in water trapped in the "pitchers" of a carnivorous plant, Sarracenia purpurea. The plants grow primarily in nitrogen-poor soil and supplement their "diet" by digesting insects, mostly ants and flies, that die when trapped by downward-pointing hairs in their tubular leaf, which the small, hovering mosquitoes easily avoid. Southern populations of the mosquitoes produce five or more generations per year; northern populations, just one.
The larvae (wigglers), which hibernate in the leaves of the host plant, use the length of the day to determine when to enter that dormant state and when to resume activity. The critical day lengths at which the mosquitoes carry out these functions are quite rigidly controlled by their genes. But over the past thirty years, the genetically controlled clocks of more northerly populations have shifted their response so that hibernation does not begin until the day length becomes even shorter. As a result, the northern populations now behave much as the southern populations do, waiting until later in the fall before hibernating. Without that shift, as a warming climate in North America lengthens the growing season, the larvae would hibernate too soon. They would need to survive on the fat they had stored—not just survive through the winter as previously, but also without feeding through a warm period at the end of summer. This would lessen their chances of being alive when warm weather returned in the spring. With the shift, their necessary hibernation is shorter, helping them to survive without eating.
Other animals have not been able to change their behavior in response to a warming climate. Some populations of pied flycatchers (Ficedula hypoleuca), an attractive brown, black, and white insect-eating bird in Europe, have declined in size by 90 percent. The reason? The peak of insect abundance is occurring ever earlier as the environment warms, and the birds' customary breeding time is now too late for their offspring to have an optimal food supply. They have not successfully adjusted to the challenge of climate change, but neither have their populations remained static; they, along with other slow adjusters, are blinking out.
DARWIN AND WALLACE'S GREAT IDEA
Why do some organisms adjust successfully to environmental change while others do not? For a biologist, there is no more basic or fascinating question, and it now has a pretty good answer—thanks in large part to scientific foundations laid by two Victorian Englishmen. In 1859, Charles Darwin (1809–82) and the brilliant polymath Alfred Russel Wallace (1823–1913) simultaneously proposed the first basically correct model for the mechanism that causes shifts such as that in mosquito hibernation. Historically, Darwin has received most of the credit for this world-changing idea—justifiably, since he supported his conjecture with an abundantly documented book, On the Origin of Species. He had formed many of his ideas on the basis of a five-year trip around the world as a naturalist on the British naval survey ship HMS Beagle (1831–36) and had subsequently corresponded about them with numerous colleagues.
It came as a shock to him when, in 1858, Wallace sent him a paper outlining Darwin's basic idea. Darwin, at the urging of friends, had his and Wallace's idea presented jointly at a meeting of a scientific society. But he followed this in 1859 with Origin, which sold out in a day and cemented his reputation as the greatest of all biologists. Like many great ideas, the Darwin-Wallace theory, which has come down to us identified by the term "natural selection," was disarmingly simple. Basically, it recognized that variation ordinarily exists among individuals within a natural population and as a result their interactions with their environments are likely to differ. Such variation was also long recognized in the characteristics of domesticated plants and animals—wheat with grain that was hard or easy to harvest, cows that gave more or less milk, dogs that could herd sheep or were hopeless at it, hogs that were fatter or thinner, and so on. Farmers (who, from the standpoint of wheat and cows, are part of the environment) selected individuals with desired characteristics and encouraged their breeding, thereby over time creating strains that were easier to reap in the case of wheat or that gave more milk in the case of cows. Eventually, wheat was produced that had seed heads so heavy their stems could barely hold them up; cows appeared that could give fifteen gallons of milk per day. Darwin himself drew much of his view of selection in nature from observing the results of selective breeding of domestic animals, including the production of fancy plumage varieties by pigeon fanciers.
Darwin and Wallace were both inspired by an observation that had been made earlier by pioneering political economist Thomas Malthus, now most famous for warning that increases in numbers could cause the human population to outstrip its food supply. Malthus noted that animals commonly produced many more offspring than could survive. Darwin and Wallace drew the conclusion that those organisms that could take best advantage of their environments would be the ones most likely to survive; and by their survival and reproductive output, they were the ones "selected" by nature to be the parents of—and pass on their biological characteristics to—the next generation. Darwin recalled in his autobiography: "In October 1838, that is, fifteen months after I had begun my systematic inquiry, I happened to read for amusement Malthus on Population, and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed.... Here, then, I had at last got a theory by which to work." "Favourable variations" here, of course, means those animals and plants most likely to survive and have many offspring.
Thus appeared the idea of natural selection, which caused a paradigm shift in the biological sciences. The widely held conceptual worldview at the time, that the diversity of organisms was created all at once and would be forever the same, gave way to another, one in which new kinds (species) were being continuously produced by a gradual process: natural selection. Furthermore (and equally revolutionary at the time), the theory included the complementary idea that species would also be going extinct.
For our purposes here, natural selection can be viewed as the differential reproduction of individuals with varying genetic endowments that are members of the same population—the individuals of a species in a given area at the same time. While natural selection is not the only process that causes organisms to change generation after generation, it is the only one that makes them appear to be "designed" to survive and thrive in a given environment. There is, we emphasize, no actual design—it just looks that way to a naïve observer. It is important to remember that those "given environments" also change in their characteristics in time and space. The construction of a housing project may dramatically alter the environment of the plants and animals in a meadow, farm field, or woodland, for example. And differences in climate from place to place—at the extremes, from the heat of the tropics to the cold of the poles—affect where particular species may thrive. Selection modifies each population of an organism to adapt it to its environment, but environments usually differ in many respects from one place to another and from one time to another. Thus the selection pressures on different populations of the same organism or on the same population at different times may vary considerably, which, as we will see later, is of great importance in natural selection's role in producing life's diversity.
ISLANDS IN TIME
Islands, it turns out, have been particularly good places to study the effects of natural selection, sometimes with surprising results. Observations of island organisms during his historic 1831–36 voyage on the Beagle subsequently helped Darwin perceive the results of natural selection and grasp its power to cause organisms to diverge from one another, first by a little bit, and then by more and more. What we call "evolution" today, he called in the first edition of Origin a combination of "descent with modification" and "much extinction." He saw, for example, that island dwellers normally do not share characteristics with one another that make them especially suited for life on islands, nor are they most similar to one another and different from mainland creatures as a whole. Instead, they usually appear to have diverged from similar species that live on the mainland closest to their island home.
But one curious aspect is shared by many island organisms. One would expect them to be very mobile, since they had to reach the island in the first place. But once they have been on the island awhile, organisms commonly have less physical ability to disperse than do their relatives that live on continents. This comparative inability to disperse means they have adjusted genetically (or, as evolutionists say, adapted) to isolated island conditions. Evolutionists interpret this sedentary tendency as the result of natural selection against anything that promotes long-distance movement. The ability to disperse 100 miles carries no reproductive advantage for a bird isolated on a 5-square-mile island 300 miles from the mainland. On the contrary, if the bird leaves the island, it is likely to drop into the sea exhausted and drown.
Species of rails (birds in the coot family) that once inhabited many Pacific islands had lost the ability to fly. By contrast, continental species of rails, the kinds that originally colonized the islands, can fly very well. Once on the islands, the rails faced no bird-eating predators such as foxes or cats, so flying was not necessary to escape being eaten. Island rails that didn't put as much energy into growing big, well-muscled wings had more energy to allocate to producing chicks. So rails that had smaller wings were "favored by selection"—just a way of saying they had more surviving offspring than rails with normal-size wings. The flightless rails survived very successfully on the islands until their environment changed dramatically a few thousand years ago—when people arrived. Then the rails were like the slow-adjusting pied flycatchers. Hungry people and the rats human beings transported around the world with them found the flightless rails easy prey and tasty—and exterminated most of them.
Recently it has been shown that selection to reduce the ability to disperse can operate over a surprisingly short time. Biologists studied annual plants of the sunflower family on about 200 small islands near Vancouver Island along the Pacific Coast in British Columbia. Populations frequently went extinct, and then the islands were recolonized from the readily dispersed mainland populations. The seeds of these plants are blown over the water attached to fluffy "parachutes" (like those of dandelions). Within a decade after plants returned to the islands, the parachutes of two species began to decrease in size over subsequent generations, and the seeds of one increased in weight—in each instance reducing dispersal ability.
Selection thus can be strong enough in some cases that dramatic results can be observed in short periods of time. That can be very good news for organisms at a time when environmental change from such factors as human modification of the climate and civilization's release of poisons into the environment threatens to exterminate many of our living companions on Earth. Unfortunately, there's an underside to the story from the human perspective; many of the organisms that today are best able to evolve rapidly are disease-causing organisms that want to make meals of us, or pests that transport those pathogens and parasites, or other pests that attack our crops.
Birds can evolve fast too, if not as quickly as pathogens. Recent work by evolutionists Peter and Rosemary Grant has revealed that the bills of Darwin's (Galápagos) finches, the birds made famous by Darwin's 1835 visit to the Galápagos islands on the Beagle, evolve in size and strength as rapid changes in climate require changes in diet. For instance, during a drought in 1977, a population of the medium ground finch (Geospiza fortis) on Daphne Major Island was subjected to intense selection pressure that favored large individuals with big bills. The drought had reduced the supply of smaller fruits and seeds, and only big-billed birds could crack the large, tough fruits that remained. Females also tended to mate with larger males, and as a result there was a detectable increase in bill size in a single generation.
In 1982 a population of large ground finches (G. magnirostris), almost twice as big as G. fortis, invaded Daphne Major. They presented little competitive problem for the medium finches until the drought of 2003 created strong competition for food. The large finches with giant bills could consume the big, tough fruits three times faster, and they physically excluded the medium finches (G. fortis) from the places where those fruits could be found. Among the medium finches, it was now those with the smallest beaks, which were best for dealing with small fruits and seeds, that had higher survival rates, not the big-beaked ones. As a result, the average beak size of the medium finches declined (figure 1-1). The Grants in this case were able to take advantage of a "natural experiment" (produced by an extended drought) to understand how the finches were evolving.
Excerpted from The Dominant Animal by Paul R. Ehrlich, Anne H. Ehrlich. Copyright © 2008 Paul R. Ehrlich and Anne H. Ehrlich. Excerpted by permission of ISLAND PRESS.
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Meet the Author
Paul R. Ehrlich is Bing Professor of Population Studies and professor of biology at Stanford University and a fellow of the Beijer Institute of Ecological Economics. The author of Human Natures, The Population Bomb, and many other books, he is a member of the National Academy of Sciences and a recipient of numerous international honors, including the Crafoord Prize and the MacArthur “genius award.”
Anne H. Ehrlich is affiliated with Stanford's Biology Department and Center for Conservation Biology, and is a member of the American Academy of Arts and Sciences. She has served on the board of the Sierra Club and other conservation organizations, has coauthored more than ten books with her husband, and is a recipient of the Tyler Prize for Environmental Achievement and the United Nations Environment Programme/Sasakawa Environment Prize.
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