21 The Evidence fo Evolution Concept outline 21.1 Fossil evidence indicates that evolution has The Fossil Record. When fossils are arranged in the order of their age a continual series of change is seen, new changes being added at each stage The Evolution of Horses. The record of horse evolution is particularly well-documented and instructive 21.2 Natural selection can produce evolutionary The beaks of darwin's Finches. Natural selection favors stouter bills in dry years, when large tough-to-crush eeds are the only food available to finches. Peppered Moths and Industrial Melanism. Natural selection favors dark-colored moths in areas of heavy pollution, while light-colored moths survive better in Artificial Selection. Artificial selection practiced in laboratory studies, agriculture, and domestication FIGURE 21.1 demonstrate that selection can produce substantia a window into the past. The fossil remains of the now extinct reptile Mesosaurus found in Permian sediments in 21.3 Evidence for evolution can be found in other Africa and South America provided one of the earliest clues to a former connection between the two continents fields of biolog Mes clearly incapal The Anatomical Record. When anatomical features of of a transatlantic swim Therefore it must have lived in the living animals are examined, evidence of shared ancestry is lakes and rivers of a formerly contiguous landmass that often later became divided as africa and South america drifted The Molecular Record. When gene or protein apart in the Cretaceous. equences from organisms are arranged, species thought to be closely related based on fossil evidence are seen to be more similar than species thought to be distantly related. O f all the major ideas of biology, the theory that to- mil.rgent and Divergent Evolution. Evolution favors days organisms evolved from now-extinct ancestors imilar forms under similar circumstances (figure 21. 1)is perhaps the best known to the general pub- lic. This is not because the average erson truly under 21.4 The theory of evolution has proven controversial. stands the basic facts of evolution, but rather because many Darwins Critics. Critics have raised seven objections people mistakenly believe that it represents a challenge to Darwins theory of evolution by natural selection their religious beliefs. Similar highly publicized crit tcisms of evolution have occurred ever since darwin, s time. For this t that. du e study of biology, you address the issue squarely: Just what is the evidence for evolution 439
439 21 The Evidence for Evolution Concept Outline 21.1 Fossil evidence indicates that evolution has occurred. The Fossil Record. When fossils are arranged in the order of their age, a continual series of change is seen, new changes being added at each stage. The Evolution of Horses. The record of horse evolution is particularly well-documented and instructive. 21.2 Natural selection can produce evolutionary change. The Beaks of Darwin’s Finches. Natural selection favors stouter bills in dry years, when large tough-to-crush seeds are the only food available to finches. Peppered Moths and Industrial Melanism. Natural selection favors dark-colored moths in areas of heavy pollution, while light-colored moths survive better in unpolluted areas. Artificial Selection. Artificial selection practiced in laboratory studies, agriculture, and domestication demonstrate that selection can produce substantial evolutionary change. 21.3 Evidence for evolution can be found in other fields of biology. The Anatomical Record. When anatomical features of living animals are examined, evidence of shared ancestry is often apparent. The Molecular Record. When gene or protein sequences from organisms are arranged, species thought to be closely related based on fossil evidence are seen to be more similar than species thought to be distantly related. Convergent and Divergent Evolution. Evolution favors similar forms under similar circumstances. 21.4 The theory of evolution has proven controversial. Darwin’s Critics. Critics have raised seven objections to Darwin’s theory of evolution by natural selection. Of all the major ideas of biology, the theory that today’s organisms evolved from now-extinct ancestors (figure 21.1) is perhaps the best known to the general public. This is not because the average person truly understands the basic facts of evolution, but rather because many people mistakenly believe that it represents a challenge to their religious beliefs. Similar highly publicized criticisms of evolution have occurred ever since Darwin’s time. For this reason, it is important that, during the course of your study of biology, you address the issue squarely: Just what is the evidence for evolution? FIGURE 21.1 A window into the past. The fossil remains of the nowextinct reptile Mesosaurus found in Permian sediments in Africa and South America provided one of the earliest clues to a former connection between the two continents. Mesosaurus was a freshwater species and so clearly incapable of a transatlantic swim. Therefore, it must have lived in the lakes and rivers of a formerly contiguous landmass that later became divided as Africa and South America drifted apart in the Cretaceous
1.1 Fossil evidence indicates that evolution has occurred t its core, the case for evolution is built upon two pillars: Dating Fossils first, evidence that natural selection can produce evolution ary change and, second, evidence from the fossil record By dating the rocks in which fossils occur, we can get an ac- that evolution has occurred. In addition, information from rocks were dated by their position with respect to one an- ent as embryology, anatomy, molecular biology, and bio- other (relative dating; rocks in deeper strata are generally species)can only be interpreted sensibly as and the rates of erosion of different kinds of sedimentary ution rocks in different environments, geologists of the nine teenth century derived a fairly accurate idea of the relative The Fossil record ages of rocks Today, rocks are dated by measuring the degree of The most direct evidence that evolution has occurred decay of certain radioisotopes contained in the rock(ab- solute dating); the older the rock, the more its isotopes have found in the fossil record. Today we have a far more com- decayed. Because radioactive isotopes decay at a constan plete understanding of this record than was available in rate unaltered by temperature or pressure, the isotopes in Darwin's time. Fossils are the preserved remains of once- rock act as an internal clock, measuring the time since the living organisms. Fossils are created when three events rock was formed This is a more accurate way of dating occur. First, the organism must become buried in sedi- rocks and provides dates stated in millions of years,rather ent; then, the calcium in bone or other hard tissue must than relative dates mineralize;and, finally, the surrounding sediment must eventually harden to form rock. The process of fossilization robably occurs rarely. Usually, animal or plant A History of Evolutionary Change will decay or be scavenged before the process can begin. In When fossils are arrayed according to their age, fro oldest to cientists. When they do become available, they are often destroyed by erosion and other natural processes before sive evolutionary change. At the largest scale, the fossil they can be collected. As a rest, only a fraction of the record documents the progression of life through time, from the origin of eukaryotic organisms, through the species that have ever existed (estimated by some to be as evolution of fishes, the rise of land-living organisms, the nany as 500 million)are known from fossils. Nonetheless, reign of the dinosaurs, and on to the origin of humans the fossils that have been discovered are sufficient to pro-(figure 21. 2) ide detailed information on the course of evolution through time Reptiles hominid Vertebrates owering plants Amphibians Extinction Colonization of land dinosaurs 1500 Millions of years ago FIGURE 21.2 Timeline of the history of life as revealed by the fossil record 440 Part vi Evolution
Dating Fossils By dating the rocks in which fossils occur, we can get an accurate idea of how old the fossils are. In Darwin’s day, rocks were dated by their position with respect to one another (relative dating); rocks in deeper strata are generally older. Knowing the relative positions of sedimentary rocks and the rates of erosion of different kinds of sedimentary rocks in different environments, geologists of the nineteenth century derived a fairly accurate idea of the relative ages of rocks. Today, rocks are dated by measuring the degree of decay of certain radioisotopes contained in the rock (absolute dating); the older the rock, the more its isotopes have decayed. Because radioactive isotopes decay at a constant rate unaltered by temperature or pressure, the isotopes in a rock act as an internal clock, measuring the time since the rock was formed. This is a more accurate way of dating rocks and provides dates stated in millions of years, rather than relative dates. A History of Evolutionary Change When fossils are arrayed according to their age, from oldest to youngest, they often provide evidence of successive evolutionary change. At the largest scale, the fossil record documents the progression of life through time, from the origin of eukaryotic organisms, through the evolution of fishes, the rise of land-living organisms, the reign of the dinosaurs, and on to the origin of humans (figure 21.2). 440 Part VI Evolution At its core, the case for evolution is built upon two pillars: first, evidence that natural selection can produce evolutionary change and, second, evidence from the fossil record that evolution has occurred. In addition, information from many different areas of biology—including fields as different as embryology, anatomy, molecular biology, and biogeography (the study of the geographic distribution of species)—can only be interpreted sensibly as the outcome of evolution. The Fossil Record The most direct evidence that evolution has occurred is found in the fossil record. Today we have a far more complete understanding of this record than was available in Darwin’s time. Fossils are the preserved remains of onceliving organisms. Fossils are created when three events occur. First, the organism must become buried in sediment; then, the calcium in bone or other hard tissue must mineralize; and, finally, the surrounding sediment must eventually harden to form rock. The process of fossilization probably occurs rarely. Usually, animal or plant remains will decay or be scavenged before the process can begin. In addition, many fossils occur in rocks that are inaccessible to scientists. When they do become available, they are often destroyed by erosion and other natural processes before they can be collected. As a result, only a fraction of the species that have ever existed (estimated by some to be as many as 500 million) are known from fossils. Nonetheless, the fossils that have been discovered are sufficient to provide detailed information on the course of evolution through time. 21.1 Fossil evidence indicates that evolution has occurred. Millions of years ago Eukaryotes Vertebrates Colonization of land Reptiles Amphibians Mammals and dinosaurs Flowering plants and first birds First hominids 1500 600 500 400 300 200 100 Extinction of the dinosaurs FIGURE 21.2 Timeline of the history of life as revealed by the fossil record
Gaps in the Fossil Record This is not to say that the fossil record is complete. Given the low likelihood of fossil preservation and recovery, it is not surprising that there are gaps in the fossil record. 10 million Nonetheless, paleontologists(the years ago scientists who study fossils)continue Modern toothed whales to fill in the gaps in the fossil record While many gaps interrupted the 20 million fossil record in darwin's era. even years ag then. scientists knew of the ar cbaeopteryx fossil transitional between 30 million dinosaurs and birds. Today the fos- years ago nd limbs could not have aided it in sil record is far more complete, par ticularly among the vertebrates; tos-40 million\ up-and-down motion, as do modem whales sils have been found linking all the years ago major groups. R seen spectacular discoveries closing ly walked on land (as do some of the major remaining gaps in 50 million our understanding of vertebrate evo- years ago flexing its backbone and paddling with lution. For example, recently a four hind limbs(as do modern otters) legged aquatic mammal was disco ered that provides important insights 60 miion concerning the evolution of whales years ago mesonychid skeleton and dolphins from land-living hoofed ancestors(figure 21.3). Simi larly, a fossil snake with legs has shed light on the evolution of snake which are descended from lizards FIGure 21.3 that gradually became more and Whale"missing links. "The recent discoveries of ambulocetus and Rodhocetus have filled more elongated with simultaneous in the gaps between the mesonychids, the hypothetical ancestral link between the reduction and eventual disappear. whales and the hoofed mammals, and present-day whales. ance of the limbs On a finer scale, evolutionary change within some types of animals is known in exceptional detail. For example, about 200 million years ago, oysters underwent a change from small curved shells to larger, flatter ones, with progressively flat- ter fossils being seen in the fossil record over a period of 12 million ' s(figure 21. 4). A host of other les all illustrate a record of successive change. The demonstr tion of this successive change is one of the strongest lines of evidence.arcuata rcuata that evolution has occurred obliquata G. mecullochil G gigantea FIGURE 21.4 record of the major evolutionary Evolution of shell shape in oysters. Over 12 million years of the Early Jurassic transitions that have occurred Period, the shells of this group of coiled oysters became larger, thinner, and flatter through time These animals rested on the ocean floor in a special position called the "life position, and it may be that the larger, flatter shells were more stable in disruptive water movements Chapter 21 The Evidence for Evolution 441
Gaps in the Fossil Record This is not to say that the fossil record is complete. Given the low likelihood of fossil preservation and recovery, it is not surprising that there are gaps in the fossil record. Nonetheless, paleontologists (the scientists who study fossils) continue to fill in the gaps in the fossil record. While many gaps interrupted the fossil record in Darwin’s era, even then, scientists knew of the Archaeopteryx fossil transitional between dinosaurs and birds. Today, the fossil record is far more complete, particularly among the vertebrates; fossils have been found linking all the major groups. Recent years have seen spectacular discoveries closing some of the major remaining gaps in our understanding of vertebrate evolution. For example, recently a fourlegged aquatic mammal was discovered that provides important insights concerning the evolution of whales and dolphins from land-living, hoofed ancestors (figure 21.3). Similarly, a fossil snake with legs has shed light on the evolution of snakes, which are descended from lizards that gradually became more and more elongated with simultaneous reduction and eventual disappearance of the limbs. On a finer scale, evolutionary change within some types of animals is known in exceptional detail. For example, about 200 million years ago, oysters underwent a change from small curved shells to larger, flatter ones, with progressively flatter fossils being seen in the fossil record over a period of 12 million years (figure 21.4). A host of other examples all illustrate a record of successive change. The demonstration of this successive change is one of the strongest lines of evidence that evolution has occurred. The fossil record provides a clear record of the major evolutionary transitions that have occurred through time. Chapter 21 The Evidence for Evolution 441 Present 10 million years ago 20 million years ago 30 million years ago 40 million years ago 50 million years ago 60 million years ago Hypothetical mesonychid skeleton Modern toothed whales Ambulocetus natans probably walked on land (as do modern sea lions) and swam by flexing its backbone and paddling with its hind limbs (as do modern otters) Rodhocetus kasrani's reduced hind limbs could not have aided it in walking or swimming. Rodhocetus swam with an up-and-down motion, as do modern whales FIGURE 21.3 Whale “missing links.” The recent discoveries of Ambulocetus and Rodhocetus have filled in the gaps between the mesonychids, the hypothetical ancestral link between the whales and the hoofed mammals, and present-day whales. G. arcuata obliquata G. arcuata incurva G. mecullochii G. gigantea FIGURE 21.4 Evolution of shell shape in oysters. Over 12 million years of the Early Jurassic Period, the shells of this group of coiled oysters became larger, thinner, and flatter. These animals rested on the ocean floor in a special position called the “life position,” and it may be that the larger, flatter shells were more stable in disruptive water movements
The Evolution of horses One of the best-studied cases in the fossil record concerns he evolution of horses. Modern-day members of the Equidae include horses, zebras, donkeys and asses, all of which are large, long-legged, fast-running animals adapted to living on open grasslands. These species, all classified in e genus Equus, are the last living descendants of a long ineage that has produced 34 genera since its origin in the Eocene Period, approximately 55 million years ago. Exam ination of these fossils has provided a particularly well documented case of how evolution has proceeded by adap tation to changing environments The First Horse FIGURE 21.5 Hyracotherium sandra, one of the earliest horses, was the The earliest known members of the horse family, species in size of a housecat. the genus Hyracotherium, didnt look much like horses at all. Small, with short legs and broad feet(figure 21.5), these species occurred in wooded habitats, where they probably browsed on leaves and herbs and escaped predators b dodging through openings in the forest vegetation. The evolutionary path from these diminutive creatures to the workhorses of today has involved changes in a variety of traits, including: Size. The first horses were no bigger than dogs, with some considerably smaller. By contrast, modern equids can weigh more than a half ton Examination of the fos- sil record reveals that horses changed lttle in size for 8250 Merychippus elr different lineages exhibited rapid and substantial in- creases. However trends toward decreased size were also exhibited among some branches of the equid evolu- tionary tree(figure 21.6) acterium Toe reduction The feet of modern horses have a sin gle toe, enclosed in a tough, bony hoof. By contrast, Hyracotherium had four toes on its front feet and three 0151050 on its hindfeet. rather than hooves. these toes were en- cased in fleshy pads. Examination of the fossils clearly shows the transition through time: increase in length of Evolutionary change in body size of horses. Lines show the the central toe, development of the bony hoof, and re- broad outline of evolutionary relationships. Although most duction and loss of the other toes(figure 21. 7). As with hange involved increases in size, some decreases also body size, these trends occurred concurrently on several occurred fferent branches of the horse evolutionary tree. At the same time as these developments, horses were evolving hanges in the length and skeletal structure of the limbs, leading to animals capable of running long distances at teeth down. Accompanying these changes have been al high speeds terations in the shape of the skull that strengthened the Tooth size and shape. The teeth of Hyracotherium skull to withstand the stresses imposed by continual were small and relatively simple in shape. Through time, chewing. As with body size, evolutionary change has not horse teeth have increased greatly in length and have de- been constant through time. Rather, much of the change veloped a complex pattern of ridges on their molars and in tooth shape has occurred within the past 20 million premolars(figure 21.7). The effect of these changes is to produce teeth better capable of chewing tough and All of these changes may be understood as adaptations to gritty vegetation, such as grass, which tends to wear changing global climates. In particular, during the late 442 Part VI Evolution
The Evolution of Horses One of the best-studied cases in the fossil record concerns the evolution of horses. Modern-day members of the Equidae include horses, zebras, donkeys and asses, all of which are large, long-legged, fast-running animals adapted to living on open grasslands. These species, all classified in the genus Equus, are the last living descendants of a long lineage that has produced 34 genera since its origin in the Eocene Period, approximately 55 million years ago. Examination of these fossils has provided a particularly welldocumented case of how evolution has proceeded by adaptation to changing environments. The First Horse The earliest known members of the horse family, species in the genus Hyracotherium, didn’t look much like horses at all. Small, with short legs and broad feet (figure 21.5), these species occurred in wooded habitats, where they probably browsed on leaves and herbs and escaped predators by dodging through openings in the forest vegetation. The evolutionary path from these diminutive creatures to the workhorses of today has involved changes in a variety of traits, including: Size. The first horses were no bigger than dogs, with some considerably smaller. By contrast, modern equids can weigh more than a half ton. Examination of the fossil record reveals that horses changed little in size for their first 30 million years, but since then, a number of different lineages exhibited rapid and substantial increases. However, trends toward decreased size were also exhibited among some branches of the equid evolutionary tree (figure 21.6). Toe reduction. The feet of modern horses have a single toe, enclosed in a tough, bony hoof. By contrast, Hyracotherium had four toes on its front feet and three on its hindfeet. Rather than hooves, these toes were encased in fleshy pads. Examination of the fossils clearly shows the transition through time: increase in length of the central toe, development of the bony hoof, and reduction and loss of the other toes (figure 21.7). As with body size, these trends occurred concurrently on several different branches of the horse evolutionary tree. At the same time as these developments, horses were evolving changes in the length and skeletal structure of the limbs, leading to animals capable of running long distances at high speeds. Tooth size and shape. The teeth of Hyracotherium were small and relatively simple in shape. Through time, horse teeth have increased greatly in length and have developed a complex pattern of ridges on their molars and premolars (figure 21.7). The effect of these changes is to produce teeth better capable of chewing tough and gritty vegetation, such as grass, which tends to wear teeth down. Accompanying these changes have been alterations in the shape of the skull that strengthened the skull to withstand the stresses imposed by continual chewing. As with body size, evolutionary change has not been constant through time. Rather, much of the change in tooth shape has occurred within the past 20 million years. All of these changes may be understood as adaptations to changing global climates. In particular, during the late 442 Part VI Evolution FIGURE 21.5 Hyracotherium sandrae, one of the earliest horses, was the size of a housecat. • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Body size (kg) Millions of years ago 50 100 150 200 250 300 350 400 450 500 550 60 55 50 45 40 35 30 25 20 15 10 5 0 Equus Hyracotherium Mesohippus Merychippus Nannippus FIGURE 21.6 Evolutionary change in body size of horses. Lines show the broad outline of evolutionary relationships. Although most change involved increases in size, some decreases also occurred
Miocene and early Oligocene(approximately 20 to 25 mil- lion years ago, grasslands became widespread in North America, where much of horse evolution occurred. As horses adapted to these habitats, long-distance and high speed locomotion probably became more important to es- cape predators and travel great distances. By contrast, the greater flexibility provided by multiple toes and shorter limbs, which was advantageous for ducking through com plex forest vegetation, was no longer beneficial. At the same time, horses were eating grasses and other vegetation that contained more grit and other hard substances, thus favoring teeth and skulls better suited for withstanding such materials Evolutionary Trend For many years, horse evolution was held up as an example of constant evolutionary change through time. Some even saw in the record of horse evolution evidence for a progres- sive,guiding force, consistently pushing evolution to move in a single direction. We now know that such views are misguided; evolutionary change over millions of years is rarely so simple Rather, the fossils demonstrate that, although there have been overall trends evident in a variety of characteristics evolutionary change has been far from constant and uni- ③ form through time. Instead, rates of evolution have varied widely, with long periods of little change and some periods of great change. Moreover, when changes happen, the often occur simultaneously in different lineages of the horse evolutionary tree. Finally, even when a trend exists exceptions, such as the evolutionary decrease in body size exhibited by some lineages, are not uncommon. These pat- terns, evident in our knowledge of horse evolution, are usu ally discovered for any group of plants and animals for which we have an extensive fossil record as we shall see Mesohippus when we discuss human evolution in chapter 23 Horse Diversity One reason that horse evolution was originally conceived of as linear through time may be that modern horse diver sity is relatively limited. Thus, it is easy to mentally pic ture a straight line from Hyracotherium to modern-day Equus. However, todays limited horse diversity--only one surviving genus-is unusual. Indeed, at the peak of horse diversity in the Miocene, as many as 13 genera of FIGURE 21.7 horses could be found in North America alone. These Evolutionary changes in horses through time. species differed in body size and in a wide variety of other haracteristics. Presumably, they lived in different habi tats and exhibited different dietary preferences. Had this diversity existed to modern times, early workers presum- The extensive fossil record for horses pr ably would have had a different outlook on horse evolu- tion, a situation that is again paralleled by the evolution of detailed view of the evolutionary diversification of this group from small forest dwellers to the large and fast modern grassland species. Chapter 21 The Evidence for Evolution 443
Miocene and early Oligocene (approximately 20 to 25 million years ago), grasslands became widespread in North America, where much of horse evolution occurred. As horses adapted to these habitats, long-distance and highspeed locomotion probably became more important to escape predators and travel great distances. By contrast, the greater flexibility provided by multiple toes and shorter limbs, which was advantageous for ducking through complex forest vegetation, was no longer beneficial. At the same time, horses were eating grasses and other vegetation that contained more grit and other hard substances, thus favoring teeth and skulls better suited for withstanding such materials. Evolutionary Trends For many years, horse evolution was held up as an example of constant evolutionary change through time. Some even saw in the record of horse evolution evidence for a progressive, guiding force, consistently pushing evolution to move in a single direction. We now know that such views are misguided; evolutionary change over millions of years is rarely so simple. Rather, the fossils demonstrate that, although there have been overall trends evident in a variety of characteristics, evolutionary change has been far from constant and uniform through time. Instead, rates of evolution have varied widely, with long periods of little change and some periods of great change. Moreover, when changes happen, they often occur simultaneously in different lineages of the horse evolutionary tree. Finally, even when a trend exists, exceptions, such as the evolutionary decrease in body size exhibited by some lineages, are not uncommon. These patterns, evident in our knowledge of horse evolution, are usually discovered for any group of plants and animals for which we have an extensive fossil record, as we shall see when we discuss human evolution in chapter 23. Horse Diversity One reason that horse evolution was originally conceived of as linear through time may be that modern horse diversity is relatively limited. Thus, it is easy to mentally picture a straight line from Hyracotherium to modern-day Equus. However, today’s limited horse diversity—only one surviving genus—is unusual. Indeed, at the peak of horse diversity in the Miocene, as many as 13 genera of horses could be found in North America alone. These species differed in body size and in a wide variety of other characteristics. Presumably, they lived in different habitats and exhibited different dietary preferences. Had this diversity existed to modern times, early workers presumably would have had a different outlook on horse evolution, a situation that is again paralleled by the evolution of humans. The extensive fossil record for horses provides a detailed view of the evolutionary diversification of this group from small forest dwellers to the large and fast modern grassland species. Chapter 21 The Evidence for Evolution 443 Hyracotherium Mesohippus Merychippus Pliohippus Equus FIGURE 21.7 Evolutionary changes in horses through time