FIGURE 21.20 A Galapagos tortoise most closely resembles South American tortoises. Isolated on these remote islands, the galapagos tortoise has evolved distinctive forms. This natural experiment is being terminated, however. Since Darwins time, much of the hatural habitat of the larger islands has been destroyed by human intrusion. Goats introduced by settlers, for example, have drastically altered the vegetation D alwin an d Patterns of recent divergence shells while others living in dry places have low, saddle 地m1m乙即论 he the head and neck. Similarly, the Galapagos find由ehre plants living on oceanic islands resemble most forms on the nearest continent-a relatie makes sense as reflecting common a he galat only evolved from a single presumptive ancestor into 13 species, each specialized in a different way. These Galapagos turtles to South american and finches have evolved in concert with the continental rtles than to those of any other continent. This kind of forms, from the same ancestors, but the two lineages have relationship strongly suggests that the island forms evolved from individuals that came from the adjacent mainland at It is fair to ask how Darwin knew that the galapagos ome time in the past. Thus, the galapagos finches of fig tortoises and finches do not represent the convergence of ure 21.8 have different beaks than their South American unrelated island and continental forms(analogues) rather han the divergence of recently isolated groups(homo- relatives. In the absence of evolution, there seems to be no logues). While either hypothesis would argue for natural logical explanation of why individual kinds of island plants and animals would be clearly related to others on the near selection, Darwin chose divergence of homologues as by far est mainland, but still have some divergent features. As the simplest explanation, because the turtles and finches Darwin pointed out, this relationship provides strong evi differ by only a few traits, and are similar in many dence that macroevolution has occurred In sum total. the evidence for macroevolution is over A similar resemblance to mainland birds can be seen in whelming. In the next chapter, we will consider Darwins an island finch Darwin never saw-a solitary finch species to macroevolutionary changes, the key argument in his the- cated 630 kilometers to the northeast of the galapagos ory that evolution occurs by natural selection This finch does not resemble the finches of Europe, Aus- tralia. Africa. or north america. Instead it resembles the Evolution favors similar forms under similar hes of Costa Rica. 500 kilometers to the east circumstances. Convergence is the evolution of similar <. Of course, because of adaptation to localized habitats,is- forms in different lineages when exposed to the same nd forms are not identical to those on the nearby conti nents. The turtles have evolved different shell shapes, for different forms in the same lineage when exposed to example; those living in moist habitats have dome-shaped different selective pressures 454 Part vI Evolution
Darwin and Patterns of Recent Divergence Darwin was the first to present evidence that animals and plants living on oceanic islands resemble most closely the forms on the nearest continent—a relationship that only makes sense as reflecting common ancestry. The Galápagos turtle in figure 21.20 is more similar to South American turtles than to those of any other continent. This kind of relationship strongly suggests that the island forms evolved from individuals that came from the adjacent mainland at some time in the past. Thus, the Galápagos finches of figure 21.8 have different beaks than their South American relatives. In the absence of evolution, there seems to be no logical explanation of why individual kinds of island plants and animals would be clearly related to others on the nearest mainland, but still have some divergent features. As Darwin pointed out, this relationship provides strong evidence that macroevolution has occurred. A similar resemblance to mainland birds can be seen in an island finch Darwin never saw—a solitary finch species living on Cocos Island, a tiny, remote volcanic island located 630 kilometers to the northeast of the Galápagos. This finch does not resemble the finches of Europe, Australia, Africa, or North America. Instead, it resembles the finches of Costa Rica, 500 kilometers to the east. Of course, because of adaptation to localized habitats, island forms are not identical to those on the nearby continents. The turtles have evolved different shell shapes, for example; those living in moist habitats have dome-shaped shells while others living in dry places have low, saddlebacked shells with the front of the shell bent up to expose the head and neck. Similarly, the Galápagos finches have evolved from a single presumptive ancestor into 13 species, each specialized in a different way. These Galápagos turtles and finches have evolved in concert with the continental forms, from the same ancestors, but the two lineages have diverged rather than converged. It is fair to ask how Darwin knew that the Galápagos tortoises and finches do not represent the convergence of unrelated island and continental forms (analogues) rather than the divergence of recently isolated groups (homologues). While either hypothesis would argue for natural selection, Darwin chose divergence of homologues as by far the simplest explanation, because the turtles and finches differ by only a few traits, and are similar in many. In sum total, the evidence for macroevolution is overwhelming. In the next chapter, we will consider Darwin’s proposal that microevolutionary changes have led directly to macroevolutionary changes, the key argument in his theory that evolution occurs by natural selection. Evolution favors similar forms under similar circumstances. Convergence is the evolution of similar forms in different lineages when exposed to the same selective pressures. Divergence is the evolution of different forms in the same lineage when exposed to different selective pressures. 454 Part VI Evolution FIGURE 21.20 A Galápagos tortoise most closely resembles South American tortoises. Isolated on these remote islands, the Galápagos tortoise has evolved distinctive forms. This natural experiment is being terminated, however. Since Darwin’s time, much of the natural habitat of the larger islands has been destroyed by human intrusion. Goats introduced by settlers, for example, have drastically altered the vegetation
21. 4 The theory of evolution has proven controversial Darwin's critics this way, the probability that everyone in a class of 50 would have the birthdays they do is(1/365)5, and yet In the century since he proposed it, Darwin's theory of here the class sits evolution by natural selection has become nearly univer- 6. Natural selection does not imply evolution. "No sally accepted by biologists, but has proven controversial scientist bas come up with an experiment wbere fisb evolve mong the general public. Darwins critics raise seven pr into frogs and leap away from predators. Is microevolu cipal objections to teaching evolution tion(evolution within a species) the mechanism that 1. Evolution is not solidly demonstrated " Evolution has produced macroevolution(evolution among is just a theory, "Darwins critics point out, as if theory species)? Most biologists that have studied the prob- meant lack of knowledge, some kind of guess. Scien- lem think so. Some kinds of animals produced by ar- tists. however, use the word the a very different tificial selection are remarkably distinctive, such as sense than the general public does. Theories are the Chihuahuas, dachshunds, and greyhounds. While all solid ground of science, that of which we are most dogs are in fact the same species and can interbreed certain. Few of us doubt the theory of gravity because laboratory selection experiments easily create forms it is"just a the at cannot interbreed and thus would in nature be 2. There are no fossil intermediates. "No one ever considered different species. Thus, production of rad saw a fin on the way to becoming a leg, "critics claim cally different forms has indeed been observed. re- pointing to the many gaps in the fossil record in Dar peatedly. To object that evolution still does not ex wins day. Since then, however, most fossil intermedi plain really differences. like between fish and ates in vertebrate evolution have indeed been found amphibians, simply takes us back to point 2-these a clear line of fossils now traces the transition be hanges take millions of years, and are seen clearly in tween whales and hoofed mammals, between reptiles the fossil record and mammals. between dinosaurs and birds. between 7. The irreducible complexity argument. Tbe in- apes and humans. The fossil evidence of evolution tricate molecular macbinery of the cell cannot be ex- between major forms is compelling plained by evolution from simpler stages. Because each 3. The intelligent design argument. Tbe organs of part of a complex cellular process like blood clotting is es- living creatures are too complex for a random process to sential to the overall process, bow can natural selection bave produced-the existence of a clock is evidence of the fashion any one part? What's wrong with this argu- existence of a clockmaker. "Biologists do not agree. ment is that each part of a complex molecular ma The intermediates in the evolution of the mam chine evolves as part of the system. Natural selection malian ear can be seen in fossils, and many interme- can act on a complex system because at every stage diate"eyes"are known in various invertebrates of its evolution, the system functions. Parts that im These intermediate forms arose because they have prove function are added, and, because of later alue-being able to detect light a little is better hanges, become essential. The mammalian blood than not being able to detect it at all. Complex clotting system, for example, has evolved from much structures like eyes evolved as a progression of slight SImpler ems stem evolved at the dawn of the vertebrates 600 million years ago, Improvements 4. Evolution violates the Second Law of Thermody and is found today in lampreys, the most primitive namics. "A jumble of soda cans doesn't by itself jump fish. One hundred million years later, as vertebrates neatly into a stack-things become more disorganized due evolved, proteins were added to the clotting system to random events, not more organized. "Biologists point making it sensitive to substances released from dam out that this argument ignores what the second law aged tissues. Fifty million years later, a third compo- really says: disorder increases in a closed system, ent was added, triggering clotting by contact with which the earth most certainly is not. Energy contin he jagged surfaces produced by injury. At each and all the processes that organize i: sun, fueling life stage as the clotting system evolved to become more complex, its overall performance came to depend on 5. Proteins are too improbable. "Hemoglobin bas 141 the added elements. Thus, blood clotting has be- amino acids. The probability that the first one would be come"irreducibly complex"-as the result of Dar leucine is 1/20, and that all 141 would be the ones they are winian evolution by chance is(1/20)4, an impossibly rare event. "This is statistical foolishness--you cannot use probability to Darwins theory of evolution has proven controversial argue backwards. The probability that a student in a among the general public, although the commonly classroom has a particular birthday is 1/365; arguing raised objections are without scientific merit Chapter 21 The Evidence for Evolution 455
Darwin’s Critics In the century since he proposed it, Darwin's theory of evolution by natural selection has become nearly universally accepted by biologists, but has proven controversial among the general public. Darwin's critics raise seven principal objections to teaching evolution: 1. Evolution is not solidly demonstrated. “Evolution is just a theory,” Darwin's critics point out, as if theory meant lack of knowledge, some kind of guess. Scientists, however, use the word theory in a very different sense than the general public does. Theories are the solid ground of science, that of which we are most certain. Few of us doubt the theory of gravity because it is "just a theory." 2. There are no fossil intermediates. “No one ever saw a fin on the way to becoming a leg,” critics claim, pointing to the many gaps in the fossil record in Darwin's day. Since then, however, most fossil intermediates in vertebrate evolution have indeed been found. A clear line of fossils now traces the transition between whales and hoofed mammals, between reptiles and mammals, between dinosaurs and birds, between apes and humans. The fossil evidence of evolution between major forms is compelling. 3. The intelligent design argument. “The organs of living creatures are too complex for a random process to have produced—the existence of a clock is evidence of the existence of a clockmaker.” Biologists do not agree. The intermediates in the evolution of the mammalian ear can be seen in fossils, and many intermediate “eyes” are known in various invertebrates. These intermediate forms arose because they have value—being able to detect light a little is better than not being able to detect it at all. Complex structures like eyes evolved as a progression of slight improvements. 4. Evolution violates the Second Law of Thermodynamics. “A jumble of soda cans doesn't by itself jump neatly into a stack—things become more disorganized due to random events, not more organized.” Biologists point out that this argument ignores what the second law really says: disorder increases in a closed system, which the earth most certainly is not. Energy continually enters the biosphere from the sun, fueling life and all the processes that organize it. 5. Proteins are too improbable. “Hemoglobin has 141 amino acids. The probability that the first one would be leucine is 1/20, and that all 141 would be the ones they are by chance is (1/20)141, an impossibly rare event.” This is statistical foolishness—you cannot use probability to argue backwards. The probability that a student in a classroom has a particular birthday is 1/365; arguing this way, the probability that everyone in a class of 50 would have the birthdays they do is (1/365)50, and yet there the class sits. 6. Natural selection does not imply evolution. “No scientist has come up with an experiment where fish evolve into frogs and leap away from predators.” Is microevolution (evolution within a species) the mechanism that has produced macroevolution (evolution among species)? Most biologists that have studied the problem think so. Some kinds of animals produced by artificial selection are remarkably distinctive, such as Chihuahuas, dachshunds, and greyhounds. While all dogs are in fact the same species and can interbreed, laboratory selection experiments easily create forms that cannot interbreed and thus would in nature be considered different species. Thus, production of radically different forms has indeed been observed, repeatedly. To object that evolution still does not explain really major differences, like between fish and amphibians, simply takes us back to point 2—these changes take millions of years, and are seen clearly in the fossil record. 7. The irreducible complexity argument. The intricate molecular machinery of the cell cannot be explained by evolution from simpler stages. Because each part of a complex cellular process like blood clotting is essential to the overall process, how can natural selection fashion any one part? What's wrong with this argument is that each part of a complex molecular machine evolves as part of the system. Natural selection can act on a complex system because at every stage of its evolution, the system functions. Parts that improve function are added, and, because of later changes, become essential. The mammalian blood clotting system, for example, has evolved from much simpler systems. The core clotting system evolved at the dawn of the vertebrates 600 million years ago, and is found today in lampreys, the most primitive fish. One hundred million years later, as vertebrates evolved, proteins were added to the clotting system making it sensitive to substances released from damaged tissues. Fifty million years later, a third component was added, triggering clotting by contact with the jagged surfaces produced by injury. At each stage as the clotting system evolved to become more complex, its overall performance came to depend on the added elements. Thus, blood clotting has become "irreducibly complex"—as the result of Darwinian evolution. Darwin’s theory of evolution has proven controversial among the general public, although the commonly raised objections are without scientific merit. Chapter 21 The Evidence for Evolution 455 21.4 The theory of evolution has proven controversial
Chapter 21 http://www.mhhe.com/raven6ehttp://www.biocourse.com Questions Media resource 21.1 Fossil evidence indicates that evolution has occurred Fossils of many extinct species have never been 1. Why do gaps exist in the fossil On Science article: discovered. Nonetheless, the fossil record is complete record: What lessons can be feather Dinosaurs enough to allow a detailed understanding of the learned from the fossil record of Book reviews: horse evolution In Search of Deep Tin evolution of life through time. The evolution of the major vertebrate groups is quite w 2. How did scientists date fossils Digging Dinosaurs by in Darwin' s day why are Although evolution of groups like horses may appear scientists today able to date Horner to be a straight-line progression, in fact there have rocks more accurately? of parallel evolution, and even 21.2 Natural selection can produce evolutionary change Natural populations provide cle ear eviden 3. Why did the average beak size Activity: Evolution of evolutionary change of the medium ground finch Darwin,s finches have different -sized beaks, which increase after a particularly dry are adaptations to eating different kinds of seeds. In particularly dry years, natural selection favors birds 4. Why did the frequency of th light-colored moths decrease a result, the average bill size becomes larger in the and that of dark-colored moths next generation increase with the advent of industrialism? What is industrial The British populations of the peppered moth, Biston melanism? betularia, consisted mostly of light-colored individuals 5. What can artificial selection before the Industrial revolution Over the last two tell us about evolution? Is centuries, populations that occur in heavily polluted artificial selection a good areas where the tree trunks are darkened with soot analogy for the selection that have come to consist mainly of dark-colored Irs in nature (melanic)individuals-a result of rapid natural 21.3 Evidence for evolution can be found in other fields of biology. Several indirect lines of evidence argue that 6. What is homology? How does macroevolution has occurred, including successive it support evolutionary theory Evolution of the hanges in homologous structures, developmental Heart 7. What is convergent patterns, vestigial structures, parallel patterns of evolution? Give examples. ·M evolution, and patterns of distribution 8. How did Darwin's studies of When differences in genes or proteins are examined, sland populations provic that are thought to be closely related based evidence for evolution the fossil record may be more similar than species Activity: Divergence thought to be distantly related ent Research On Science articles Darwinism at the Was darw 21.4 The theory of evolution has proven controversial. The objections raised by Darwins critics are easily 9.Is“ Darwinism” really science? On Science articl Book reviews. 456 Part vI Evolution Mr. Darwin's Shooter
456 Part VI Evolution Chapter 21 Summary Questions Media Resources 21.1 Fossil evidence indicates that evolution has occurred. • Fossils of many extinct species have never been discovered. Nonetheless, the fossil record is complete enough to allow a detailed understanding of the evolution of life through time. The evolution of the major vertebrate groups is quite well known. • Although evolution of groups like horses may appear to be a straight-line progression, in fact there have been many examples of parallel evolution, and even reversals from overall trends. 1. Why do gaps exist in the fossil record? What lessons can be learned from the fossil record of horse evolution? 2. How did scientists date fossils in Darwin’s day? Why are scientists today able to date rocks more accurately? • Natural populations provide clear evidence of evolutionary change. • Darwin’s finches have different-sized beaks, which are adaptations to eating different kinds of seeds. In particularly dry years, natural selection favors birds with stout beaks within one species, Geospiza fortis. As a result, the average bill size becomes larger in the next generation. • The British populations of the peppered moth, Biston betularia, consisted mostly of light-colored individuals before the Industrial Revolution. Over the last two centuries, populations that occur in heavily polluted areas, where the tree trunks are darkened with soot, have come to consist mainly of dark-colored (melanic) individuals—a result of rapid natural selection. 3. Why did the average beak size of the medium ground finch increase after a particularly dry year? 4. Why did the frequency of light-colored moths decrease and that of dark-colored moths increase with the advent of industrialism? What is industrial melanism? 5. What can artificial selection tell us about evolution? Is artificial selection a good analogy for the selection that occurs in nature? 21.2 Natural selection can produce evolutionary change. • Several indirect lines of evidence argue that macroevolution has occurred, including successive changes in homologous structures, developmental patterns, vestigial structures, parallel patterns of evolution, and patterns of distribution. • When differences in genes or proteins are examined, species that are thought to be closely related based on the fossil record may be more similar than species thought to be distantly related. 6. What is homology? How does it support evolutionary theory? 7. What is convergent evolution? Give examples. 8. How did Darwin’s studies of island populations provide evidence for evolution? 21.3 Evidence for evolution can be found in other fields of biology. • The objections raised by Darwin’s critics are easily answered. 9. Is “Darwinism” really science? Explain. 21.4 The theory of evolution has proven controversial. • On Science Article: featherd Dinosaurs Book Reviews: • In Search of Deep Time by Gee • Digging Dinosaurs by Horner • Activity: Evolution of Fish • Exploration: Evolution of the Heart • Molecular Clock • Activity: Divergence • Student Research: Evolution of Insect Diets On Science Articles: • Darwinism at the Cellular Level • Was Darwin Wrong? • On Science Article: Answering Evolution’s Critics • Bioethics Case Study: Creationism Book Reviews: • Mr. Darwin’s Shooter by McDonald http://www.mhhe.com/raven6e http://www.biocourse.com
22 The Origin of species Concept Outline 22.1 Species are the basic units of evolution. The Nature of Species. Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups and that maintain connectedness over geographic distances 22.2 Species maintain their genetic distinctiveness through barriers to reproduction. Prezygotic Isolating Mechanisms. Some breeding barriers prevent the formation of zygotes Postzygotic Isolating Mechanisms. Other breeding barriers prevent the proper development or reproduction of the zygote after it forms 22.3 We have learned a great deal about how species form Reproductive Isolation May Evolve as a By-Product of Evolutionary Change. Speciation can occur in the absence of natural selection, but reproductive isolation FIGURE 22.1 generally occurs more quickly when populations are a group of Galapagos iguanas bask in the sun on their adapting to different environments isolated island. How does geographic isolation contribute to the The Geography of Speciation. Speciation occurs most formation of new species readily when populations are geographically isolated. Sympatric speciation can occur by polyploidy and, perhaps, A Ithough Darwin titled his book On the Origin of Species, 22. 4 Clusters of species reflect rapid evolution e never actually discussed what he referred to as that mystery of mysteries"of how one species gives rise to an Darwin's Finches. Thirteen species of finches, all other. Rather, his argument concerned evolution by natural descendants of one ancestral finch, occupy diverse niches selection; that is, how one species evolves through time to Hawaiian Drosophila. More than a quarter of the worlds adapt to its changing environment. Although of fundamen fruit fly species are found on the Hawaian Islands Lake victoria Cichlid Fishes. Isolation has led to tal importance to evolutionary biology, the process of adap extensive species formation among these small fishes tation does not explain how one species becomes another New Zealand Alpine Buttercups. Repeated glaciations (figure 22. 1); much less can it explain how one species can have fostered waves of species formation in alpine plants give rise to many descendant species. As we shall see, adap- Diversity of Life through Time. The number of species tation may be involved in this process of speciation, but it has increased through time, despite a number of mass need not be extincton events The Pace of Evolution. The idea that evolution occurs in spurts is controversial Problems with the Biological Species Concept. This 457
457 22 The Origin of Species Concept Outline 22.1 Species are the basic units of evolution. The Nature of Species. Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups and that maintain connectedness over geographic distances. 22.2 Species maintain their genetic distinctiveness through barriers to reproduction. Prezygotic Isolating Mechanisms. Some breeding barriers prevent the formation of zygotes. Postzygotic Isolating Mechanisms. Other breeding barriers prevent the proper development or reproduction of the zygote after it forms. 22.3 We have learned a great deal about how species form. Reproductive Isolation May Evolve as a By-Product of Evolutionary Change. Speciation can occur in the absence of natural selection, but reproductive isolation generally occurs more quickly when populations are adapting to different environments. The Geography of Speciation. Speciation occurs most readily when populations are geographically isolated. Sympatric speciation can occur by polyploidy and, perhaps, by other means. 22.4 Clusters of species reflect rapid evolution. Darwin’s Finches. Thirteen species of finches, all descendants of one ancestral finch, occupy diverse niches. Hawaiian Drosophila. More than a quarter of the world’s fruit fly species are found on the Hawaiian Islands. Lake Victoria Cichlid Fishes. Isolation has led to extensive species formation among these small fishes. New Zealand Alpine Buttercups. Repeated glaciations have fostered waves of species formation in alpine plants. Diversity of Life through Time. The number of species has increased through time, despite a number of mass extinction events. The Pace of Evolution. The idea that evolution occurs in spurts is controversial. Problems with the Biological Species Concept. This concept is not as universal as previously thought. Although Darwin titled his book On the Origin of Species, he never actually discussed what he referred to as that “mystery of mysteries” of how one species gives rise to another. Rather, his argument concerned evolution by natural selection; that is, how one species evolves through time to adapt to its changing environment. Although of fundamental importance to evolutionary biology, the process of adaptation does not explain how one species becomes another (figure 22.1); much less can it explain how one species can give rise to many descendant species. As we shall see, adaptation may be involved in this process of speciation, but it need not be. FIGURE 22.1 A group of Galápagos iguanas bask in the sun on their isolated island. How does geographic isolation contribute to the formation of new species?
2.1 Species are the basic units of evolution The Nature of Species Occasionally, two species occur together that appear to be nearly identical, and are thus called sibling species. In Before we can discuss how one species gives rise to another, most cases, however, our inability to distinguish the two re- e need to understand exactly what a species is. Even flects our own reliance on vision as our primary sense though definition of what constitutes a species is of funda- When the mating calls or chemicals exuded by such species mental importance to evolutionary biology, this issue has are examined, they usually reveal great differences. In other still not been completely settled and is currently the subject words, even though we have trouble separating them, the of considerable research and debate. However, any concept animals themselves have no such difficulties! of a species must account for two phenomena: the distinc- tiveness of species that occur together at a single locality, and the connection that exists among populations of the Geographic Variation within Species same species that are geographically separated Within the units classified as species, populations that occur in different areas may be more or less distinct from one an- The Distinctiveness of Sympatric Species other. Such groups of distinctive individuals may be classi fied taxonomically as subspecies or varieties(the vague Put out a birdfeeder on your balcony or back porch and term"race"has a similar connotation, but is no longer com- you will attract a wide variety of different types of birds(es- monly used). In areas where these populations approach one pecially if you put out a variety of different kinds of foods ). another, individuals often exhibit combinations of features In the midwestern United States, for example, you migl characteristic of both populations. In other words, even routinely see cardinals, blue jays, downy woodpeckers, though geographically distant populations may appear dis- that are intermediate in their characteristics (figure 22/ons house finches-even hummingbirds in the summer(figure tinct, they usually are connected by intervening populated 22.2). Although it might take a few days of careful observa tion, you would soon be able to readily distinguish the many different species. The reason is that species that The Biological Species Concept "same"and patria for "species")are distinctive entities that What can account both for the distinctiveness of sympatric are phenotypically different, utilize different parts of the species and the connectedness of geographic populations of habitat, and behave separately. This observation is gener he same species? One obvious possibility is that each ally true not only for birds, but also for most other types of species exchanges genetic material only with other mem organisms in most places. bers of its species. If sympatric species commonly ex umminrgbtrd Northern cardinal Blue jay House finch FIGURE 22.2 Common birds in the midwestern United States. No one would doubt that these birds are distinct species. Each can be distinguished from the others by many ecological, behavioral, and phenotypic traits. 458 Part vI Evolution
Occasionally, two species occur together that appear to be nearly identical, and are thus called sibling species. In most cases, however, our inability to distinguish the two reflects our own reliance on vision as our primary sense. When the mating calls or chemicals exuded by such species are examined, they usually reveal great differences. In other words, even though we have trouble separating them, the animals themselves have no such difficulties! Geographic Variation within Species Within the units classified as species, populations that occur in different areas may be more or less distinct from one another. Such groups of distinctive individuals may be classified taxonomically as subspecies or varieties (the vague term “race” has a similar connotation, but is no longer commonly used). In areas where these populations approach one another, individuals often exhibit combinations of features characteristic of both populations. In other words, even though geographically distant populations may appear distinct, they usually are connected by intervening populations that are intermediate in their characteristics (figure 22.3). The Biological Species Concept What can account both for the distinctiveness of sympatric species and the connectedness of geographic populations of the same species? One obvious possibility is that each species exchanges genetic material only with other members of its species. If sympatric species commonly ex- 458 Part VI Evolution The Nature of Species Before we can discuss how one species gives rise to another, we need to understand exactly what a species is. Even though definition of what constitutes a species is of fundamental importance to evolutionary biology, this issue has still not been completely settled and is currently the subject of considerable research and debate. However, any concept of a species must account for two phenomena: the distinctiveness of species that occur together at a single locality, and the connection that exists among populations of the same species that are geographically separated. The Distinctiveness of Sympatric Species Put out a birdfeeder on your balcony or back porch and you will attract a wide variety of different types of birds (especially if you put out a variety of different kinds of foods). In the midwestern United States, for example, you might routinely see cardinals, blue jays, downy woodpeckers, house finches—even hummingbirds in the summer (figure 22.2). Although it might take a few days of careful observation, you would soon be able to readily distinguish the many different species. The reason is that species that occur together (termed sympatric from the Greek sym for “same” and patria for “species”) are distinctive entities that are phenotypically different, utilize different parts of the habitat, and behave separately. This observation is generally true not only for birds, but also for most other types of organisms in most places. 22.1 Species are the basic units of evolution. Northern cardinal Blue jay Downy woodpecker House finch Ruby-throated hummingbird FIGURE 22.2 Common birds in the midwestern United States. No one would doubt that these birds are distinct species. Each can be distinguished from the others by many ecological, behavioral, and phenotypic traits