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I.1 What Is ? Jonathan Losos

OUTLINE . The process in which individuals with a particular trait tend to leave more offspring in the 1. What is evolution? next than do individuals with a different 2. Evolution: Pattern versus process trait. 3. Evolution: More than changes in the pool Approximately 375 million years ago, a large and vague- 4. In the light of evolution ly salamander-like creature plodded from its aquatic 5. Critiques and the evidence for evolution home and began the invasion of land, setting 6. The pace of evolution forth the chain of evolutionary events that led to the 7. Evolution, , and that fill our skies, the beasts that walk our soil, me writing this chapter, and you reading it. This was, of course, just Evolution refers to change through as be- one episode in ’s saga: millions of years earlier, plants come modified and diverge to produce multiple descen- had come ashore, followed soon thereafter—or perhaps dant species. Evolution and natural selection are often simultaneously—by . We could go back much conflated, but evolution is the historical occurrence of earlier, 4 billion years or so, to that fateful day when the change, and natural selection is one —in most first replicated itself, an important milestone in cases the most important—that can cause it. Recent of life and the beginning of the evolutionary years have seen a flowering in the field of evolutionary pageant. Moving forward, the last few hundred million , and much has been learned about the causes and years have also had their highs and lows: the origins of consequences of evolution. The two main pillars of our frogs and , the end-Permian when 90 knowledge of evolution come from knowledge of the percent of all species perished, and the rise and fall of the historical record of evolutionary change, deduced directly . from the record and inferred from examination of These vignettes are a few of many waypoints in the phylogeny, and from study of the process of evolutionary evolutionary chronicle of life on . Evolutionary change, particularly the effect of natural selection. It is try to understand this history, explaining how now apparent that when selection is strong, evolution can and why life has taken its particular path. But the study proceed considerably more rapidly than was generally of evolution involves more than looking backward to try envisioned by . As a result, are realizing to understand the past. Evolution is an ongoing process, that it is possible to conduct evolutionary experiments in one possibly operating at a faster rate now than in real time. Recent developments in many areas, including past in this -dominated world. Consequently, molecular and , have greatly ex- is also forward looking: it includes panded our knowledge and reaffirmed evolution’s central the study of evolutionary processes in action today— place in the understanding of biological diversity. how they operate, what they produce—as well as in- vestigation of how evolution is likely to proceed in the GLOSSARY future. Moreover, evolutionary biology is not solely an Evolution. Descent with modification; transformation academic matter; evolution affects humans in many of species through time, including both changes that ways, from coping with the of agricultural occur within species, as well as the origin of new pests and disease-causing to understanding species. the workings of our own . Indeed, evolutionary

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4 Introduction has broad relevance, playing an important role in per breeding event. The logic behind natural selection is advances in many areas, from computer programming unassailable. If some trait variant is causally related to to to engineering. greater reproductive , then more members of the will have that variant in the next generation; continued over many , such selection can 1. WHAT IS EVOLUTION? greatly change the constitution of a population. Look up the word “evolution” in the online version of the But there is a catch. Natural selection can occur with- Oxford English Dictionary, and you will find 11 defini- out leading to evolution if differences among individuals tions and numerous subdefinitions, ranging from math- are not genetically based. For natural selection to cause ematical (“the successive transformation of a curve by evolutionary change, trait variants must be transmitted the alteration of the conditions which define it”) to chem- from parent to offspring; if that is the case, then offspring ical (“the emission or release of gas, heat, light, etc.”) to will resemble their parents and the trait variants possessed military (“a manoeuvre executed by troops or ships to by the parents that produce the most offspring will in- adopt a different tactical formation”). Even with ref- crease in frequency in the next generation. erence to biology, there are several definitions, including However, offspring do not always resemble their “emergence or release from an envelope or enclosing parents. In some cases, individuals vary phenotypically structure; (also) protrusion, evagination,” not to men- not because they are different genetically, but because tion “rare” and “historical” usage related to the concept they experienced different environments during growth of preformation of embryos. Even among evolutionary (this is the “nurture” part of the versus nurture biologists, evolution is defined in different ways. For debate; see chapters III.10 and VII.1). If, in , varia- example, one widely read refers to evolution as tion in a population is not genetically based, then se- “changes in the properties of groups of organisms over lection will have no evolutionary consequence; in- the course of generations” (Futuyma 2005), whereas dividuals surviving and producing many offspring will another defines it as “changes in frequencies over not differ genetically from those that fail to prosper, and time” (Freeman and Herron 2007). as a result, the of the population will not One might think that—as in so many other areas of change. Nonetheless, much of the phenotypic variation evolutionary biology—we could look to Darwin for within a population is, in fact, genetically based; con- clarity. But in the first edition of On the Origin of Spe- sequently, natural selection often does lead to evolu- cies, the term “evolution” never appears (though the last tionary change. word of the book is “evolved”); not until the sixth edi- But that does not mean that the occurrence of evo- tion does Darwin use “evolution.” Rather, Darwin’s lutionary change necessarily implies the action of nat- term of choice is “descent with modification,” a simple ural selection. Other processes—especially , phrase that captures the essence of what evolutionary , and immigration of individuals with dif- biology is all about: the study of the transformation of ferent genetic constitutions—also can cause a change in species through time, including both changes that occur the genetic makeup of a population from one generation within species, as well as the origin of new species. to the next (see Section IV: Evolutionary Processes). In other words, natural selection can cause adaptive evo- lutionary change, but not all evolution is adaptive. 2. EVOLUTION: PATTERN VERSUS PROCESS These caveats notwithstanding, 150 years of research Many people—sometimes even biologists—equate evo- have made clear that natural selection is a powerful force lution with natural selection, but the two are not the responsible for much of the significant evolutionary same. Natural selection is one process that can cause change that has occurred over the . As the evolutionary change, but natural selection can occur chapters in Section II: and the History of without producing evolutionary change. Conversely, Life, and Section III: Natural Selection and , processes other than natural selection can lead to demonstrate, natural selection can operate in many evolution. ways, and scientists have correspondingly devised many Natural selection within refers to the sit- methods to detect it, both through studies of the phe- uation in which individuals with one variant of a trait notype and of DNA itself (see also chapter V.14). (say, blue ) tend to leave more offspring that are healthy and fertile in the next generation than do in- 3. EVOLUTION: MORE THAN CHANGES IN THE dividuals with an alternative variant of the trait. Such GENE POOL selection can occur in many ways, for example, if the variant leads togreater longevity,greaterattractiveness to During the heyday of population in the middle members of the other , or greater number of offspring decades of the last century, many biologists equated

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What Is Evolution? 5 evolution with changes from one generation to the next Concurrently, a more fundamental revolution em- in gene frequencies (gene frequency refers to the fre- phasizing the historical perspective has taken place over quencies of different of a gene; for background the last 30 years with the realization that on on , see chapter I.4). The “Modern phylogenetic relationships—that is, the of life,the Synthesis” of the 1930s and 1940s led to several decades pattern of descent and relationship among species—is in which the field was primarily concerned with the ge- critical in interpreting all aspects of evolution above the netics of populations with an emphasis on natural se- population level. Beginning with a transformation in the lection (see chapter I.2). This focus was sharpened by the field of concerning how phylogenetic re- advent of molecular approaches to studying evolution. lationships are inferred, this “tree-thinking” approach Starting in 1960 with the application of elec- now guides study not only of all aspects of macroevolu- trophoresis techniques, biologists could, for the first tion but also of many population-level phenomena. time, directly assess the extent of genetic variation within Finally, life is hierarchically organized. are lo- populations. To everyone’s surprise, populations were cated within individuals, individuals within populations, found to contain much more variation than expected. populations within species, and species within This finding both challenged the view that natural se- (a consists of an ancestral species and all its de- lection was the dominant force guiding evolutionary scendants). concerns what happens change (see discussion of “neutralists” in chapters I.2 among individuals within a population, but evolutionary and V.1), yet further directed attention to the genetics of change can occur at all levels. For example, why are there populations. With more advanced molecular techniques more than 2000 species of rodents but only 3 species of available today, the situation has not changed. There is monotremes (the and echidnas), a much older much more variation than we first suspected. clade of ? One cannot look at questions con- The last 35 years have seen a broadening of evolu- cerning natural selection within a population to answer tionary inquiry as the field has recognized that there is this question. Rather, one must inquire about properties more to understanding evolutionary change than study- of entire species. Is there some attribute of rodents that ing what happens to genes within populations—though makes them particularly prone to speciate or to avoid this area remains a critically important part of evolu- extinction? Similarly, why is there so much seemingly tionary inquiry. Three aspects of expansion in evolu- useless noncoding DNA in the of many species tionary thinking are particularly important. (see chapter V.2)? One possibility is that some genes are First, phenotypic evolution results from evolutionary particularly adept at mutating to multiply the number of change in the developmental process that transforms a copies of that gene within a genome; such DNA might single-celled fertilized into an adult . Al- increase in frequency in the genome even if such multi- though under genetic control, is an in- plication has no benefit to the individual in whose body tricate process that cannot be understood by examina- the DNA resides. Just as selection among individual or- tion of DNA sequences alone. Rather, understanding ganisms on heritable traits canleadtoevolutionary how evolve, and the extent to which devel- change within populations, selection among entities at opmental systems constrain and direct evolutionary other levels (species, genes) can also lead to evolutionary change, requires detailed molecular and embryological change, as long as those entities have traits that are knowledge (see chapters V.10 and V.11). transmitted to their offspring (be they descendant species Second, history is integral to understanding evolution or genes) and affect the number of descendants they pro- (see introduction to Section II: Phylogenetics and the duce. The upshot is that evolution occurs at multiple levels History of Life). The study of — of the hierarchy of life; to understand its rich complexity provides the primary, almost exclusive, direct evidence we must study evolution at these distinct levels as well as of life in the past. Somewhat moribund in the middle of the interactions among them. What happens, for example, the last century, paleontology has experienced a resur- when a trait that benefits an individual within a popula- gence in recent decades owing to both dramatic new dis- tion (perhaps cannibalism—more , fewer competi- coveries stemming from an upsurge in paleontological tors!) has detrimental effects at the level of species? exploration, and new about evolution inspired by Although evolutionary biology has expanded in and primarily testable with fossil data, such as scope, genetic change is still its fundamental foundation. concerning and stasis, species Nonetheless, in recent years attention has focused on selection, and mass extinction. Initially critical in the variation that is not genetically based. Phenotypic plas- development and acceptance of evolutionary , ticity—the ability of a single to produce dif- paleontology has once again become an important and ferent phenotypes when exposed to different environ- vibrant part of evolutionary biology (see chapter II.9 and ments—may itself be adaptive (see chapter III.10). If others in Section II). individuals in a population are likely to experience

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6 Introduction different conditions as they develop, then the evolution of obvious to many biologists, some of whom considered a genotype that could produce appropriate phenotypes Dobzhansky’s claim to be self-serving hype. One could depending on circumstances would be advantageous. argue, for example, that the enormous growth in our Although selection on these different phenotypes would understanding of from 1950 to 2000 not lead to evolutionary change, the degree of plasticity was made with little involvement or insight from evo- itself can evolve if differences in extent of plasticity lead lutionary biology. Indeed, to the practicing molecular to differences in the number of surviving offspring. In- in the 1980s and 1990s, evolutionary biology deed, an open question is, why don’t populations evolve was mostly irrelevant. to become infinitely malleable, capable of producing the Now, nothing could be further from the truth. When appropriate for any environment? Presumably, results of the sequencing project first plasticity has an associated cost such that adaptation to appeared in 2000, many initially believed that a thorough different environments often occurs by genetic differ- understanding of would soon follow, entiation rather than by the evolution of a single genotype answering questions about the genetic basis of human that can produce different phenotypes. Such costs, how- diseases and phenotypic variation among individuals. ever, have proven difficult to demonstrate. These hopes were quickly dashed—the genetic code, after Differences observed among populations may also all, is nothing more than a long list of letters (A, C, G, and reflect plastic responses to different environmental con- T, the abbreviations of the four building ditions and thus may not reflect genetic differentiation. blocks of DNA). Much of the genome of many species However, if consistently transmitted from one genera- seems to have no and is just, in some sense, tion to the next, such nongenetic differences may lead to functionless filler; as a result, picking out where the genes divergent selective pressures on traits that are genetically in this 4 billion–long string of alphabet spaghetti, much determined, thus promoting evolutionary divergence less figuring out how these genes function, is not easy. between the populations. One particular example con- So where did molecular biologists turn? To the field of cerns behavior, which is highly variable in response to evolutionary biology! Genomicists soon realized that the the environment—an extreme manifestation of plastic- best way to understand the human genome was to study ity (see chapter VIII.10). Learned behaviors that are it in the context of its evolutionary history, by comparing transmitted from one generation to the next—often human sequences with those of other species in a phy- called traditions or culture—occur not only in humans logenetic framework. One method for locating genes, but in other , not only our near relatives the for example, is to examine comparable parts of the but also cetaceans, birds, and others. Such behavioral genome of different species. The underlying rationale is differences among populations would not reflect genetic that genes evolve more slowly than other parts of the differentiation, but they might the stage for genetic genome. Specifically, nonfunctioning stretches of DNA divergence in traits relating to the behaviors. One can tend to evolve differences through time as random mu- easily envision, for example, how popula- tations become established (the process of genetic drift; tions that use different tools—such as delicate twigs to see chapter IV.1), but functioning genes tend to diverge probe mounds, or heavy stones to pound nuts— less, because natural selection removes deleterious mu- might evolve different morphological features to en- tations when they arise, keeping the DNA sequence sim- hance the effectiveness of these behaviors. A concrete ilar among species. As a result, examination of the example involves human populations that tend cattle— amount of divergence between two species relative to the surely a nongenetically based behavior—and have amount of time since they shared a common can evolved genetic changes to permit the digestion of milk pinpoint stretches of DNA where evolution has occurred in adults. slowly, thus identifying the position of functional genes. Moreover, how a gene functions can often be deduced by comparing its function with that of homologous genes in 4. IN THE LIGHT OF EVOLUTION other species and using a phylogeny to reconstruct the In a 1964 address to the American Society of Zoologists, gene’s evolutionary history (see chapter V.14). the distinguished Russian-born biologist Theodosius And thus was born the effort to sequence the ge- Dobzhansky proclaimed “nothing makes sense in biol- nomes of other species (see chapter V.3). At first, the ogy except in the light of evolution.” Ever since, evolu- nascent field of comparative focused on pri- tionary biologists have trotted out this phrase (or some mates and model laboratory species such as mice and permutation of it) to emphasize the centrality of evolu- fruit flies, the former to permit comparisons of the tion in understanding the biological world. Nonetheless, human genome with that of our close evolutionary rel- for much of the twentieth century, the pervasive im- atives, the latter to take advantage of the great under- portance of an evolutionary perspective was not at all standing of the genomic systems of well-studied species.

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What Is Evolution? 7

More recently, the phylogenetic scope has broadened as The public debate is ironic given that manifestation of it has become evident that useful knowledge can be evolution has so many important societal consequences gained by examining genomes across the — (see chapter VIII.1). Evolutionary adaptation of disease- knowledge of the genetic causes of Parkinson’s disease causing organisms has rendered many drugs ineffective, in humans, for example, can be gained from studying leading to a huge public health toll as diseases thought the comparable gene in fruit flies, and much of relevance to have been vanquished have reemerged as deadly to humans can be learned from understanding the ge- scourges (see chapter VIII.3). A recent example is the netic basis of differences among dog breeds. evolution of resistance to in the bacterium Dobzhansky would not have been surprised. Evolu- Staphylococcus aureus, which leads to more than tionary biology turns out to be integral to understanding 100,000 infections and 19,000 fatalities a year in the the workings of DNA and the genome, just as it is key to United States. A similar story exists about pest understanding so many other aspects of our biological species that devour our crops and spread diseases. In the world (see chapter I.3). United States alone, the evolution of resistance results in agricultural losses totaling between $3 billion and $8 billion per year. Perhaps most scary is the reali- 5. CRITIQUES AND THE EVIDENCE FOR EVOLUTION zation that the human population is an enormous re- Unique among the , evolutionary biology’s source to many organisms and that natural selection foundation—that species evolve through time—is not continually pushes these species to become more adept at accepted by a considerable number of nonscientists, making use of this potential bonanza. Ebola, AIDS, in- especially in the United States, , and a few other fluenza—all are diseases caused by that adapt to countries. Public opinion polls repeatedly reveal that take advantage of us; a particularly worrisome concern is most Americans are either unsure about or do not be- that some form of avian flu could evolve to become more lieve in evolution. One yearly poll conducted for more virulent to or transmissible between humans, with the than 30 years, for example, consistently finds that about potential to produce a pandemic that could kill millions 40 percent of the US population that God cre- (see chapter VIII.2). All these problems are the result of ated humans in their form in the recent past. evolutionary phenomena, and all are studied using the Yet, the scientific data for evolution is overwhelming tools of evolutionary biology. (summarized in chapter I.3). Just like the composition and structure of genomes, many other biological phe- 6. THE PACE OF EVOLUTION nomena are explicable only in an evolutionary context. Why, if evolution had not occurred, would whales have For more than a century after the publication of On the tiny vestiges of a pelvis buried deep within their blubber? Origin of Species, biologists thought that evolution Why would cave fish and crickets have eyes that are usually proceeded slowly. To a large extent, this think- missing some parts and could not function even if there ing was a result of Darwin’s writing—“We see nothing were light? Why do humanfetuses develop, and thenlose, of these slow changes in , until the of time fur and a tail? All these, and many other phenomena, are has marked the long lapse of ages” (On the Origin of easily understood as a result of the evolutionary heritage Species, chap. 4, 1859). Darwin was, after all, right of species but are inexplicable in the absence of evolution. about so many things, big and small, from accurately The case for evolution is built on two additional pil- deducing the manner in which coral atolls form to cor- lars. First is the fossil record, which documents both the rectly predicting the existence of an unknown moth with major and minor transitions in the history of life (see a 12-inch proboscis from the of a Malagasy chapters II.9–II.18); each year, exciting new discoveries orchid. Hence, biologists have learned that it doesn’t further narrow the gaps in our understanding of life’s generally pay to disagree with what Darwin said. chronicle. Second is our understanding of evolutionary Nonetheless, Darwin was not right about everything. process, in particular, natural selection, the primary driv- One major mistake was the mechanism of , not er of evolutionary divergence. Studies in the laboratory surprising, since Mendel’s work was unknown to him, and in human-directed clearly dem- and the discovery that DNA is the genetic material was onstrate the efficacy of selection in driving substantial still a century in the future. A second concerned the genetic and phenotypic divergence; one need look no pace at which evolution occurs. Darwin expected that further than the enormous diversity of dog breeds to natural selection would be weak and consequently that appreciate the power of sustained selection. Moreover, evolutionary change would happen slowly, taking many scientists are increasingly documenting the occurrence of thousands or millions of years to cause detectable natural selection in nature and its ability to transform change. Of course, in his day there were no actual data species, sometimes over quite short periods of time. underlying this conclusion; rather, this expectation

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8 Introduction sprang from Darwin’s appreciation of the view pro- species to our own ends (see chapter VIII.5); conversely, mulgated by his mentor, the geologist , that wild species are responding to human-caused changes in the slow accumulation of changes caused by weak forces the environment, adapting to our efforts to control them would lead in the fullness of geologic time to major and responding to new opportunities (see chapter changes, a position in agreement with the prevailing VIII.3). Consequently, it’s no surprise that knowledge of Victorian about the slow and gradual manner in evolution is important for efforts to improve artificial which change occurs—or should occur—in both nature selection and combat our evolutionary foes. What is and human civilization. more surprising, perhaps, is the diversity of areas in Darwin’s view influenced evolutionary biologists for which an understanding of evolutionary processes is more than a century—well into the 1970s, most thought relevant to human society. These include not only that evolution usually occurred at a snail’s pace. Spurred medicine (see chapters VIII.1 and VIII.2), conservation by the results of long-term field studies of natural se- (see chapter VIII.6), and criminal forensics (see chapter lection that began in earnest around that time, we now VIII.4), but also important human pursuits such as know that Darwin was far off the mark. Many studies creating new in the laboratory (see chapter now clearly indicate that selection in nature is often VIII.7) and devising algorithms to solve analytically in- strong, and that as a result, evolutionary change often tractable problems (see chapter VIII.8). occurs very rapidly (see chapter III.7). Beyond purely utilitarian functions, an understanding One important consequence of this realization is that of evolution can tell us much about ourselves: where we we can observe evolution in real time. Pioneered by the came from and where we may be going, perhaps even study of Gala´pagos finches by , shedding light on what it means to be human. In recent who documented rapid evolutionary change in these birds years, a series of important fossil discoveries have (appropriately named after Darwin) from one generation brought into focus many aspects of the human evolu- to the next in response to weather-induced environmental tionary story, from our early roots to our recent changes, the study of real-time evolutionary change in past. Sequencing of the genomes of humans past and nature has become a cottage industry, with hundreds, or present and of our close primate relatives has com- perhaps now thousands, of well-documented examples. plemented these findings in important ways and in some This work not only clearly demonstrates the occurrence of cases has led to unexpected discoveries, such as evidence evolution but also provides great insights into the processes of lineages, like the Denisovans, for which little fossil (usually, but not always, natural selection) that cause it. data exist (see chapters II.18 and V.15). Perhaps most exciting, the rapidity by which evolution But what about our evolutionary future? When I was can occur has opened the door to evolutionary experi- a boy, the public service television station ran short filler ments in which researchers can alter environmental con- promos speculating that in the future, humans would ditions and test evolutionary hypotheses over a several- have a bulbous, -packed with tiny eyes and year period. Work at the forefront in this area involved nostrils. Where this came from I have no idea, but it studies on the color of in Trinidad. Observing probably represented a mixture of orthogenetic thinking that the fish were generally much more colorful when — has been marked by rapid increase in they occurred in streams without predators, John Endler and so must continue in that direction—with a moved some fish from streams with predators to nearby misguided notion that evolution equals progress, and areas lacking them; very quickly, the populations evolved because intelligence is the hallmark of the human spe- exuberant coloration, apparently a result of a cies, it would surely continue to evolve into the future. preference for brighter males, which, left unchecked by the Even then, I could sense that something was not quite absence of predators, led to rapid evolution over 14 gen- right about this prediction, and today, in fact, many erations. Subsequent studies have shown that the guppies believe that human evolution has ended because selec- freed from evolve many other differences, such tion no longer operates on phenotypic traits: not only has as in growth and reproductive rates (see chapter III.11). medical care ameliorated the negative consequences of Many similar studies are now ongoing, and it is a safe many genetic traits, but human cultural practices such as prediction that field experiments will be an important tool birth control may have severed the positive link between for understanding evolutionary processes in the future. beneficial traits (e.g., physical strength, intelligence) and reproductive output. Although these points have validity, they are not ab- 7. EVOLUTION, HUMANS, AND SOCIETY solute. In much of the developing world, selective agents Evolution has important implications for humans in a such as malaria can still exert strong selective pressure in number of ways. Some have already been discussed: the absence of adequate medical care. Moreover, new humans have used evolutionary principles to alter many diseases, such as AIDS, for which, at least initially, no

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What Is Evolution? 9 treatment exists, continue to emerge and may impose trically as a great evolutionary progression leading ulti- selection on populations in all parts of the world. Even in mately to us, but microbial species adapted to a great the developed world, evidence suggests that some ge- diversity of extreme environments—Yellowstone’s hot netically based traits are correlated with survival and springs, deep-sea hydrothermal vents—might see things reproductive success, and thus that natural selection is differently. Second, the dinosaurs—members of the class still leading to evolutionary change (see chapters VII.11 Reptilia—dominated the earth for more than 150 million and VIII.12). Finally, natural selection is only one of years. For most of that time, they cohabited with our several evolutionary processes. Surely, the increased mammalian , who were generally small-bodied, mobility of humans is increasing the homogenizing ef- minor players in Mesozoic . Conventional fects of gene flow and diminishing the diversifying effects wisdom has it that our ancestors, thanks to their of genetic drift that acts in small and isolated popula- large and warm-blooded , outcom- tions. Human populations never existed as discretely peted dinosaurs, and ultimately would have displaced identifiable genetic “races” (see chapter VIII.11), but them. However, evidence for this view is slender; right ongoing genetic exchange is diminishing the geographic before the end of their reign, dinosaurs were thriving and variation that was the result of our past evolutionary showed no evidence of being pushed out by mammals. It history (see chapter VIII.12). is thought provoking to contemplate what the world Although selection has been important in shaping would be like—where we would be today—had an as- human evolution, that does not mean that natural selection teroid not slammed into the earth 65.3 million years ago, can explain all aspects of the . Many wiping out the dinosaurs and clearing the way for the human traits—our large brain, altruistic behavior, keen evolutionary diversification of mammals, including our sense of smell—may have evolved as , but own species. others may represent or may have evolved for nonadaptive . The field of focuses particularly on human behavior and is FURTHER READING very controversial; some see in most human behavior evi- Coyne, J. A. 2009. Why Evolution Is True. New York: dence for adaptation to conditions past or present, but Viking. others are more skeptical (see chapter VII.12). Darwin, C. 1859. by Means of Many look to evolution to help address issues about Natural Selection, or the Preservation of Favoured Races what it means to be human. Those questions are pri- in the Struggle for Life. London: John Murray. marily in the realm of rather than evolution- Dawkins, R. 2009. The Greatest Show on Earth: The Evi- ary biology and for the most part do not fall within the dence for Evolution. New York: Free Press. purview of this volume or this chapter. Nonetheless, I will Futuyma, D. J. 2013. Evolution. 2nd ed. Sunderland, MA: end with two observations. First, recent advances make Sinauer. clear that plants and animals occupy only a small part of Grant, P. R., and R. Grant. 2008. How and Why Species Multiply: The Radiation of Darwin’s . Princeton, the evolutionary tree of life; a great variety of microbial NJ: Princeton University Press. species constitute most of life’s diversity. As a result, the Reznick, D. N. 2009. The “Origin” Then and Now: An human species is just one of millions of tiny branches on Interpretive Guide to the “Origin of Species.” Princeton, the evolutionary tree, and these microbial species are as NJ: Princeton University Press. well adapted to their ecological niches as we are to ours. It Zimmer, C. 2009. The Tangled Bank: An Introduction to is easy for humans to view life’s history anthropocen- Evolution. Greenwood Village, CO: Roberts & Company.

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