Domestication Artificial selection has also been responsible for the great variety of breeds of cats, dogs PIgeons, C mestic animals. In some cases, breeds have been developed for particular purposes. Grey hound dogs, for example, were bred by select ing for maximal running abilities, with the end result being an animal with long legs and tail (the latter used as a rudder), an arched back(to increase the length of its stride), and great muscle mass. By contrast, the odd proportions of the ungainly basset hound resulted from se- lection for dogs that could enter narrow holes in pursuit of rabbits and other small game. In other cases, breeds have been developed pri- marily for their appearance, such as the many stiff colorful and ornamented varieties of pigeons or the breeds of cats Domestication also has led to unintentional selection for some traits. In recent years, as part of an attempt to domesticate the silver fox, Russian scientists each generation have chosen the most docile animals and allowed Dachshund them to reproduce. Within 40 years, the vast docile, not only allowing themselves to be pet- FIGURE 21.14 ted, but also whimpering to get attention and Breeds of dogs. The differences between these dogs are greater than the sniffing and licking their caretakers. In many differences displayed between any wild species of canids respects, they had become no different than domestic dogs! However, it was not only be- havior that changed. These foxes also began to exhibit dif- ample, the breeds of dogs, all of which have been pro- ferent color patterns, floppy ears, curled tails, and shorter duced since wolves were first domesticated, perhaps legs and tails. Presumably, the genes responsible for docile 10,000 years ago. If the various dog breeds did not exis ehavior have other effects as well (the phenomenon of and a paleontologist found fossils of animals similar to pleiotropy discussed in the last chapter); as selection has fa- dachshunds, greyhounds, mastiffs, Chihuahuas, and vored docile animals. it has also led to the evolution of iranians, there is no question that they would be co these other traits sidered different species. Indeed, these breeds are so dif- ferent that they would probably be classified in different Can Selection Produce Major Evolutionary genera. In fact, the diversity exhibited by dog breeds far Ch outstrips the differences observed among wild members of he fa Given that we can observe the results of selection operating olves. Consequently, the claim that artificial selection over relatively short periods of time, most scientists believe produces only minor changes is clearly incorrect. Indeed, that natural selection is the process responsible for the evo- if selection operating over a period of only 10,000 years lutionary changes documented in the fossil record. Some can produce such substantial differences, then it would critics of evolution accept that selection can lead to changes seem powerful enough, over the course of many millions within a species, but contend that such changes are rela of years, to produce the diversity of life we see around us tively minor in scope and not equivalent to the substantial today changes documented in the fossil record. In other words, it is one thing to change the number of bristles on a fruit fly or the size of a corn stalk, and quite another to produce an Artificial selection often leads to rapid and substantial entirely new species results over short periods of time, thus demonstrating This argument does not fully appreciate the extent of the power of selection to produce major evolutionary hans change produced by artificial selection. Consider, for ex Chapter 21 The Evidence for Evolution 449
Domestication Artificial selection has also been responsible for the great variety of breeds of cats, dogs (figure 21.14), pigeons, cattle and other domestic animals. In some cases, breeds have been developed for particular purposes. Greyhound dogs, for example, were bred by selecting for maximal running abilities, with the end result being an animal with long legs and tail (the latter used as a rudder), an arched back (to increase the length of its stride), and great muscle mass. By contrast, the odd proportions of the ungainly basset hound resulted from selection for dogs that could enter narrow holes in pursuit of rabbits and other small game. In other cases, breeds have been developed primarily for their appearance, such as the many colorful and ornamented varieties of pigeons or the breeds of cats. Domestication also has led to unintentional selection for some traits. In recent years, as part of an attempt to domesticate the silver fox, Russian scientists each generation have chosen the most docile animals and allowed them to reproduce. Within 40 years, the vast majority of foxes born were exceptionally docile, not only allowing themselves to be petted, but also whimpering to get attention and sniffing and licking their caretakers. In many respects, they had become no different than domestic dogs! However, it was not only behavior that changed. These foxes also began to exhibit different color patterns, floppy ears, curled tails, and shorter legs and tails. Presumably, the genes responsible for docile behavior have other effects as well (the phenomenon of pleiotropy discussed in the last chapter); as selection has favored docile animals, it has also led to the evolution of these other traits. Can Selection Produce Major Evolutionary Changes? Given that we can observe the results of selection operating over relatively short periods of time, most scientists believe that natural selection is the process responsible for the evolutionary changes documented in the fossil record. Some critics of evolution accept that selection can lead to changes within a species, but contend that such changes are relatively minor in scope and not equivalent to the substantial changes documented in the fossil record. In other words, it is one thing to change the number of bristles on a fruit fly or the size of a corn stalk, and quite another to produce an entirely new species. This argument does not fully appreciate the extent of change produced by artificial selection. Consider, for example, the breeds of dogs, all of which have been produced since wolves were first domesticated, perhaps 10,000 years ago. If the various dog breeds did not exist and a paleontologist found fossils of animals similar to dachshunds, greyhounds, mastiffs, Chihuahuas, and pomeranians, there is no question that they would be considered different species. Indeed, these breeds are so different that they would probably be classified in different genera. In fact, the diversity exhibited by dog breeds far outstrips the differences observed among wild members of the family Canidae—such as coyotes, jackals, foxes, and wolves. Consequently, the claim that artificial selection produces only minor changes is clearly incorrect. Indeed, if selection operating over a period of only 10,000 years can produce such substantial differences, then it would seem powerful enough, over the course of many millions of years, to produce the diversity of life we see around us today. Artificial selection often leads to rapid and substantial results over short periods of time, thus demonstrating the power of selection to produce major evolutionary change. Chapter 21 The Evidence for Evolution 449 Greyhound Mastiff Dachshund Chihuahua FIGURE 21.14 Breeds of dogs. The differences between these dogs are greater than the differences displayed between any wild species of canids
21.3 Evidence for evolution can be found in other fields of biology. The anatomical Re ecorc Much of the power of the theory of evolution is its ability to provide a sensible framework for understanding the diversity of life. Many observa- ons from a wide variety of fields of biology simply cannot be understood in any meaningful way except as a re- sult of evolution As vertebrates evolved. the same Cat Horse bones were sometimes put to differ ent uses. Yet the bones are still seen. FIGURE 21.15 their presence betraying their evolu- Homology among the bones of the forelimb. Although these structures show ary past. For example the fore considerable differences in form and function, the same basic bones are present in limbs of vertebrates are all homolo- the forelimbs of humans, cats, bats, porpoises, and horses gous structures, that is, structures th different appearances and func tions that all derived from the same body part in a common ancestor can see in figure 21.15 how the bones Gill slits Gill sl of the forelimb have been modified different ways for different verter- bates. Why should these very differ. ent structures be composed of the same bones? if evolution had not oc- curred. this would indeed be a riddle. But when we consider that all of these animals are descended from a common ancestor, it is easy to under stand that natural selection has modi- fied the same initial starting blocks to Some of the strongest anatomical evi de pporting evolution ce Human from comparisons of how organism FIGURE 21.16 tionary history of an organism can be Our embryos show or evolutionary history. The embryos ofvarious groups of seen to unfold during its develop- gill slits(in purple)and a tail ment,with the embryo exhibiting characteristics of the embryos of its ancestors(figure 21. 16). For example, early in their development, human embryos possess gill ossess a fine fur(called lanugo) during the fifth month of as the coccyx at the end of our spine. Human fetuses even structions layered on top of old ones evolved forms suggest slits, like a fish; at a later stage, every human embryo has a development. These relict developmental long bony tail, the vestige of which we carry to adulthood strongly that our development has 450 Part VI Evolution
The Anatomical Record Much of the power of the theory of evolution is its ability to provide a sensible framework for understanding the diversity of life. Many observations from a wide variety of fields of biology simply cannot be understood in any meaningful way except as a result of evolution. Homology As vertebrates evolved, the same bones were sometimes put to different uses. Yet the bones are still seen, their presence betraying their evolutionary past. For example, the forelimbs of vertebrates are all homologous structures, that is, structures with different appearances and functions that all derived from the same body part in a common ancestor. You can see in figure 21.15 how the bones of the forelimb have been modified in different ways for different verterbates. Why should these very different structures be composed of the same bones? If evolution had not occurred, this would indeed be a riddle. But when we consider that all of these animals are descended from a common ancestor, it is easy to understand that natural selection has modified the same initial starting blocks to serve very different purposes. Development Some of the strongest anatomical evidence supporting evolution comes from comparisons of how organisms develop. In many cases, the evolutionary history of an organism can be seen to unfold during its development, with the embryo exhibiting characteristics of the embryos of its ancestors (figure 21.16). For example, early in their development, human embryos possess gill slits, like a fish; at a later stage, every human embryo has a long bony tail, the vestige of which we carry to adulthood as the coccyx at the end of our spine. Human fetuses even possess a fine fur (called lanugo) during the fifth month of development. These relict developmental forms suggest strongly that our development has evolved, with new instructions layered on top of old ones. 450 Part VI Evolution 21.3 Evidence for evolution can be found in other fields of biology. Human Cat Bat Porpoise Horse FIGURE 21.15 Homology among the bones of the forelimb. Although these structures show considerable differences in form and function, the same basic bones are present in the forelimbs of humans, cats, bats, porpoises, and horses. Gill slits Tail Fish Reptile Bird Human Tail Gill slits FIGURE 21.16 Our embryos show our evolutionary history. The embryos of various groups of vertebrate animals show the features they all share early in development, such as gill slits (in purple) and a tail
The observation that seemingly different organisms may exhibit similar embryological forms pro- vides indirect but convincing evi- dence of a past evolutionary rela- tionship Slugs and squids, for example, do not bear much superficial resemblance to their embryological forms pro- vides convincing evidence that they are both mollusks Vestigial Structures Many organisms possess vestigial FIGURE 21.17 structures that have no apparent Vestigial features. The skeleton of a baleen whale, a representative of the group of function, but that resemble struc- mammals that contains the largest living species, contains pelvic bones. These bones tures their presumed ancestors resemble those of other mammals, but are only weakly developed in the whale and have had. Humans, for example, possess no apparent function a complete set of muscles for gling their ears, just as a coyote does(table 21.1). Boa constrictors have hip bones and rudimentary hind legs. Manatees(a moval, the appendix may burst, allowing the contents of type of aquatic mammal often referred to as"sea cows") the gut to come in contact with the lining of the body cav have fingernails on their fins(which evolved from legs). ity, a potentially fatal event. It is difficult to understand ves- Figure 21 17 illustrates the skeleton of a baleen whale, tigial structures such as these as anything other than evolu which contains pelvic bones, as other mammal skeletons tionary relicts, holdovers from the evolutionary past. The do, even though such bones serve no known function in the argue strongly for the common ancestry of the members of whale.The human vermiform appendix is apparently vesti the groups that share them, regardless of how different gial; it represents the degenerate terminal part of the they have subsequently become ecum, the blind pouch or sac in which the large intestine begins.In other mammals such as mice, the cecum is the Comparisons of the anatomy of different living animals largest part of the large intestine and functions in storage- often reveal evidence of shared ancestry. In some usually of bulk cellulose in herbivores. Although some sug instances, the same organ has evolved to carry out gestions have been made, it is difficult to assign any current different functions, in others, an organ loses its function function to the vermiform appendix. In many respects, it is altogether. Sometimes, different organs evolve in dangerous organ: quite often it becomes infected, leading similar ways when exposed to the same selective to an inflammation called appendicitis; without surgical re- pressures able 21.1 Some Vestigial Traits in Humans Trait Ear-wiggling muscles Three small muscles around each ear that are large and important in some mammals, such as dogs, turning the ears toward a source of sound. Few people can wiggle their ears, and none can turn them toward Tail Present in human and all vertebrate embryos. In humans, the tail is reduced; most adults only have three to five tiny tail bones and, occasionally, a trace of a tail-extending muscle Structure which presumably had a digestive function in some of our ancestors, like the cecum of some herbivores. In humans, it varies in length from 5-15 cm, and some people are born without one Wisdom teeth Molars that are often useless and sometimes even trapped in the jawbone. Some people never develop Based on a suggestion by Dr Leslie Dendy, Department of Science and Technology, University of New Mexico, Los Alamos. Chapter 21 The Evidence for Evolution 451
The observation that seemingly different organisms may exhibit similar embryological forms provides indirect but convincing evidence of a past evolutionary relationship. Slugs and giant ocean squids, for example, do not bear much superficial resemblance to each other, but the similarity of their embryological forms provides convincing evidence that they are both mollusks. Vestigial Structures Many organisms possess vestigial structures that have no apparent function, but that resemble structures their presumed ancestors had. Humans, for example, possess a complete set of muscles for wiggling their ears, just as a coyote does (table 21.1). Boa constrictors have hip bones and rudimentary hind legs. Manatees (a type of aquatic mammal often referred to as “sea cows”) have fingernails on their fins (which evolved from legs). Figure 21.17 illustrates the skeleton of a baleen whale, which contains pelvic bones, as other mammal skeletons do, even though such bones serve no known function in the whale. The human vermiform appendix is apparently vestigial; it represents the degenerate terminal part of the cecum, the blind pouch or sac in which the large intestine begins. In other mammals such as mice, the cecum is the largest part of the large intestine and functions in storage— usually of bulk cellulose in herbivores. Although some suggestions have been made, it is difficult to assign any current function to the vermiform appendix. In many respects, it is a dangerous organ: quite often it becomes infected, leading to an inflammation called appendicitis; without surgical removal, the appendix may burst, allowing the contents of the gut to come in contact with the lining of the body cavity, a potentially fatal event. It is difficult to understand vestigial structures such as these as anything other than evolutionary relicts, holdovers from the evolutionary past. They argue strongly for the common ancestry of the members of the groups that share them, regardless of how different they have subsequently become. Comparisons of the anatomy of different living animals often reveal evidence of shared ancestry. In some instances, the same organ has evolved to carry out different functions, in others, an organ loses its function altogether. Sometimes, different organs evolve in similar ways when exposed to the same selective pressures. Chapter 21 The Evidence for Evolution 451 FIGURE 21.17 Vestigial features. The skeleton of a baleen whale, a representative of the group of mammals that contains the largest living species, contains pelvic bones. These bones resemble those of other mammals, but are only weakly developed in the whale and have no apparent function. Table 21.1 Some Vestigial Traits in Humans Trait Description Ear-wiggling muscles Three small muscles around each ear that are large and important in some mammals, such as dogs, turning the ears toward a source of sound. Few people can wiggle their ears, and none can turn them toward sound. Tail Present in human and all vertebrate embryos. In humans, the tail is reduced; most adults only have three to five tiny tail bones and, occasionally, a trace of a tail-extending muscle. Appendix Structure which presumably had a digestive function in some of our ancestors, like the cecum of some herbivores. In humans, it varies in length from 5–15 cm, and some people are born without one. Wisdom teeth Molars that are often useless and sometimes even trapped in the jawbone. Some people never develop wisdom teeth. Based on a suggestion by Dr. Leslie Dendy, Department of Science and Technology, University of New Mexico, Los Alamos
The Molecular record Traces of our evolutionary past are also evident at the molecular level. If you think about it, the fact that organ Human Bird isms have evolved successively from Lamprey relatively simple ancestors implies that a record of evolutionary change is pre- sent in the cells of each of us in our DNA. When an ancestral species gives rise to two or more descendants. those descendants will initially exhibit fairl high overall similarity in their DNA However, as the descendants evolve in- dependently, they will accumulate more and more differences in their DNA. Consequently, organisms that are more distantly related would be ex- pected to accumulate a greater number of evolutionary differences, whereas two species that are more closely re lated should share a greater portion of their dna To examine this hypothesis, we Number of amino acid differences between this hemoglobin polypeptide and a human one need an estimate of evolutionary rela- tionships that has been developed FIGURE 21.18 from data other than DNA (it would Molecules reflect evolutionary divergence. You can see that the greater the be a circular argument to use DNa to amino acid differences in the vertebrate hemoglobin polypeptide the number of estimate relationships and then con- clude that closely related species are more similar in their dna than are distantly related species). Such an hypothesis of evolu Why should closely related species be similar in DNA tionary relationships is provided by the fossil record, Because DNA is the genetic code that produces the struc- which indicates when particular types of organisms ture of living organisms, one might expect species that are evolved. In addition, by examining the anatomical struc- similar in overall appearance and structure, such as humans tures of fossils and of modern species, we can infer how and chimpanzees, to be more similar in DNA than are closely species are related to each other more dissimilar species, such as humans and frogs. This ex When degree of genetic similarity is compared with pectation would hold true even if evolution had not oc- our ideas of evolutionary relationships based on fossils, a lowever, moglobin polypeptide is compared to the corresponding tion and appear to serve no purpose. If evolution had not molecule in other species, closely related species are occurred, there would be no reason to expect simila found to be more similar. Chimpanzees, gorillas, orang- appearing species to be similar in their junk DNA. How- itans, and macaques, vertebrates thought to be more ever, comparisons of such stretches of dNA provide the closely related to humans, have fewer differences from same results as for other parts of the genome: more closely humans in the 146-amino-acid hemoglobin B chain than related species are more similar, an observation that only do more distantly related mammals, like dogs. Nonmam makes sense if evolution has occurred malian vertebrates differ even more. and nonvertebrate hemoglobins are the most different of all(figure 21.18 Similar patterns are also evident when the DNa itself Comparison of the DNA of different species provides compared. For example, chimps and humans, which are strong evidence for evolution. Species deduced from thought to have descended from a common ancestor that the fossil record to be closely related are more similar ived approximately 6 million years ago, exhibit few differ in their dNA than are species thought to be more ences in their dna distantly related. 452 Part vI Evolution
The Molecular Record Traces of our evolutionary past are also evident at the molecular level. If you think about it, the fact that organisms have evolved successively from relatively simple ancestors implies that a record of evolutionary change is present in the cells of each of us, in our DNA. When an ancestral species gives rise to two or more descendants, those descendants will initially exhibit fairly high overall similarity in their DNA. However, as the descendants evolve independently, they will accumulate more and more differences in their DNA. Consequently, organisms that are more distantly related would be expected to accumulate a greater number of evolutionary differences, whereas two species that are more closely related should share a greater portion of their DNA. To examine this hypothesis, we need an estimate of evolutionary relationships that has been developed from data other than DNA (it would be a circular argument to use DNA to estimate relationships and then conclude that closely related species are more similar in their DNA than are distantly related species). Such an hypothesis of evolutionary relationships is provided by the fossil record, which indicates when particular types of organisms evolved. In addition, by examining the anatomical structures of fossils and of modern species, we can infer how closely species are related to each other. When degree of genetic similarity is compared with our ideas of evolutionary relationships based on fossils, a close match is evident. For example, when the human hemoglobin polypeptide is compared to the corresponding molecule in other species, closely related species are found to be more similar. Chimpanzees, gorillas, orangutans, and macaques, vertebrates thought to be more closely related to humans, have fewer differences from humans in the 146-amino-acid hemoglobin β chain than do more distantly related mammals, like dogs. Nonmammalian vertebrates differ even more, and nonvertebrate hemoglobins are the most different of all (figure 21.18). Similar patterns are also evident when the DNA itself is compared. For example, chimps and humans, which are thought to have descended from a common ancestor that lived approximately 6 million years ago, exhibit few differences in their DNA. Why should closely related species be similar in DNA? Because DNA is the genetic code that produces the structure of living organisms, one might expect species that are similar in overall appearance and structure, such as humans and chimpanzees, to be more similar in DNA than are more dissimilar species, such as humans and frogs. This expectation would hold true even if evolution had not occurred. However, there are some noncoding stretches of DNA (sometimes called “junk DNA”) that have no function and appear to serve no purpose. If evolution had not occurred, there would be no reason to expect similarappearing species to be similar in their junk DNA. However, comparisons of such stretches of DNA provide the same results as for other parts of the genome: more closely related species are more similar, an observation that only makes sense if evolution has occurred. Comparison of the DNA of different species provides strong evidence for evolution. Species deduced from the fossil record to be closely related are more similar in their DNA than are species thought to be more distantly related. 452 Part VI Evolution Number of amino acid differences between this hemoglobin polypeptide and a human one 10 20 30 40 50 60 70 67 125 45 32 8 80 90 100 110 120 Time Human Macaque Dog Bird Frog Lamprey FIGURE 21.18 Molecules reflect evolutionary divergence. You can see that the greater the evolutionary distance from humans (white cladogram), the greater the number of amino acid differences in the vertebrate hemoglobin polypeptide
Convergent and Niche Placental Mammal Australian Marsupials Divergent evolution Different geographical areas some-Burrower Marsupial mole times exhibit groups of plants and an nals of strikingly Mole even though the organisms may be only distantly related. It is difficult to Lesser anteater Numbat (anteater) explain so many similarities as the re Anteater sult of coincidence. Instead. natural selection appears to have favored par- allel evolutionary adaptations in simi lar environments because selection Marsupial in these instances has tended to favor mouse changes that made the two groups M ouse more alike, their phenotypes have converged. This form of evolutionary change is referred to as convergent Climber evolution, or sometimes, parallel Lemu evolution The Marsupial-Placental Convergence In the best-known case of conver Flying squirrel Flying phal langer gent evolution, two major groups of mammals, marsupials and placentals, have evolved in a very similar way, even though the two lineages have Ocelot Tasmanian"tiger cat been living independently on sepa rate continents. Australia separated from the other continents more than 50 million years ago, after marsupi als had evolved but before the ap- Tasmanian pearance of placental mammals. As a result, the only mammals in Aus- tralia(other than bats and a few col nizing rodents) have been marsupi FIGURE 21 19 ls, members of a group in which the Convergent evolution. Marsupials in Australia resemble placental mammals in the young are born in a very immature rest of the world. They evolved in isolation after Australia separated from other condition and held in a pouch until continents they are ready to emerge into the utside world. Thus, even though placental mammals are the dominant mammalian group Homology versus Analogy throughout most of the world, marsupials retained su- premacy in Australia How do we know when two similar characters are gous and when they are analogous? As we have seer shing degree, they resemble marsupials like? To an aston- tation favoring different functions can obscure hom What are the australian the placental mammals living today on the other continents(figure 21. 19). The similarity while convergent evolution can create analogues that ap- between some individual members of these two sets of pear as similar as homologues. There is no hard-and-fast answer to this question; the determination of homologues mammals argues strongly that they are the result of conver- is often a thorny issue in biological classification. as isolated areas because of similar selective pressures in simi lar environments omparing slugs and squids, studies of embryonic develop ment often reveal features not apparent when studying adult organisms. In general, the more complex two struc tures are, the less likely they evolved independently Chapter 21 The Evidence for Evolution 453
Convergent and Divergent Evolution Different geographical areas sometimes exhibit groups of plants and animals of strikingly similar appearance, even though the organisms may be only distantly related. It is difficult to explain so many similarities as the result of coincidence. Instead, natural selection appears to have favored parallel evolutionary adaptations in similar environments. Because selection in these instances has tended to favor changes that made the two groups more alike, their phenotypes have converged. This form of evolutionary change is referred to as convergent evolution, or sometimes, parallel evolution. The Marsupial-Placental Convergence In the best-known case of convergent evolution, two major groups of mammals, marsupials and placentals, have evolved in a very similar way, even though the two lineages have been living independently on separate continents. Australia separated from the other continents more than 50 million years ago, after marsupials had evolved but before the appearance of placental mammals. As a result, the only mammals in Australia (other than bats and a few colonizing rodents) have been marsupials, members of a group in which the young are born in a very immature condition and held in a pouch until they are ready to emerge into the outside world. Thus, even though placental mammals are the dominant mammalian group throughout most of the world, marsupials retained supremacy in Australia. What are the Australian marsupials like? To an astonishing degree, they resemble the placental mammals living today on the other continents (figure 21.19). The similarity between some individual members of these two sets of mammals argues strongly that they are the result of convergent evolution, similar forms having evolved in different, isolated areas because of similar selective pressures in similar environments. Homology versus Analogy How do we know when two similar characters are homologous and when they are analogous? As we have seen, adaptation favoring different functions can obscure homologies, while convergent evolution can create analogues that appear as similar as homologues. There is no hard-and-fast answer to this question; the determination of homologues is often a thorny issue in biological classification. As we have seen in comparing vertebrate embryos, and again in comparing slugs and squids, studies of embryonic development often reveal features not apparent when studying adult organisms. In general, the more complex two structures are, the less likely they evolved independently. Chapter 21 The Evidence for Evolution 453 Niche Placental Mammals Australian Marsupials Burrower Mole Lesser anteater Mouse Lemur Flying squirrel Ocelot Wolf Tasmanian wolf Tasmanian "tiger cat" Flying phalanger Spotted cuscus Numbat (anteater) Marsupial mole Marsupial mouse Anteater Mouse Climber Glider Cat Wolf FIGURE 21.19 Convergent evolution. Marsupials in Australia resemble placental mammals in the rest of the world. They evolved in isolation after Australia separated from other continents