part The Origin of living Things Unraveling the Mystery of How Geckos Defy gravity Defying gravity. This gecko lizard is able to climb walls and walk upside down across ceilings. Learning how geckos do this is a fascinating bit of experimental science. Science is most fun when it tickles your imagination. This is particularly true when you see something your common sense tells you just can't be true. Imagine, for example, you polar molecules. This suggests that geckos are tapping are lying on a bed in a tropical hotel room. a little lizard, a directly into the molecular structure of the surfaces they blue gecko about the size of a toothbrush, walks up the wall walk on beside you and upside down across the ceiling, stopping for Tracking down this clue. Kellar Autumn of lewis a few moments over your head to look down at you, and Clark College in Portland, Oregon, and Robert Full of the then trots over to the far wall and down University of California, Berkeley, took a closer look at There is nothing at all unusual in what you have just gecko feet. Geckos have rows of tiny hairs called setae on agined. Geckos are famous for strolling up walls in this the bottoms of their feet, like the bristles of some trendy fashion. How do geckos perform this gripping feat? Investi- toothbrush. When you look at these hairs under the micro- gators have puzzled over the adhesive properties of geckos scope, the end of each seta is divided into 400 to 1000 fine for decades. What force prevents gravity from dropping the projections called spatulae. There are about half a million of yo these setae on each foot, each only one-tenth the diameter The most reasonable hypothesis seemed suction- salamanders' feet form suction cups that let them climb Autumn and Full put together an interdisciplinary team alls, so maybe geckos' do too. The way to test this is to see of scientists and set out to measure the force produced by a if the feet adhere in a vacuum, with no air to create suction. single seta. To do this, they had to overcome two significant Salamander feet don't, but gecko feet do It's not suction experimental challenges How about friction? Cockroaches climb using tiny hooks that grapple onto irregularities in the surface, much as rock Isolating a single seta. No one had ever isolated a single climbers use crampons. Geckos, however, happily run up seta before. They succeeded in doing this by surgically Q walls of smooth polished glass that no cockroach can climb plucking a hair from a gecko foot under a microscope and It's not friction bonding the hair onto a microprobe. The microprobe Electrostatic attraction? Clothes in a dryer stick together was fitted into a specially designed micromanipulator that because of electrical charges created by their rubbing to- can move the mounted hair in various ways. gether. You can stop this by adding a"static remover"like a Measuring a very small force. Previous research had Cling-free sheet that is heavily ionized. But a gecko's feet shown that if you pull on a whole gecko, the adhesive still adhere in ionized air. It,s not electrostatic attraction force sticking each of the gecko,s feet to the wall is about Could it be glue? Many insects use adhesive secretions 10 Newtons(N), which is like supporting 1 kg. Because from glands in their feet to aid climbing. But there are no each foot has half a million setae, this predicts that a sin glands cells in the feet of a gecko, no secreted chemicals, no gle seta would produce about 20 micro Newtons of force footprints left behind. It's not glue Thats a very tiny amount to measure. To attempt the There is one tantalizing clue. however the kind that ex- measurement, Autumn and Full recruited a mechanica perimenters love Gecko feet seem to get stickier on some engineer from Stanford, Thomas Kenny. Kenny is an ex- surfaces than others. They are less sticky on low-energy pert at building instruments that can measure forces at surfaces like Teflon, and more sticky on surfaces made of the atomic level
1 Unraveling the Mystery of How Geckos Defy Gravity Science is most fun when it tickles your imagination. This is particularly true when you see something your common sense tells you just can’t be true. Imagine, for example, you are lying on a bed in a tropical hotel room. A little lizard, a blue gecko about the size of a toothbrush, walks up the wall beside you and upside down across the ceiling, stopping for a few moments over your head to look down at you, and then trots over to the far wall and down. There is nothing at all unusual in what you have just imagined. Geckos are famous for strolling up walls in this fashion. How do geckos perform this gripping feat? Investigators have puzzled over the adhesive properties of geckos for decades. What force prevents gravity from dropping the gecko on your nose? The most reasonable hypothesis seemed suction— salamanders’ feet form suction cups that let them climb walls, so maybe geckos’ do too. The way to test this is to see if the feet adhere in a vacuum, with no air to create suction. Salamander feet don’t, but gecko feet do. It’s not suction. How about friction? Cockroaches climb using tiny hooks that grapple onto irregularities in the surface, much as rockclimbers use crampons. Geckos, however, happily run up walls of smooth polished glass that no cockroach can climb. It’s not friction. Electrostatic attraction? Clothes in a dryer stick together because of electrical charges created by their rubbing together. You can stop this by adding a “static remover” like a Cling-free sheet that is heavily ionized. But a gecko’s feet still adhere in ionized air. It’s not electrostatic attraction. Could it be glue? Many insects use adhesive secretions from glands in their feet to aid climbing. But there are no glands cells in the feet of a gecko, no secreted chemicals, no footprints left behind. It’s not glue. There is one tantalizing clue, however, the kind that experimenters love. Gecko feet seem to get stickier on some surfaces than others. They are less sticky on low-energy surfaces like Teflon, and more sticky on surfaces made of polar molecules. This suggests that geckos are tapping directly into the molecular structure of the surfaces they walk on! Tracking down this clue, Kellar Autumn of Lewis & Clark College in Portland, Oregon, and Robert Full of the University of California, Berkeley, took a closer look at gecko feet. Geckos have rows of tiny hairs called setae on the bottoms of their feet, like the bristles of some trendy toothbrush. When you look at these hairs under the microscope, the end of each seta is divided into 400 to 1000 fine projections called spatulae. There are about half a million of these setae on each foot, each only one-tenth the diameter of a human hair. Autumn and Full put together an interdisciplinary team of scientists and set out to measure the force produced by a single seta. To do this, they had to overcome two significant experimental challenges: Isolating a single seta. No one had ever isolated a single seta before. They succeeded in doing this by surgically plucking a hair from a gecko foot under a microscope and bonding the hair onto a microprobe. The microprobe was fitted into a specially designed micromanipulator that can move the mounted hair in various ways. Measuring a very small force. Previous research had shown that if you pull on a whole gecko, the adhesive force sticking each of the gecko’s feet to the wall is about 10 Newtons (N), which is like supporting 1 kg. Because each foot has half a million setae, this predicts that a single seta would produce about 20 microNewtons of force. That’s a very tiny amount to measure. To attempt the measurement, Autumn and Full recruited a mechanical engineer from Stanford, Thomas Kenny. Kenny is an expert at building instruments that can measure forces at the atomic level. Part I The Origin of Living Things Defying gravity. This gecko lizard is able to climb walls and walk upside down across ceilings. Learning how geckos do this is a fascinating bit of experimental science. Real People Doing Real Science
Begin p Seta pulled on sensor Time(s) The sliding step experiment. The adhesive force of a single seta was measured. An initial push perpendicularly put the seta in contact with the sensor. Then, with parallel pulling, Closeup look at a gecko's foot. The setae on a gecko's foot are continued to increase over time to a value of 60 microNewtons arranged in rows, and point backwards, away from the toenail. (after this, the seta began to slide and pulled off the sensor). In a Each seta branches into several hundred spatulae (inset photo) large number of similar experiments, adhesion forces typically ach 200 microNewtons Two hundred micro Newtons is a tiny force, but stupen dous for Igh to ho The Experiment up an ant. a million hairs could support a small child. a little ecko, ceiling walking with 2 million of them(see photos Once this team had isolated a seta and placed it in Kenny's above), could theoretically carry a 90-pound backpack--talk device, "We had a real nasty surprise, "says Autumn. Fc about being over-engineered wo months, pushing individual seta against a surface, they If a gecko's feet stick that good, how do geckos ever couldnt get the isolated hair to stick at all become unstuck? The research team experimented with This forced the research team to stand back and think unattaching individual seta; they used yet another micro- bit. Finally it hit them. Geckos don,t walk by pushing their instrument, this one designed by engineer Ronald Fearin feet down, like we do. Instead, when a gecko takes a step, it also from U C. Berkeley, to twist the hair in various way pushes the palm of the foot into the surface, then uncurls They found that tipped past a critical angle, 30 degrees, its toes, sliding them backwards onto the surface. this he attractive forces between hair and surface atoms shoves the forest of tips sideways against the surface. weaken to nothing. The trick is to tip a foot hair until its Going back to their instruments, they repeated their ex- projections let go. Geckos release their feet by curling up riment,but this time they oriented the seta to approach each toe and peeling it off, just the way we remove ta the surface from the side rather than head-on. This had the What is the source of the powerful adhesion of gecko feet? effect of bringing the many spatulae on the tip of the seta The experiments do not reveal exactly what the attractive into direct contact with the surface force is. but it seems almost certain to involve interactions at To measure these forces on the seta from the side, as well the atomic level. For a geckos foot to stick, the hundreds of as the perpendicular forces they had already been measur- spatulae at the tip of each seta must butt up squarely against ing, the researchers constructed a micro-electromechanical the surface, so the individual atoms of each spatula can come cantilever. The apparatus consisted of two piezoresistive into play. When two atoms approach each other very yers deposited on a silicon cantilever to detect force in closely--closer than the diameter of an atom-a subtle nu- both parallel and perpendicular angles clear attraction called Van der Waals forces comes into play These forces are individually very weak, but when lots of The results them add their little bits, the sum can add up to quite a lot. Might robots be devised with feet tipped with artificial With the seta oriented properly, the experiment yielded re- setae, able to walk up walls? Autumn and Full are working sults. Fantastic results. The attachment force measured by with a robotics company to find out. Sometimes science is the machine went up 600-fold from what the team had not only fun, but can lead to surprising advances en measuring before. A single seta produced not the microNewtons of force predicted by the whole-foot m To explore this experiment further surements, but up to an astonishing 200 micro Newtons go to the virtual lab at (see graph above)! Measuring many individual seta, adhe www.mhhe.com/raven6/vlabl.mhtml sive forces averaged 194+25 microNewtons
The Experiment Once this team had isolated a seta and placed it in Kenny’s device, “We had a real nasty surprise,” says Autumn. For two months, pushing individual seta against a surface, they couldn’t get the isolated hair to stick at all! This forced the research team to stand back and think a bit. Finally it hit them. Geckos don’t walk by pushing their feet down, like we do. Instead, when a gecko takes a step, it pushes the palm of the foot into the surface, then uncurls its toes, sliding them backwards onto the surface. This shoves the forest of tips sideways against the surface. Going back to their instruments, they repeated their experiment, but this time they oriented the seta to approach the surface from the side rather than head-on. This had the effect of bringing the many spatulae on the tip of the seta into direct contact with the surface. To measure these forces on the seta from the side, as well as the perpendicular forces they had already been measuring, the researchers constructed a micro-electromechanical cantilever. The apparatus consisted of two piezoresistive layers deposited on a silicon cantilever to detect force in both parallel and perpendicular angles. The Results With the seta oriented properly, the experiment yielded results. Fantastic results. The attachment force measured by the machine went up 600-fold from what the team had been measuring before. A single seta produced not the 20 microNewtons of force predicted by the whole-foot measurements, but up to an astonishing 200 microNewtons (see graph above)! Measuring many individual seta, adhesive forces averaged 194+25 microNewtons. Two hundred microNewtons is a tiny force, but stupendous for a single hair only 100 microns long. Enough to hold up an ant. A million hairs could support a small child. A little gecko, ceiling walking with 2 million of them (see photos above), could theoretically carry a 90-pound backpack—talk about being over-engineered. If a gecko’s feet stick that good, how do geckos ever become unstuck? The research team experimented with unattaching individual seta; they used yet another microinstrument, this one designed by engineer Ronald Fearing also from U.C. Berkeley, to twist the hair in various ways. They found that tipped past a critical angle, 30 degrees, the attractive forces between hair and surface atoms weaken to nothing. The trick is to tip a foot hair until its projections let go. Geckos release their feet by curling up each toe and peeling it off, just the way we remove tape. What is the source of the powerful adhesion of gecko feet? The experiments do not reveal exactly what the attractive force is, but it seems almost certain to involve interactions at the atomic level. For a gecko’s foot to stick, the hundreds of spatulae at the tip of each seta must butt up squarely against the surface, so the individual atoms of each spatula can come into play. When two atoms approach each other very closely—closer than the diameter of an atom—a subtle nuclear attraction called Van der Waals forces comes into play. These forces are individually very weak, but when lots of them add their little bits, the sum can add up to quite a lot. Might robots be devised with feet tipped with artificial setae, able to walk up walls? Autumn and Full are working with a robotics company to find out. Sometimes science is not only fun, but can lead to surprising advances. To explore this experiment further, go to the Virtual Lab at www.mhhe.com/raven6/vlab1.mhtml 1 2 Time (s) 345 20 0 -20 40 60 Force (µN) 80 0 Begin parallel pulling Seta pulled off sensor The sliding step experiment. The adhesive force of a single seta was measured. An initial push perpendicularly put the seta in contact with the sensor. Then, with parallel pulling, the force continued to increase over time to a value of 60 microNewtons (after this, the seta began to slide and pulled off the sensor). In a large number of similar experiments, adhesion forces typically approach 200 microNewtons. Closeup look at a gecko’s foot. The setae on a gecko’s foot are arranged in rows, and point backwards, away from the toenail. Each seta branches into several hundred spatulae (inset photo)
The science of biology Concept Outline 1.1 Biology is the science of life Properties of life. Biology is the science that studies living organisms and how they interact with one another and heir environment 1. 2 Scientists form generalizations from observations. The Nature of Science. Science employs both deductive reasoning and inductive reasoning How Science Is Done. Scientists construct hypotheses from systematically collected objective data. They then perform experiments designed to disprove the hypotheses. 1.3 Darwins theory of evolution illustrates how science Darwins Theory of Evolution. On a round-the-world oyage Darwin made observations that eventually led him to formulate the hypothesis of evolution by natural selection Darwins Evidence. The fossil and geographic patterns of life he observed convinced Darwin that a process of evolution FIGURE 1.1 had occurred A replica of the Beagle, off the southern coast of South Inventing the Theory of Natural Selection. The Malthus idea that populations cannot grow unchecked led set forth on H.M.S. Beagle in 1831, at the age of 22.n, Darwin, and another naturalist named Wallace, to propose he hypothesis of natural selection Evolution After Darwin: More Evidence. In the century since Darwin, a mass of experimental evidence has supported ou are about to embark on a journey-a journey of his theory of evolution, now accepted by practically all pr: discovery about the nature of life. Nearly 180 years go, a young English naturalist named Charles Darwin sail on a similar journey on board H M.S. Beagle(figure 1. 4 This book is organized to help you learn biology 1. 1 shows a replica of the Beagle). What Darwin learned on Core Principles of Biology. The first half of this text is his five-year voyage led directly to his development of the devoted to general principles that apply to all organisms, the theory of evolution by natural selection, a theory that has second half to an examination of particula ar or become the core of the science of biology. Darwins voyage seems a fitting place to begin our exploration of biology the scientific study of living organisms and how they hav volved. Before we begin, however, let,s take a moment to think about what biology is and why it's important
3 1 The Science of Biology Concept Outline 1.1 Biology is the science of life. Properties of Life. Biology is the science that studies living organisms and how they interact with one another and their environment. 1.2 Scientists form generalizations from observations. The Nature of Science. Science employs both deductive reasoning and inductive reasoning. How Science Is Done. Scientists construct hypotheses from systematically collected objective data. They then perform experiments designed to disprove the hypotheses. 1.3 Darwin’s theory of evolution illustrates how science works. Darwin’s Theory of Evolution. On a round-the-world voyage Darwin made observations that eventually led him to formulate the hypothesis of evolution by natural selection. Darwin’s Evidence. The fossil and geographic patterns of life he observed convinced Darwin that a process of evolution had occurred. Inventing the Theory of Natural Selection. The Malthus idea that populations cannot grow unchecked led Darwin, and another naturalist named Wallace, to propose the hypothesis of natural selection. Evolution After Darwin: More Evidence. In the century since Darwin, a mass of experimental evidence has supported his theory of evolution, now accepted by practically all practicing biologists. 1.4 This book is organized to help you learn biology. Core Principles of Biology. The first half of this text is devoted to general principles that apply to all organisms, the second half to an examination of particular organisms. You are about to embark on a journey—a journey of discovery about the nature of life. Nearly 180 years ago, a young English naturalist named Charles Darwin set sail on a similar journey on board H.M.S. Beagle (figure 1.1 shows a replica of the Beagle). What Darwin learned on his five-year voyage led directly to his development of the theory of evolution by natural selection, a theory that has become the core of the science of biology. Darwin’s voyage seems a fitting place to begin our exploration of biology, the scientific study of living organisms and how they have evolved. Before we begin, however, let’s take a moment to think about what biology is and why it’s important. FIGURE 1.1 A replica of the Beagle, off the southern coast of South America. The famous English naturalist, Charles Darwin, set forth on H.M.S. Beagle in 1831, at the age of 22
1.1 Biology is the science of life. Properties of life VITHIN CELLS In its broadest sense, biology is the study of living things-the science of life. Living things come in an astounding variety of shapes and forms, and biologists study life in many differ ent ways. They live with gorillas, collect fossils, and listen to whales. They isolate viruses, grow mushrooms, and ex- amine the structure of fruit flies. They read the messages encoded in the long molecules of heredity and count how many times a hummingbird's wings beat each second What makes something"alive"? Anyone could deduce that a galloping horse is alive and a car is not, but wby? We cannot say, "If it moves, it's alive, "because a car can move, and gelatin can wiggle in a bowl. They certainly are ne alive. What characteristics do define life? All living organ isms share five basic characteristics: 1. Order. All organisms consist of one or more cells with highly ordered structures: atoms make up mole Cell cules, which construct cellular organelles, which are contained within cells. This hierarchical organization continues at higher levels in multicellular organisms and among organisms(figure 1. 2) 2. Sensitivity. All organisms respond to stimuli. Plants grow toward a source of light, and your pupils dilate when you walk into a dark room 3. Growth, development, and reproduction. All or ganisms are capable of growing and reproducing, and they all possess hereditary molecules that are passed to their offspring, ensuring that the offspring are of the same species. Although crystals also"grow, "their growth does not involve hereditary molecules 4. Regulation. All organisms have regulatory mecha nisms that coordinate the organisms internal func- tions. These functions include supplying cells with nu- trients, transporting substances through the organism, ers 5. Homeostasis. All organisms maintain relatively constant internal conditions different from their envi- ronment, a process called homeostasis. All living things share certain key characteristics: order, Macromolecule sensitivity, growth, development and reproduction, egulation, and homeostasis. FIGURE 1.2 Hierarchical organization of living things. Life is highly orga nized-from small and simple to large and complex, within cells, within multicellular organisms, and among organisms Part I The Origin of Living things
4 Part I The Origin of Living Things Properties of Life In its broadest sense, biology is the study of living things—the science of life. Living things come in an astounding variety of shapes and forms, and biologists study life in many different ways. They live with gorillas, collect fossils, and listen to whales. They isolate viruses, grow mushrooms, and examine the structure of fruit flies. They read the messages encoded in the long molecules of heredity and count how many times a hummingbird’s wings beat each second. What makes something “alive”? Anyone could deduce that a galloping horse is alive and a car is not, but why? We cannot say, “If it moves, it’s alive,” because a car can move, and gelatin can wiggle in a bowl. They certainly are not alive. What characteristics do define life? All living organisms share five basic characteristics: 1. Order. All organisms consist of one or more cells with highly ordered structures: atoms make up molecules, which construct cellular organelles, which are contained within cells. This hierarchical organization continues at higher levels in multicellular organisms and among organisms (figure 1.2). 2. Sensitivity. All organisms respond to stimuli. Plants grow toward a source of light, and your pupils dilate when you walk into a dark room. 3. Growth, development, and reproduction. All organisms are capable of growing and reproducing, and they all possess hereditary molecules that are passed to their offspring, ensuring that the offspring are of the same species. Although crystals also “grow,” their growth does not involve hereditary molecules. 4. Regulation. All organisms have regulatory mechanisms that coordinate the organism’s internal functions. These functions include supplying cells with nutrients, transporting substances through the organism, and many others. 5. Homeostasis. All organisms maintain relatively constant internal conditions, different from their environment, a process called homeostasis. All living things share certain key characteristics: order, sensitivity, growth, development and reproduction, regulation, and homeostasis. 1.1 Biology is the science of life. FIGURE 1.2 Hierarchical organization of living things. Life is highly organized—from small and simple to large and complex, within cells, within multicellular organisms, and among organisms. Organelle Macromolecule Molecule Cell WITHIN CELLS
WITHIN MULTICELLULAR ORGANISMS AMONG ORGANISMS Organ Chapter 1 The Science of Biology 5
Chapter 1 The Science of Biology 5 AMONG ORGANISMS Ecosystem Community Species Population WITHIN MULTICELLULAR ORGANISMS Tissue Organ Organ system Organism
1.2 Scientists form generalizations from observations The Nature of science FIGURE 1.3 Deductive reasoning: How Eratosthenes estimated the cir- Biology is a fascinating and important subject, because it cumference of the earth using deductive reasoning. 1. On a dramatically affects our daily lives and our futures. Many day when sunlight shone straight down a deep well at Syene in es, such as the worlds rapidly expanding population and diseases like cancer and AIDS. The knowledge these biolo- 2. The shadow's length and the obelisk,'s height formed two sides of a triangle. Using the recently developed principles of Euclidean gists gain will be fundamental to our ability to manage the geometry, he calculated the angle, a, to be 7o and 12, exactly t of orlds resources in a suitable manner, to prevent or cure circle(360 ).3. If angle a= so of a circle, then the distance diseases, and to improve the quality of our lives and those between the obelisk(in Alexandria)and the well (in Syene)must of our children and grandchildren equal so of the circumference of the earth. 4. Eratosthenes had Biology is one of the most successful of the "natural sc heard that it was a 50-day camel trip from Alexandria to Syene ences, "explaining what our world is like. To understand Assuming that a camel travels about 18.5 kilometers per day, he biology, you must first understand the nature of science. estimated the distance between obelisk and well as 925 kilometers The basic tool a scientist uses is thought. To understand (using different units of the nature of science, it is useful to focus for a moment on measure, of course) how scientists think. They reason in two ways: deductively 5. Eratosthenes thus de- duced the circumference and inductively of the earth to be 50 x 25=46,250 Deductive reasoning kilometers. Modern Deductive reasoning applies general principles to predict distance from the well to obelisk specific results. Over 2200 years ago, the Greek Era the obelisk at just over Length of tosthenes used deductive reasoning to accurately estimate 800 kilometers. Employ he circumference of the earth. At high noon on the longest ing a distance of 800 day of the year, when the suns rays hit the bottom of a kilometers, Era deep well in the city of Syene, Egypt, Eratosthenes mea sosthenes,s value would sured the length of the shadow cast by a tall obelisk in Al ave been50×800= exandria. about 800 kilometers to the north. Because he 40.000 kilometers The actual circumference is knew the distance between the two cities and the height of 40.075 kilometers the obelisk, he was able to employ the principles of euclid ean geometry to correctly deduce the circumference of the earth(figure 1. 3). This sort of analysis of specific cases us- ing general principles is an example of deductive reasoning It is the reasoning of mathematics and philosophy and is used to test the validity of general ideas in all branches of knowledge. General principles are constructed and then used as the basis for examining specific cases. Inductive Reasoning you release an apple from your hand, what happens? The apple falls to the ground. From a host of simple, specific Inductive reasoning uses specific observations to construct observations like this, Newton inferred a general principle general scientific principles. Webster's Dictionary defines sci- all objects fall toward the center of the earth. What New ence as systematized knowledge derived from observation ton did was construct a mental model of how the world and experiment carried on to determine the principles un- works, a family of general principles consistent with what derlying what is being studied. In other words, a scientist he could see and learn. Scientists do the same today. They determines principles from observations, discovering gen- use specific observations to build general models, and then eral principles by careful examination of specific cases. In- test the models to see how well they work ductive reasoning first became important to science in the 1600s in Europe, when Francis Bacon, Isaac Newton, and Science is a way of viewing the world that focuses on others began to use the results of particular experiments to objective information, putting that information to work infer general principles about how the world operates. If to build understanding Part I The Origin of Living things
6 Part I The Origin of Living Things The Nature of Science Biology is a fascinating and important subject, because it dramatically affects our daily lives and our futures. Many biologists are working on problems that critically affect our lives, such as the world’s rapidly expanding population and diseases like cancer and AIDS. The knowledge these biologists gain will be fundamental to our ability to manage the world’s resources in a suitable manner, to prevent or cure diseases, and to improve the quality of our lives and those of our children and grandchildren. Biology is one of the most successful of the “natural sciences,” explaining what our world is like. To understand biology, you must first understand the nature of science. The basic tool a scientist uses is thought. To understand the nature of science, it is useful to focus for a moment on how scientists think. They reason in two ways: deductively and inductively. Deductive Reasoning Deductive reasoning applies general principles to predict specific results. Over 2200 years ago, the Greek Eratosthenes used deductive reasoning to accurately estimate the circumference of the earth. At high noon on the longest day of the year, when the sun’s rays hit the bottom of a deep well in the city of Syene, Egypt, Eratosthenes measured the length of the shadow cast by a tall obelisk in Alexandria, about 800 kilometers to the north. Because he knew the distance between the two cities and the height of the obelisk, he was able to employ the principles of Euclidean geometry to correctly deduce the circumference of the earth (figure 1.3). This sort of analysis of specific cases using general principles is an example of deductive reasoning. It is the reasoning of mathematics and philosophy and is used to test the validity of general ideas in all branches of knowledge. General principles are constructed and then used as the basis for examining specific cases. Inductive Reasoning Inductive reasoning uses specific observations to construct general scientific principles. Webster’s Dictionary defines science as systematized knowledge derived from observation and experiment carried on to determine the principles underlying what is being studied. In other words, a scientist determines principles from observations, discovering general principles by careful examination of specific cases. Inductive reasoning first became important to science in the 1600s in Europe, when Francis Bacon, Isaac Newton, and others began to use the results of particular experiments to infer general principles about how the world operates. If you release an apple from your hand, what happens? The apple falls to the ground. From a host of simple, specific observations like this, Newton inferred a general principle: all objects fall toward the center of the earth. What Newton did was construct a mental model of how the world works, a family of general principles consistent with what he could see and learn. Scientists do the same today. They use specific observations to build general models, and then test the models to see how well they work. Science is a way of viewing the world that focuses on objective information, putting that information to work to build understanding. 1.2 Scientists form generalizations from observations. FIGURE 1.3 Deductive reasoning: How Eratosthenes estimated the circumference of the earth using deductive reasoning. 1. On a day when sunlight shone straight down a deep well at Syene in Egypt, Eratosthenes measured the length of the shadow cast by a tall obelisk in the city of Alexandria, about 800 kilometers away. 2. The shadow’s length and the obelisk’s height formed two sides of a triangle. Using the recently developed principles of Euclidean geometry, he calculated the angle, a, to be 7° and 12′, exactly 1 50 of a circle (360°). 3. If angle a = 1 50 of a circle, then the distance between the obelisk (in Alexandria) and the well (in Syene) must equal 1 50 of the circumference of the earth. 4. Eratosthenes had heard that it was a 50-day camel trip from Alexandria to Syene. Assuming that a camel travels about 18.5 kilometers per day, he estimated the distance between obelisk and well as 925 kilometers (using different units of measure, of course). 5. Eratosthenes thus deduced the circumference of the earth to be 50 925 46,250 kilometers. Modern measurements put the distance from the well to the obelisk at just over 800 kilometers. Employing a distance of 800 kilometers, Eratosthenes’s value would have been 50 × 800 40,000 kilometers. The actual circumference is 40,075 kilometers. Sunlight at midday c D iti is e t s anc = 8 e 0 between 0 km Well Light rays parallel Height of obelisk Length of shadow a a
How Science Is done the light switch is turned off. An alternative hypothes might be, "There is no light in the room because the light How do scientists establish which general principles are bulb is burned out. " And yet another alternative hypothe true from among the many that might be true? They do sis might be, "I am going blind. "To evaluate these hy this by systematically testing alternative proposals. If these potheses, you would conduct an experiment designed to proposals prove inconsistent with experimental observa- eliminate one or more of the hypotheses. For example, you tions, they are rejected as untrue. After making careful ob- might test your hypotheses by reversing the position of the servations concerning a particular area of science, scien ight switch. If you do so and the light does not come on, tists construct a hypothesis, which is a suggested you have disproved the first hypothesis. Something other explanation that accounts for those observations. A hy- than the setting of the light switch must be the reason for pothesis is a proposition that might be true. Those hy he darkness. Note that a test such as this does not prove potheses that have not yet been disproved are retained. that any of the other hypotheses are true; it merely dem- They are useful because they fit the known facts, but they onstrates that one of them is not. A successful experiment are always subject to future rejection if, in the light of new is one in which one or more of the alternative hypotheses information they are found to be incorrect. is demonstrated to be inconsistent with the results and is Testing Hypotheses As you proceed through this text, you will encounter many hypotheses that have withstood the test of experiment We call the test of a hypothesis an experiment(figure Many will continue to do so; others will be revised as new 1. 4). Suppose that a room appears dark to you. To under- observations are made by biologists. Biology, like all science, stand why it appears dark, you propose several hypotheses. is in a constant state of change, with new ideas appearing The first might be, "There is no light in the room because and replacing old ones Question Hypothesis 1 Hypothesis 2 hypotheses Hypothesis 5 1 and 4 HHH Experiment 2 and 3 FIGURE 1.4 How science is done. This diagram il- Remaining lustrates the way in which scientific in- hypotheses vestigations proceed. First, scientists make observations that raise a particular question. They develop a number of potential explanations Predictions (hypotheses)to answer the question Next, they carry out experiments in ar attempt to eliminate one or more of these hypotheses. Then, predictions are Experiment 1)(Experiment 2 made based on the remaining hypotheses, and further experiments are carried out to test these predictions. As a result of this process, the least confirmed unlikely hypothesis is selected Chapter 1 The Science of Biology 7
How Science Is Done How do scientists establish which general principles are true from among the many that might be true? They do this by systematically testing alternative proposals. If these proposals prove inconsistent with experimental observations, they are rejected as untrue. After making careful observations concerning a particular area of science, scientists construct a hypothesis, which is a suggested explanation that accounts for those observations. A hypothesis is a proposition that might be true. Those hypotheses that have not yet been disproved are retained. They are useful because they fit the known facts, but they are always subject to future rejection if, in the light of new information, they are found to be incorrect. Testing Hypotheses We call the test of a hypothesis an experiment (figure 1.4). Suppose that a room appears dark to you. To understand why it appears dark, you propose several hypotheses. The first might be, “There is no light in the room because the light switch is turned off.” An alternative hypothesis might be, “There is no light in the room because the lightbulb is burned out.” And yet another alternative hypothesis might be, “I am going blind.” To evaluate these hypotheses, you would conduct an experiment designed to eliminate one or more of the hypotheses. For example, you might test your hypotheses by reversing the position of the light switch. If you do so and the light does not come on, you have disproved the first hypothesis. Something other than the setting of the light switch must be the reason for the darkness. Note that a test such as this does not prove that any of the other hypotheses are true; it merely demonstrates that one of them is not. A successful experiment is one in which one or more of the alternative hypotheses is demonstrated to be inconsistent with the results and is thus rejected. As you proceed through this text, you will encounter many hypotheses that have withstood the test of experiment. Many will continue to do so; others will be revised as new observations are made by biologists. Biology, like all science, is in a constant state of change, with new ideas appearing and replacing old ones. Chapter 1 The Science of Biology 7 FIGURE 1.4 How science is done. This diagram illustrates the way in which scientific investigations proceed. First, scientists make observations that raise a particular question. They develop a number of potential explanations (hypotheses) to answer the question. Next, they carry out experiments in an attempt to eliminate one or more of these hypotheses. Then, predictions are made based on the remaining hypotheses, and further experiments are carried out to test these predictions. As a result of this process, the least unlikely hypothesis is selected. Observation Question Experiment Hypothesis 1 Hypothesis 2 Hypothesis 3 Hypothesis 4 Hypothesis 5 Potential hypotheses Remaining possible hypotheses Last remaining possible hypothesis Reject hypotheses 1 and 4 Reject hypotheses 2 and 3 Experiment Experiment 1 Hypothesis 2 Hypothesis 3 Hypothesis 5 Hypothesis 5 Predictions Predictions confirmed Experiment 1 Experiment 2 Experiment 3 Experiment 4
Establishing Controls set of ideas about the nature of the universe, explains ex- Often we are interested in learning about processes that are perimental facts, and serves as a guide to furtl ther questions influenced by many factors, or variables. To evaluate alter and experiments. To a scientist, such theories are the solid ground of sci- native hypotheses about one variable, all other variables ence, that of which we are most certain. In contrast to the must be kept constant. This is done by carrying out two ex- general public, tbeory implies just the opposite--a lack of periments in parallel: in the first experiment, one variable is knowledge, or a guess. Not surprisingly, this difference altered in a specific way to test a particular hypothesis; in the often results in confusion. In this text, theory will always be second experiment, called the control experiment, that used in its scientific sense, in reference to an accepted gen- variable is left unaltered. In all other respects the two exper- eral principle or body of knowledge iments are identical, so any difference in the outcomes of the two experiments must result from the influence of the cL. To suggest, as many critics outside of science do, that lution is"just a theory"is misleading. The hypothesis variable that was changed. Much of the challenge of experi- that evolution has occurred is an accepted scientific fact; it is isolate a particular variable from other factors that might in- supported by overwhelming evidence. Modern evolutionary eory is a complex body of ideas whose importance spreads ce a p far beyond explaining evolution; its ramifications permeate all areas of biology, and it provides the conceptual frame Using Predictions work that unifies biology as a science. A successful scientific hypothesis needs to be not only valid Research and the Scientific Method out useful-it needs to tell you something you want to know. a hypothesis is most useful when it makes predic- It used to be fashionable to speak of the"scientific meth- ns,because those predictions provide a way to test the va- od"as consisting of an orderly sequence of logical"ei lidity of the hypothesis. If an experiment produces results ther/or"steps. Each step would reject one of two mutually inconsistent with the predictions, the hypothesis must be re- incompatible alternatives, as if trial-and-error testing jected. On the other hand, if the predictions are supported would inevitably lead one through the maze of uncertain by experimental testing, the hypothesis is supported. The ty that always impedes scientific progress. If this were in- more experimentally supported predictions a hypothesis deed so, a computer would make a good scientist. But sci- makes, the more valid the hypothesis is. For example, Ein- ence is not done this way. As British philosopher Karl steins hypothesis of relativity was at first provisionally ac- Popper has pointed out, successful scientists without ex- epted because no one could devise an experiment that in- ception design their experiments with a pretty fair idea of validated it. The hypothesis made a clear prediction: that how the results are going to come out. They have what the sun would bend the path of light passing by it. When Popper calls an"imaginative preconception"of what the this prediction was tested in a total eclipse, the light from truth might be. A hypothesis that a successful scientist background stars was indeed bent. Because this result tests is not just any hypothesis; rather, it is an educated unknown when the hypothesis was being formulated, it pro- less or a hunch, in which the scientist integrates all that vided strong support for the hypothesis, which was then ac- he or she knows and allows his or her imagination full cepted with more confidence play, in an attempt to get a sense of what might be true see Box: How Biologists Do Their Work). It is because Developing theories insight and imagination play such a large role in scientific progress that some scientists are so much better at science Scientists use the word theory in two main ways. A"theo- than others, just as Beethoven and Mozart stand out ry"is a proposed explanation for some natural phenome- among most other composers non,often based on some general principle. Thus one ome scientists perform what is called basic research speaks of the principle first proposed by Newton as the which is intended to extend the boundaries of what we "theory of gravity. "Such theories often bring together know. These individuals typically work at universities, and concepts that were previously thought to be unrelated, their research is usually financially supported by their in- and offer unified explanations of different phenomena. stitutions and by external sources, such as the government, Newton's theory of gravity provided a single explanation industry, and private foundations. Basic research is as di for objects falling to the ground and the orbits of planets verse as its name implies Some basic scientists attempt to around the sun. "Theory"is also used to mean the body find out how certain cells take up specific chemicals, while of interconnected concepts, supported by scientific rea others count the number of dents in tiger teeth. The infor- soning and experimental evidence, that explains the facts mation generated by basic research contributes to the in some area of study. Such a theory provides an indis- growing body of scientific knowledge, and it provides the pensable framework for organizing a body of knowledge. scientific foundation utilized by applied research. Scien For example, quantum theory in physics brings together a tists who conduct applied research are often employed in Part I The Origin of Living things
Establishing Controls Often we are interested in learning about processes that are influenced by many factors, or variables. To evaluate alternative hypotheses about one variable, all other variables must be kept constant. This is done by carrying out two experiments in parallel: in the first experiment, one variable is altered in a specific way to test a particular hypothesis; in the second experiment, called the control experiment, that variable is left unaltered. In all other respects the two experiments are identical, so any difference in the outcomes of the two experiments must result from the influence of the variable that was changed. Much of the challenge of experimental science lies in designing control experiments that isolate a particular variable from other factors that might influence a process. Using Predictions A successful scientific hypothesis needs to be not only valid but useful—it needs to tell you something you want to know. A hypothesis is most useful when it makes predictions, because those predictions provide a way to test the validity of the hypothesis. If an experiment produces results inconsistent with the predictions, the hypothesis must be rejected. On the other hand, if the predictions are supported by experimental testing, the hypothesis is supported. The more experimentally supported predictions a hypothesis makes, the more valid the hypothesis is. For example, Einstein’s hypothesis of relativity was at first provisionally accepted because no one could devise an experiment that invalidated it. The hypothesis made a clear prediction: that the sun would bend the path of light passing by it. When this prediction was tested in a total eclipse, the light from background stars was indeed bent. Because this result was unknown when the hypothesis was being formulated, it provided strong support for the hypothesis, which was then accepted with more confidence. Developing Theories Scientists use the word theory in two main ways. A “theory” is a proposed explanation for some natural phenomenon, often based on some general principle. Thus one speaks of the principle first proposed by Newton as the “theory of gravity.” Such theories often bring together concepts that were previously thought to be unrelated, and offer unified explanations of different phenomena. Newton’s theory of gravity provided a single explanation for objects falling to the ground and the orbits of planets around the sun. “Theory” is also used to mean the body of interconnected concepts, supported by scientific reasoning and experimental evidence, that explains the facts in some area of study. Such a theory provides an indispensable framework for organizing a body of knowledge. For example, quantum theory in physics brings together a set of ideas about the nature of the universe, explains experimental facts, and serves as a guide to further questions and experiments. To a scientist, such theories are the solid ground of science, that of which we are most certain. In contrast, to the general public, theory implies just the opposite—a lack of knowledge, or a guess. Not surprisingly, this difference often results in confusion. In this text, theory will always be used in its scientific sense, in reference to an accepted general principle or body of knowledge. To suggest, as many critics outside of science do, that evolution is “just a theory” is misleading. The hypothesis that evolution has occurred is an accepted scientific fact; it is supported by overwhelming evidence. Modern evolutionary theory is a complex body of ideas whose importance spreads far beyond explaining evolution; its ramifications permeate all areas of biology, and it provides the conceptual framework that unifies biology as a science. Research and the Scientific Method It used to be fashionable to speak of the “scientific method” as consisting of an orderly sequence of logical “either/or” steps. Each step would reject one of two mutually incompatible alternatives, as if trial-and-error testing would inevitably lead one through the maze of uncertainty that always impedes scientific progress. If this were indeed so, a computer would make a good scientist. But science is not done this way. As British philosopher Karl Popper has pointed out, successful scientists without exception design their experiments with a pretty fair idea of how the results are going to come out. They have what Popper calls an “imaginative preconception” of what the truth might be. A hypothesis that a successful scientist tests is not just any hypothesis; rather, it is an educated guess or a hunch, in which the scientist integrates all that he or she knows and allows his or her imagination full play, in an attempt to get a sense of what might be true (see Box: How Biologists Do Their Work). It is because insight and imagination play such a large role in scientific progress that some scientists are so much better at science than others, just as Beethoven and Mozart stand out among most other composers. Some scientists perform what is called basic research, which is intended to extend the boundaries of what we know. These individuals typically work at universities, and their research is usually financially supported by their institutions and by external sources, such as the government, industry, and private foundations. Basic research is as diverse as its name implies. Some basic scientists attempt to find out how certain cells take up specific chemicals, while others count the number of dents in tiger teeth. The information generated by basic research contributes to the growing body of scientific knowledge, and it provides the scientific foundation utilized by applied research. Scientists who conduct applied research are often employed in 8 Part I The Origin of Living Things
How Biologists Do when the days get short enough in the fall, each leaf responds independently by falling Their Work Hypothesis 3: A strong wind arose the night before Nemerov made his observation blowing all the leaves off the ginkgo trees The Consent Next, the scientist attempts to eliminate Late in November, on a single night one or more of the hypotheses by conduct Not even near to freezing, the ginkgo trees ng an experiment. In this case, one might That stand along the walk drop all their leaves cover some of the leaves so that they can In one consent. and neitber to rain nor to win not use light to sense day length. If hypoth But as thougb to time alone: the golden and sis 2 is true, then the covered leaves green should not fall when the others do. because Leaves litter the lawn today, that yesterde they are not receiving the same informa Had spread aloft tbeir fluttering fans of ligbt. 4 tion. Suppose, however, that despite the What signal fiom the stars? Wbat senses took it overing of some of the leaves, all the leaves still fall together. This result would What in those wooden motives so decided eliminate hypothesis 2 as a possibility. Ei- To strike their leaves, to down their leaves. ther of the other hypotheses, and many Rebellion or surrender? And if this others, remain possibilites Can bappen thus, wbat race sball be exempt? s simple experiment with ginkgoes What use to learn the lessons taught by time, points out the essence of scientific If a star at amy time may tell us: Now progress: science does not prove that cer- Howard Nemerov FIGURE 1.A tain explanations are true; rather, it proves A ginkgo tree that others are not. Hypotheses that are inconsistent with experime cannot control or even identify. It is the job first formulate several possible answers, sional s, while hypotheses that are not may be rejected in the future when more he poses, to identify and try to understand Hypothesis 1: Ginkgo trees possess an inter- information becomes available, if they are those things that influence life nal clock that times the release of leaves to inconsistent with the new information. Just Nemerov asks why ginkgo trees(figure match the season. On the day Nemerov de- as finding the correct path through a maze 1.A)drop all their leaves at once. To find scribes, this clock sends a"drop"signal by trying and eliminating false paths an answer to questions such as this, biolo- (perhaps a chemical) to all the leaves at the entists work to find the correct explana gists and other scientists pose possible an- same time swers and then try to determine which an- ns of natural phenomena by eliminatin swers are false. Tests of alternative Hypothesis 2: The individual leaves of ginkgo possibilities are called experiments. To trees are each able to sense day length, and some kind of industry. Their work may involve the manu- The explosive growth in scientific research during the facturing of food additives, creating of new drugs, or test- second half of the twentieth century is reflected in the published, it must be reviewed and accepted by other scien- ma pole al =co scientific journals now in existence ing the quality of the environment. enormous number of After developing a hypothesis and performing a series of Science and nature are devoted to experiments, a scientist writes a paper carefully describing a wide range of scientific disciplines, most are extrem the experiment and its results. He or she then submits the specialized: Cell Motility and the Cytoskeleton, Glycoconju paper for publication in a scientific journal, but before it is gate four lutation Research, and Synapse are just a few tists who are familiar with that particular field of research This process of careful evaluation, called peer review, lies at the heart of modern science, fostering careful work, precise The scientific process involves the rejection of description, and thoughtful analysis. When an important hypotheses that are inconsistent with experimental discovery is announced in a paper, other scientists attempt results or observations. Hypotheses that are consist to reproduce the result, providing a check on accuracy and with available data are conditiona lly accepted. Theent honesty. Nonreproducible results are not taken seriously formulation of the hypothesis often involves creative Chapter 1 The Science of Biology 9
some kind of industry. Their work may involve the manufacturing of food additives, creating of new drugs, or testing the quality of the environment. After developing a hypothesis and performing a series of experiments, a scientist writes a paper carefully describing the experiment and its results. He or she then submits the paper for publication in a scientific journal, but before it is published, it must be reviewed and accepted by other scientists who are familiar with that particular field of research. This process of careful evaluation, called peer review, lies at the heart of modern science, fostering careful work, precise description, and thoughtful analysis. When an important discovery is announced in a paper, other scientists attempt to reproduce the result, providing a check on accuracy and honesty. Nonreproducible results are not taken seriously for long. The explosive growth in scientific research during the second half of the twentieth century is reflected in the enormous number of scientific journals now in existence. Although some, such as Science and Nature, are devoted to a wide range of scientific disciplines, most are extremely specialized: Cell Motility and the Cytoskeleton, Glycoconjugate Journal, Mutation Research, and Synapse are just a few examples. The scientific process involves the rejection of hypotheses that are inconsistent with experimental results or observations. Hypotheses that are consistent with available data are conditionally accepted. The formulation of the hypothesis often involves creative insight. Chapter 1 The Science of Biology 9 How Biologists Do Their Work learn why the ginkgo trees drop all their leaves simultaneously, a scientist would first formulate several possible answers, called hypotheses: Hypothesis 1: Ginkgo trees possess an internal clock that times the release of leaves to match the season. On the day Nemerov describes, this clock sends a “drop” signal (perhaps a chemical) to all the leaves at the same time. Hypothesis 2: The individual leaves of ginkgo trees are each able to sense day length, and when the days get short enough in the fall, each leaf responds independently by falling. Hypothesis 3: A strong wind arose the night before Nemerov made his observation, blowing all the leaves off the ginkgo trees. Next, the scientist attempts to eliminate one or more of the hypotheses by conducting an experiment. In this case, one might cover some of the leaves so that they cannot use light to sense day length. If hypothesis 2 is true, then the covered leaves should not fall when the others do, because they are not receiving the same information. Suppose, however, that despite the covering of some of the leaves, all the leaves still fall together. This result would eliminate hypothesis 2 as a possibility. Either of the other hypotheses, and many others, remain possibilities. This simple experiment with ginkgoes points out the essence of scientific progress: science does not prove that certain explanations are true; rather, it proves that others are not. Hypotheses that are inconsistent with experimental results are rejected, while hypotheses that are not proven false by an experiment are provisionally accepted. However, hypotheses may be rejected in the future when more information becomes available, if they are inconsistent with the new information. Just as finding the correct path through a maze by trying and eliminating false paths, scientists work to find the correct explanations of natural phenomena by eliminating false possibilities. The Consent Late in November, on a single night Not even near to freezing, the ginkgo trees That stand along the walk drop all their leaves In one consent, and neither to rain nor to wind But as though to time alone: the golden and green Leaves litter the lawn today, that yesterday Had spread aloft their fluttering fans of light. What signal from the stars? What senses took it in? What in those wooden motives so decided To strike their leaves, to down their leaves, Rebellion or surrender? And if this Can happen thus, what race shall be exempt? What use to learn the lessons taught by time, If a star at any time may tell us: Now. Howard Nemerov What is bothering the poet Howard Nemerov is that life is influenced by forces he cannot control or even identify. It is the job of biologists to solve puzzles such as the one he poses, to identify and try to understand those things that influence life. Nemerov asks why ginkgo trees (figure 1.A) drop all their leaves at once. To find an answer to questions such as this, biologists and other scientists pose possible answers and then try to determine which answers are false. Tests of alternative possibilities are called experiments. To FIGURE 1.A A ginkgo tree
1.3 Darwins theory of evolution illustrates how science works Darwin's Theory of Evolution Darwin's theory of evolution explains and describes how organisms on earth have changed over time and acquired a diversity of new forms. This famous theory provides a good example of how a scientist develops a hypothesis and how a scientific theory grows and wins acceptance Charles Robert Darwin(1809-1882; figure 1.5)was an English naturalist who, after 30 years of study and obser vation wrote one of the most famous and influential books of all time. This book, On the Origin of Species by Means of Natural Selection, or Tbe Preservation of Favoured Races in the Struggle for Life, d a sensation when it ished, and the ideas Darwin expressed in it have played a central role in the development of human thought ever sInce. In Darwins time, most people be lieved that the various kinds of organ isms and their individual structures re sulted from direct actions of the Creator and to this day many people still believe this to be true). Species were thought to be specially created and unchangeable, or immutable. over the course of time. In contrast to these views a number of earlier philosophers had presented the view that living, history of life on hings must have changed during th earth. Darwin proposed a concept he called natural selection as a coherent logical explanation for this process, and he brought his ideas to wide public at tention. His book as its title indicates presented a conclusion that differed FIGURE 1.5 sharply from conventional wisdom. Al- Charles Darwin. This newly rediscovered photograph taken in 1881, the year before though his theory did not challenge the Darwin died, appears to be the last ever taken of the great biologist. existence of a Divine Creator. Darwin argued that this Creator did not simply create things and then leave them fore er unchanged. Instead, Darwin's go expressed Himself through the operation of natural laws tionary theory deeply troubled not only many of his con- that produced change over time, or evolution. These temporaries but Darwin himself. views put Darwin at odds with most people of his time, The story of Darwin and his theory begins in 1831, when who believed in a literal interpretation of the bible and he was 22 years old. On the recommendation of one of his epted the idea of a fixed and constant world. His revolu- professors at Cambridge University, he was selected to serve 10 Part I The Origin of living things
10 Part I The Origin of Living Things Darwin’s Theory of Evolution Darwin’s theory of evolution explains and describes how organisms on earth have changed over time and acquired a diversity of new forms. This famous theory provides a good example of how a scientist develops a hypothesis and how a scientific theory grows and wins acceptance. Charles Robert Darwin (1809–1882; figure 1.5) was an English naturalist who, after 30 years of study and observation, wrote one of the most famous and influential books of all time. This book, On the Origin of Species by Means of Natural Selection, or The Preservation of Favoured Races in the Struggle for Life, created a sensation when it was published, and the ideas Darwin expressed in it have played a central role in the development of human thought ever since. In Darwin’s time, most people believed that the various kinds of organisms and their individual structures resulted from direct actions of the Creator (and to this day many people still believe this to be true). Species were thought to be specially created and unchangeable, or immutable, over the course of time. In contrast to these views, a number of earlier philosophers had presented the view that living things must have changed during the history of life on earth. Darwin proposed a concept he called natural selection as a coherent, logical explanation for this process, and he brought his ideas to wide public attention. His book, as its title indicates, presented a conclusion that differed sharply from conventional wisdom. Although his theory did not challenge the existence of a Divine Creator, Darwin argued that this Creator did not simply create things and then leave them forever unchanged. Instead, Darwin’s God expressed Himself through the operation of natural laws that produced change over time, or evolution. These views put Darwin at odds with most people of his time, who believed in a literal interpretation of the Bible and accepted the idea of a fixed and constant world. His revolutionary theory deeply troubled not only many of his contemporaries but Darwin himself. The story of Darwin and his theory begins in 1831, when he was 22 years old. On the recommendation of one of his professors at Cambridge University, he was selected to serve 1.3 Darwin’s theory of evolution illustrates how science works. FIGURE 1.5 Charles Darwin. This newly rediscovered photograph taken in 1881, the year before Darwin died, appears to be the last ever taken of the great biologist
