When kettlewell recaptured the marked moths what did he find

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___ e, H A P T E R 1 7

The Evolution of Life

17 .1 The Origin of Life INTEGRATED SCIENCE 17A ASTRONOMY Oid life on Earth Originate on Mars?

17 .2 Early Llf e on Earth

17 .3 Charles Darwin and The Origin of Species

17 .4 How Natural Selection Works HISTORY OF SCIENCE The Peppered Moth

17 .s Adaptation SCIENCE AND SOCIETY Antibiotic-Resistant Baderia INTEGRATED SCIENCE 17B PHYSICS Staying Wann and Keeping Cool

17 .6 Evolution and Genetics

17.7 How Species Form

17 .8 Evidence of Evolution INTEGRATED SCIENCE 17C EARTH SCIENCE Fossils: Earth's Tangible Evidence of Evolution

17. 9 The Evolution of Humans

MARINJE :LGWAN..AS swim th.ro~ seawatem wifru

S?ines mat prate~ them £tom atwna:ls. 'Tih.ese @.d~t@.tions, and the eounfless other -ways iru

whicli organisms are st:tU.eturcd to SlliiYJ.EVe

and reproduee, maJce '\JP fille •llilGreoiale stOl\y of evolutien. ;BJow do living 6hings dbange over tiifte irt response fo iheii; environments?

Mter all, a giraffe ~an:~ grow a long tD.eGlc justi because it wan-ts to. So, how ao ada,ptations (sudi. as a•gifaffe's long nd ). aGtUa.lly. Gome

about? Poes the same ,proeess ,ex:plain !how

new types of living things- new ~eeies-

originare? Also, if all organisms today evolved from· earlier organisms, then, how did, life get

startea in the fust place? Read oft O a.isgover these seorets of life.

CHAPTER 17 THE EVOLUTION OF LIFE 487

• • •

•M Charles Darwin and The Origin of Species EXPLAIN TH 15 How did the Galapagos finches contribute to Darwin's ideas about evolution?

H ow has life on Earth changed over time? For example, how did we get from tiny, primitive cells to humans, hippos, redwoods, and all the amazing diversity of life on Earth today? For thousands of years, people believed that life on Earth did not change. They

believed that Earth had always had the same species, and always would. Then fossils were discovered in Earth's rocks, and people began to wonder. Fossils sug- gested that the kinds of species living on Earth changed over time-old species disappeared, and new species appeared. Also interesting was that fossil organisms sometimes showed a distinct resemblance to modern species (Figure 17.8). Could some fossils actually be the ancestors of modern species?

French naturalise Jean-Baptiste Lamarck (1744-1829) was one of the first to argue that chis was the case. Lamarck believed that modern species were descended from ancestors that had evolved-changed over time-to become better adapted co the environments they lived in. According to Lamarck, organisms acquired new characteristics during their lifetimes and then passed these characteristics to their offspring. For example, ancestral giraffes screeched their necks to grab the high leaves on a tree, and their necks became longer. They then passed these longer necks to their offspring. The offspring reached for even higher leaves, stretching their necks even further, and so on (Figure 17.9a). Lamarck's theory for how change occurs, called the inheritance of acquired characteristics, proved to be incorrect: Organisms cannot pass characteristics acquired during their lifetimes to their offspring because these acquired characteristics are not genetic. However, Lamarck's fierce support for the idea that organisms evolve sec the stage for Charles Darwin.

Ancestral giraff~ stretched

their necks.

FIGURE 17 , 9

Their offspring inherited the

stretched necks.

This happened repeatedly over

generations.

(b} I Darwin

Among ancestral giraffes, some individuals had longer necks than others.

(a) Lamarck believed that organisms acquired new characteristics during their lifetimes and passed these characteristics to their offspring. (b) In his theory of evolution by natural selection, Darwin argued that organisms with advantageous traits left more offspring than organisms with other traits. As a result, advantageous traits became more common in a population.

LEARNING OBJECT I VE Describe some of the Influences and events that brought Darwin to his theory of evolution through natural selection.

FIGURE 17.B Could fossils be the ancestors of modern spei:ies? This fossilt found in Germany, is about 50 million years old. It has a clear resemblance co a horse. yet is only the size of a fox.

UNIFYING CONCEPT

• 1he Scin,tiftc Method Secdon 1.3

Those with longer necks left more

offspring, also with long necks.

This h11ppened repeatedly over

generations.

488 PART THREE BIOLOGY

FIGURE 17.10 Charles Darwin developed the theory of evolution by natural selection.

There's an expression: Genius is 1 % Inspiration and 99% perspiration. Darwin's genius reflects a lot of perspiration. While on the Beagle, Darwin collected 1529 alcohol• preserved specimens and 3907 skins, bones, and dried specimens. He also took 2000 pages of notes on plants, animals, and geology. It's no wonder that when he wrote down his theory, he was able to support It with a wide variety of well-considered examples.

FIGURE 17.11 The finches Darwin saw on the Gala- pagos Islands-now called Darwin's finches-show remarkable variation in the size and shape of their beaks. Each is suited to a different diet. (a) The cacms finch has a pointy beak that it uses co eat cactus pulp and Aowers. (b) The large ground finch has a blunt, powerful beak that it uses to crack seeds. (c) The woodpecker finch has a woodpecker-like beak chat it uses to drill holes in wood. It then uses a cactus spine to pry out insects.

English naturalist C harles Darwin (1809-1882), shown in Figure 17.10, sec forth the theory of evolution in his book The Origin of Species by Means of Natu- ral Selection, published in 1859. Darwin proposed that evolution-inherited changes in populations of organisms over time-had produced all the living forms on Earth.

Darwin's theory of evolution grew out of the observations he made as the official naturalist aboard the H.M.S. Beagle, which sailed around South America from 1831 co 1836. During these years, Darwin studied South American species, collecting large numbers of plants, animals, and fossils. Darwin became increas- ingly intrigued by the question of how species got to be the way they were. He was particularly struck by the living things he encountered on the Galapagos Islands, 950 kilometers from the South American continent. Darwin cook particular note of the 13 species of Galapagos finches-now known as Darwin's finches. Darwin's finches showed remarkable variation in the size and shape of their beaks, with each beak being suited to; and used for, a different diet (Figure 17.11). How had the beaks of these finches come co differ in this way? Darwin wrote, "Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy chat from an original paucity of birds in this archipelago, one species had been taken and modified for different ends."*

Darwin was also inspired by the work of two of his contemporaries, Charles Lyell and Thomas Malthus. Lyell, a geologist, argued that Earth's geological fea- tures were created not by major catastrophic events-the favored theory of the time-but by gradual processes that produced their effects over long time peri- ods. For example, the formation of a deep canyon did not require a cataclysmic flood, but could result from a river's slow erosion of rock over millennia. Darwin realized chis could be true for organisms as well: The accumulation of gradual changes over long periods could produce all che diversity of living organisms as well as all their remarkable features.

The economist Thomas Malthus was a second important influence for Darwin, and the one who led Darwin to his great idea on the cause of evolutionary change. Malthus observed that human populations grow much faster than avail- able food supplies, and he concluded, wich despair, chat famine was an inevitable feature of human existence. Darwin applied Malchus's idea to the natural world and argued that, because there are not enough resources for all organisms co survive and to reproduce as much as they can, living organisms are involved in an intense "struggle for existence." As a result, organisms with advantageous traits leave more offspring than organisms with other traits, causing populations to change over time. To go back to the giraffe's long neck: Darwin argued chat

*Charles Darwin, The Voyage of the Beagle, 1909.

(a) (cl

CHAPTER 17 THE EVOLUTION OF LIFE 489

ancestral giraffes with longer necks were better ac reaching the high leaves on trees. Because longer-necked giraffes got more food, they were able to survive and leave more offspring than ancestral giraffes with shorter necks. This happened repeatedly over generations. Over time, there were more longer-necked giraffes in the giraffe population (Figure 17.9b). This process, which Darwin called natural selection, is the major driving force behind evolution.

CHECK YOURSELF 1. If Lamarck had been correct and evolutionary change occurred through

the inheritance of acquired characteristics, what trait might a bodybuilder pass to his offspring?

2. Many animals that live in the Arctic, such as Arctic hares, have white fur. How could natural selection explain the evolution of their white fur color?

CHECK YOUR ANSWERS 1. If Lamarck were correct, the bodybuilder's children would inherit the

increased muscle mass that the bodybuilder had acquired over a lifetime of weightlifting. Because Lamarck's theory turned out to be incorrect, however, the children will have to do their own bodybuilding.

2. Animals that were harder to see in their snowy environments had an advantageous trait-predators were less likely to spot them. Arctic hares with whiter fur were more likely to survive to adulthood, reproduce, and leave offspring. These offspring would also have inherited whiter fur. As a result, whiter fur became more common in the Arctic hare population. Over many generations, natural selection produced a white coat that matches the Arctic snow.

•fl• How Natural Selection Works EXPLAIN THIS What does it mean to say that one rabbit has greater fitness than another?

R abbics were introduced into Australia in 1859, when a man named Thomas Austin released 24 individuals onto his property in the southeastern part of the continent. The rabbits quickly became pests, devastating farmlands

and natural habitats (Figure 17.12). Breeding "like rabbits," they spread across the continent in such large numbers chat they were described as a "gray blanker" that covered the land. Many attempts were made to control the rabbit population, including the construction of an 1822-kilometer-long "rabbit-proof' fence- still the longest fence in the world. Unfortunately, by the time the fence was completed in 1907, the rabbits had already passed through. (The fence wouldn't have worked anyway-even after it was completed, rabbits would pile up so thickly behind it that some were eventually able to walk right over their companions' backs to the ocher side.)

In the early 1950s, the government decided to try to control the rabbit popu- lation by releasing myxoma virus, a virus deadly to rabbits. Initially, the virus was a wonder, killing more than 99.9% of infected rabbits. Within a few years, however, fewer rabbits were dying. What had happened? Within che original rabbit oooulation, a small number of individuals happened co be resistant co the

MasteringPhysics" TUTORIAL: Darwin and the Galapagos Islands VIDEO: Galapagos Islands Overview

VIDEO: Galapagos Marine Iguana

LEARNING OBJECTIVE Explain how natural selection results in populations becoming adapted to their environments.

Mastering Physics· TUTORIAL: Causes of Mi croevol u ti on

490 PART THREE BIOLOGY

(a)

(bl

FIGURE 17.12 (a) Rabbits introduced into Australia caused widespread destruction, includ- ing here on Phillip Island. (b) This photo shows the same area after rabbits were eradicated. The vegetation has grown back.

myxoma virus. These resistant individuals survived the disease and reproduced, producing more disease-resistant offspring (Figure 17.13). Over time, the num- ber of disease-resistant rabbits increased, and the virus became less and less effec- tive. The rabbit population had evolved resistance to the myxoma virus through natural selection.

Natural selection occurs when organisms with advantageous traits leave more offspring than organisms with ocher traits, causing populations to change over time. Let's look more carefully at the process of natural selection.

1. Variation. In any population of organisms, individuals have many traits chat show variation-that is, they vary from individual to individual. In humans, some variable traits are height, hair color, hairstyle, foot size, and blood type.

2. Heritability. Many traits are determined at lease parcly by genes and so are heritable-that is, they are passed from parents to offspring. Which of the human traits listed above are heritable? All of them arc heritable except hair- style. Hairstyle is not heritable because it is not genetically determined.

3. Natural selection. Some variable heritable traits are advantageous. The organisms that possess these advantageous traits are able co leave more offspring than organisms without che advantageous traits. The fitness of an organism describes the number of offspring it leaves over its lifetime compared co ocher individuals in the population. An organism chat leaves more offspring than other individuals in the population is said co have greater fitness.

4. Adaptation. Because organisms with advantageous traits leave more offspring, advantageous traits are "selected for" and become more common in a popula- tion. What is the result? The population evolves to become better adapted to its environment.

Figure 17.14 summarizes the process of natural selection. Note chat, although natural selection acts on individuals within a population, allowing some indi- viduals to leave more offspring chan others, it is the population as a whole chat evolves and becomes adapted to its environment.

FIGURE 17.13

.. ' ,, .

. :u: ~~: .. ,,. ,, ......

•••

Disease-resistant rabbit

At first, the myxoma virus killed 99.9% of infected rabbits. However, a small number of naturally disease-resistant rabbits (blue) survived and reproduced, passing their myxoma· resistant genes to their offspring. The population became more resistant, and the virus became less effective.

CHAPTER 17 THE EVOLUTION OF LIFE 491

(1} VARIATION

Organisms have lots of traits, many of which show variation.

(3) NATURAL SELECTION

Variation in heritable traits can result in some organisms leaving more offspring

than others. This is called natural selection.

CHECK YOURSELF

(2} HERITABILITY

Some traits are heritable. They are determined by genes and so are passed from parents to offspring.

(4) ADAPTATION

Natural selection causes advantageous traits to become more common in a

population. In this way, entire populations become adapted to their environments.

1. (a) Which of these traits are variable In cats: fur color, tall length, number of eyes? (b) Which of the traits are heritable?

2. The cheetah Is the fastest land animal on Earth. It can run 112 kllometersjhour (70 mllesjhour)I Cheetahs prey on Thomson's gazelles that can run almost as fast, 80 kilometersjhour (50 mllesjhour). How might natural selection have pro- duced the cheetah's fast running speed?

CHECK YOUR ANSWERS 1. (a) Fur color varies among cats-there are tabby cats, black cats, gray cats,

and so on. Tail length also varies-not all cats' tails are exactly the same length. But there is no variation in the number of eyes-all cats have two eyes. (b) All three traits are heritable because all are determined genetically.

2. Faster cheetahs were better at catching Thomson's gazelles. Being better at catching food made faster cheetahs better at surviving and reproducing. As a result, faster cheetahs left more offspring, which were also fast. This resulted in a cheetah population with faster Individuals. Over many generations, natural selection produced the remarkably fast cheetah we know today.

FIGURE 17.14 How natural selection works.

492 PART THREE BIOLOGY

The Peppered Moth During the Industrial Revolution, coal was the primary fud in England. Burning coal slathered dark soot on trCG, rocks, and ground. And then a startling thing happened to the moths.

Peppered moths in England had always been light in color, with the scattering of dark peppery flecks that gave them their name. Their coloration made them hard to sec in a habitat oflichcn-covered trees and rocks. (Lichens are fungi that grow with photosynthetic algae or bacteria; they form crustlike growths on rocks, trees, and other surfaces.) It was believed that this camouflage protected the moths from birds, their main predators.

As the Industrial Revolution progressed, pollution killed the lichens, leaving the trees first bare and then darkened with soot. In 1848, the first dark peppered moth was found in the industrial center of Manchester, England. Dark moths had probably always existed in the population, but they had been extremely rare. Over the next decades, as more coal burned and the environment became increasingly sooty, more and more dark moths were seen. By 1895, 98% of peppered moths in industrialii.ed areas were dark. Then, in the second half of the 20th century, antipollution laws were passed and soot disappeared. Light moths increased in number, and today the dark moths have all but disappeared.

Did natural selection cause the coloration shifts in the pep- pered moth? Biologists hypothcsii.ed that in lichen-covered habitats, natural sdcction favored light moths because they were better camouflaged. In sooty habitats, natural selec- tion favored dark moths. A series of experiments by Bernard Kettlewell tested this hypothesis, Kettlewell released equal numbers of marked dark and light moths in polluted and un- polluted areas. After a while, he tried to rccaprure the moths. In polluted areas, Kettlewell recaptured more dark moths than light moths, which suggested that dark moths had survived better. The opposite was true in unpolluted habitats, where he

Can you find the moths? Light peppered moths arc well camouflaged on lichc:n- covcrcd trees.

rccipturcd more light moths. Kettlewell also placed moths on tree trunks and filmed birds eating the moths. He found that birds ate what they could sec: Birds ate more light moths in polluted habitats and more dark moths in unpolluted habitats.

Kettlewell's work became a classic example of natural sdcction. Evcnrually, however, certain aspects of his experi- ments were challenged. For example, moth experts pointed out that peppered moths don't usually sit on tree trunks, where Kettlewell had placed them. Instead, they usually rest on the undersides of branches. In addition, Kettlewell released the normally nocturnal moths during the daytime. This may have affected the moths' ability to find resting spots. Finally, Kettlewell used a mix oflab-raised and wild-caught moths, which could differ in their behavior. These doubts led Michael Majerus of Cambridge University to conduct a new set of experiments belWcen 2001 and 2007. Majerus's work confirmed that bird predation was the key factor affecting the relative numbers oflight and dark peppered moths. It is also interesting that a shift from light to dark forms in polluted areas (and back again, as pollution is cleaned up) has been rcponcd in more than 70 other moth species in England and the United States alone.

LEARNING OBJECTIVE Use examples to describe different kinds of adaptations found in living organisms.

•flj Adaptation EXPLAIN THIS Why do some birds have bright feathers despite the fact that the vivid colors make them more visible to predators?

Natural selection leads to the evolution of adaptations-traits that make organisms well suited co living and reproducing in their environments. The Check Yourself question in the preceding section gave an example of an adaptation- the cheetah's speed. The cheetah's speed helps it catch the food it needs co survive and reproduce.

Adaptations can relate co various aspects of an organism's life. Some adapta- tions help organisms survive. Survival is, after all, usually an important flrst step in successful reproduction. Survival requires that organisms be able to acquire food and other necessary resources. It also requires that organisms avoid becom- ing food for someone else (Figure 17.15). Anti-predator adaptations include cam- ouflage, toxicity, or just the ability to hide or run away.

CHAPTER 17 THE EVOLUTION OF LIFE 493

{a) {b)

Other adaptations have evolved to help organisms acquire maces. These include the beautiful feathers of male peacocks and birds of paradise (Figure 17.16a), the sexy "rib-bits" of male frogs, and che enchanting songs of many male birds. Males have evolved these "sexy" traits because females of the species find chem attrac· tive. In ocher species, females don't choose their mates based on attractive traits. Instead, males fight with other males to obtain mates. The adaptations of these males may include large size, great strength, or fighting structures such as ant- lers (Figure 17.16b). Natural selection chat favors individuals best able to acquire maces is also called sexual selection.

(a) (b)

Finally, some adaptations relate co bearing and raising young. Figure 17.17 shows one such adaptation- parental care. Parental care evolved because natural selection favored organisms chat were able to help their offspring survive and thrive. Parental care is found in many animals, including humans.

Natural selection has produced remarkable adaptations over time. Nature does not plan ahead- it does not plan to make a falcon or a polar bear. Instead, adaptations are built step by step, through the never-ending selection of the most successful forms.

FIGURE 17.15 Almost every organism has adaptations that help prevent it from becoming food for someone else. (a) The spines of this cactus prevent most animals from eating it. (b) When threatened, this octopus releases a cloud of dark ink that may confuse a predator long enough for the octopus to escape.

The peacock may be.the organism with the most famous adaptation for attracting mates. The male peacock's great fan of colorful tall feathers not only Is admired by people but, more Important; Impresses peahens.

And speaking of bright colors- the bold colors of organisms such as wasps, coral snakes, and poison dart frogs evolved to warn potential predators that th~y are dangerous.

FIGURE 17.16 Some adaptations for acquiring mates. (a) The beautiful feathers of this male bird of paradise (shown here displaying his wings) help attract female mates. (b) These male deer arc fighting for control of territory as well as mates.

FIGURE 17.17 Parental care occurs in many species. This male poison dart frog is carry- ing his tadpoles on his back.

494 PART THREE BIOLOGY

- --

•• It

' CHECK YOURSELF / ., ' ; , I • -~. ' '

' ,,.~· ~.

Mating Is very dangerous for a male praying mantis. Quite often, the female will eat him as he mates with her.

" ,:-. ' \\. , ~, 1. What advantage does the female get from eating the male7 2. Would It be more advantageous {#adaptive") for the male not to mate at all7 -~

CHECK YOUR ANSWERS When a male praying mantis (the: smaller insect on top) mates with a female:, he: is in danger of having his head bitten off.

1. The female gets nutrients when she eats the male. 2. A male praying mantis that never mates is more likely to survive to old

age. But, if he doesn't mate, he won't leave any offspring. Remember

Antibiotic-Resistant Bacteria A patient is ill with pneumonia and gets a prescription fur penicillin. After three: days, he: feels better and stops mking his pills. A few days later, his symptoms return. He: quickly finds his pills and starts ta1cing them again, but this time they have no c:lkct. What happened? This frightening phenomenon is called antibiotic resistance. Antibiotic resistance: is caused by natural selection: Penicillin killed most of the pneumonia bacteria, but a few penicillin-resistant bacteria survived. These bacteria multiplied, and the: patient's infection came: back.- only this time:, the bacteria are resistant to penicillin.

Antibiotics arc wonder drugs. When penicillin, the first antibiotic, appeared, it dramatically cut the number of ill- nesses and deaths resulting from bacterial infections. After only a decade: of use:, however, the: first penicillin-resistant bacterial strains appeared. Since then, antibiotic resistance: has spread, with more and more bacterial populations be- coming resistant to more and more: different antibiotics. Diseases once easy to trc:at-cubc:rculosis, pneumonia, even common childhood ailments such as car infc:ctionr.-arc: now often resistant co multiple: antibiotics. In 2011 the World Health Organization reported that about 440,000 new cases of multi-drug-rc:sisranc cubc:rculosis appear each year, resulting in at least 150,000 deaths.

Some of the most dangerous antibiotic-resistant bacteria are found in hospitals, where the use of many different types of antibiotics allows widc:ly resistant strains ro evolve:. The Centers fur Disc:asc: Control reported that in 2005, methicillin- rc:sistant Staphykicoccus aureus (MRSA), a bacterial strain that is resistant to most of the: antibiotics currently available, was responsible for more than 94,000 life-threatening infec- tions and 18,650 deaths in the United States alone:. And, some MRSA strains arc: beginning to show resistance ro the antibiotic vancomycin, often considered "the: drug oflasr resort." Another worrisome: development is the emergence of MRSA in the: wider community. Community-based MRSA infections usually start as skin infections and spread through skin-to-skin contact. Some: of these cases turn inro "flc:sh-c:ating" disc:asc:, and ochers arc halted only by drastic measures such as amputation. Environments with a higher

risk for community•basc:d MRSA infections include athletic fucilitic:s, dorms, prisons, and day-care: centers, Compared to people whose infections respond to antibiotics, people who have: antibiotic-resistant infections require: longer hospital stays and are more: likc:ly to die from their infections.

All antibiotic use has the potential of contributing co resistance. However, resistance has bc:c:n greatly accdc:rated by the overuse of antibiotics. Under pressure: from patients, physicians may prescribe antibiotics for illnc:ssc:s that arc not caused by bacteria. (Many common illnesses, such as colds, flus, and masc sore throats, arc caused by viruses.) These anti- biotics select for resistance: in the normal (non-disc:asc:- causing) bacterial populations in our bodies, making it possible for resistant genes to be transferred to discasc:-causing bacteria that later invade the body. The: fuct that patients sometimes stop taking their medications too soon contributes to the: problem; chis selects for antibiotic-resistant strains without providing the sustained dose that would accually kill all the bacteria. Antibiotics arc also used heavily in the livestock industry, where animals arc given antibiotics regu- larly-even when they arc healthy-to promote: growth. Unfurtunatc:ly, this practice: greatly promotes the evolution of antibiotic resistance:. In recent years, reports of food-borne illnesses caused by antibiotic-resistant bacteria have become regular items in the news. For example:, in August 2011, an outbrc:alc of antibiotic-resistant salmonella in ground turkey caused at least 79 illnc:ssc:s and one death.

What can be done about antibiotic resistance? First, humans must learn to use antibiotics wisdy, taking them only when they arc needed-that is, for bacterial infections-- and then taking the entire course of treatment. Second, physicians and veterinarians can promote: a socially respon• siblc: approach to antibiotics by educating patients and agriculturalists on the proper application of these drugs. Third, antibiotics should not be: used to promote: growth in livestock. In 2012, steps were finally taken to ban the agricultural use of certain antibiotics. Finally, since many antibiotics are less effective: now because: of resistance, scientists must search for new antibiotics to take the place of those char no longer do the job.

CHAPTER 17 THE EVOLUTION OF LIFE 495

that adaptations are traits that make organisms good at living and repro- ducing in their environments. It's not enough to survive-you also have to reproduce! This male praying mantis may not have long to live, but at least he has a good chance of leaving offspring .

- 496 PART THREE BIOLOGY

LEARNING OBJECTIVE Explaln how an understanding of genetics produced Insights about the mechanisms of evolution and the origin of genetic diversity.

• • •

• • •

•f&I Evolution and Genetics • • •

S o far, we've seen how natural selection acts on organisms' traits-giraffe neck length, cheetah speed, peppered moth color, and so on. Traits are only pare of the story, though, because what gees passed from parents co offspring

CHAPTER 17 THE EVOLUTION OF LIFE 497

are not traits, but genes. The incorporation of modern genetics (see Chapter 16) into Darwin's theory of evolution took place in the middle of the twentieth cen- tury and produced many new insights about how populations evolve.

The focus on genes led to a description of evolution as changes in the allele frequencies of genes over time. Allele frequencies describe how common different alleles are in a population. For example, the peppered moths we discussed earlier have a light allele (a} and a dark allele (A) for color. A population with many light moths and few dark moths might have allele frequencies of92% a and 8% A. As the habitat becomes more polluted, dark moths become more common, and the dark allele increases in frequency. In a polluted area, the allele frequencies might change to 5 % a and 95 % A.

We can describe natural selection in terms of allele frequencies as well: (1) There is variation in a gene when multiple alleles for that gene exist within a population. For example, in peppered moths there are two alleles for color, A and a. (2) A specific allele may give an organism an advantage chat allows it to reproduce more than other organisms in the population. In a polluted habitat, for example, the A allele is advantageous. (3) As a result, more copies of the advantageous allele are passed to the next generation, and the frequency of the advantageous allele increases in the population. In a polluted habitat, the frequency of the A allele increases.

Notice that, although natural selection affects genes and allele frequencies, natural selection does not act directly on genes. Another way to say this is: Natural selection acts on an organism's phenotype (traits), not on its genotype (genes). To see why, let's go back to the peppered moth. In peppered moths, the dark allele (A) is dominant AA and the light allele (a) is recessive. This means that both AA moths and Aa moths have dark wings (F~gure 17.21). Whether a bird is likely to eat the moth depends on the moth's phenotype (whether it is dark or light), not its geno- type. A bird is equally likely to eat a dark moth whether it has genotype AA or Aa.

• • •

Aa

FIGURE 17.21 Natural selection acts on phenotype, not genotype. In the case of these two dark moths, it's the phenotype (dark color} that matters, not the genotype (AA versus Aa} .

498 PART THREE BIOLOGY • • • Where Variation Comes From Natural selection cannot happen without variation. Furthermore, populations with more variation have a better chance of adapting co a changing environ- ment. This is because with more variation, it is more likely chat somewhere in the population there are alleles chat will allow some individuals co survive under the new conditions. For instance, what would have happened to peppered moths during the Industrial Revolution if all the moths had been light and none were dark? In polluted areas, populations with only light moths might have died out. (In Chapter 21, we'll see that having many kinds of species in a habitat also in- creases the chance char at lease some organisms will survive major changes in the environment.)

CHAPTER 17 THE EVOLUTION OF LIFE 499

But where does variation come from? An understanding of genetics enabled biologists co answer chis question. Genetic mutations (see Chapter 16) constantly create new variations within populations. For example, when a genetic mutation changes the amino acids in a protein, it may produce a new allele for a given gene. Sexual reproduction also contributes co variation by bringing together alleles for different traits in new combinations.

• • •

I .......

CHAPTER 17 THE EVOLUTION OF LIFE 503

•fl=• Evidence of Evolution EXPLAIN TH Is How do corn on the cob, a dog's dewclaw, and the human hand provide evidence for evolution?

A 11 scientific theories make predictions about what we should observe in nature (see Chapter 1). If these predictions are confirmed, the theory is supported. The theory of evolution has been tested repeatedly against observations of the natural world, and the evidence for evolution is overwhelm- ing. Eight main kinds of evidence support the idea that evolution produced the diversity <>f life on Earth: (1) observations of natural selection in action, (2) arti- ficial selection, (3) similarities in body structures, (4) vestigial organs, (5) DNA and molecular evidence, (6) patterns of development, (7) hierarchical organiza- tion of living things, (8) biogeography, and (9) fossils. We will look at the first eight topics here, and then consider fossils in Integrated Science 17C.

l. Observations of natural selection in action. In many cases, scientists have seen natural selection produce evolutionary changes in populations; they have observed and measured the actual changes in populations. Examples include some of the cases we have looked at: Australian rabbits evolved resistance to the myxoma virus, so that over time a smaller and smaller fraction of individuals died from the disease. Peppered moths evolved to become better camouflaged in their environments-dark moths became more and more common as habitats became polluted, and then became less and less common as pollution was cleaned up. Bacteria evolved resistance co certain antibiotics, so that these antibiotics no longer controlled infections. Scientists have also studied how the beaks of Darwin's finches evolve after a drought, how insects evolve resistance to pesticides, and natural selection in a wide variety of other populations.

2. Artificial selection. Artificial selection is the selective breeding of organisms with desirable traits in order to obtain organisms with similar traits. Humans artificially select for desirable traits in domesticated animals and crops all the time: We breed fast racehorses to try to get faster racehorses; different types of dogs to produce superior hunters, herders, or sled-pullers (Figure 17.29); and varieties of strawberries to grow the largest and sweetest fruit. In artifi- cial selection, humans control the reproductive success of different organisms and bring about distinct evolutionary changes in populations over time. These changes can be dramatic-think how much a Chihuahua differs from the ani- mal it is descended from, the wolf Or look ac Figure 17.30 co see the difference between the corn we eat today and teosinte, the plant from which corn was bred. Artificial selection has produced countless forms of domestic animals and crops, all with traits valued by humans.

3. Similarities in body structures. We see evidence of the evolutionary histories of species in the structures of their bodies. Consider, for example, the limbs of different mammals. Different mammals use their front limbs for differ- ent purposes: Humans use theirs as arms and hands for manipulating tools, cats use theirs to walk on, whales use theirs as flippers, and bats use theirs as wings. If each of these animals had originated independently, we would expect their limbs to look completely different. Each limb would have been designed from scratch to best perform its function. Bue, despite the differ- ent functions of human hands, cat legs, whale flippers, and bat wings, all these limbs show the same arrangement of bones (Figure 17.31). This suggests that the limbs were inherited from a common ancestor and then modified through natural selection for different functions.

LEARNING OBJECTIVE List and provide examples of the main kinds of evidence that support the theory of evolution.

UNIFYING CONCEPT

• 'Ihe Scientific Method Section 1.3

FIGURE 17.29 Artifkial selection has produced great diversity in dogs.

FIGURE 17.30 Corn (below), one of the most important agricultural crops in the world, was laboriously bred through artificial selection from teosinte (above). Teosime has tiny cobs, only a few rows of kernels, and inedible hard coverings on its seeds.

504 PART THREE BIOLOGY

A mouse ~nd a whale are about as different as two mammals can be. Yet just about every bone In a mouse corresponds to a specific bone In a whale. These slmllarltles suggest that mice and whales had a common ancestor and that their skeletons were mod I fled over time by natur~ selection to flt different environments and ways of Ufe.

Maste ringPhysics • TUTORIAL: Reconstructing Forelimbs

FIGURE 17.32 The Texas blind salamander lives in lighdcss caves. It has tiny vestigial eyes (dark dots in the photo) chat are covered by skin.

A dog's dewclaw Is a vestigial organ. The dewclaw Is a digit that appears on the Inside of the front paws. It does not reach the ground and has no function. It Is just what remains of a formerly functiona l toe.

Human

FIGURE 17.31

Cat Whale Bat

Although these mammalian limbs arc used for different activities, they arc composed of the same sec of bones, evidence chat they were inherited from a common ancestor.

4. Vestigial organs. An organism's evolutionary history often leaves traces in its body. Some organisms have vestigial organs. Vestigial organs are not functional- they are just the remains of an organ found in the orga,11ism's ancestor. For example, we think of snakes as legless. But did you know that certain snakes actually have tiny, partial hind legs? The tiny stubs have no purpose-they are just the remains of what once were bigger limbs. A snake's vestigial hind legs provide evidence that snakes evolved from animals with legs. in the same way, many blind cave species lack functional eyes in their lightless habitats but retain vestigial eyes (Figure 17.32). These vestigial organs suggest that cave species evolved from animals with eyes.

5. DNA and molecular evidence. The DNA of related species have similar nucle- otide (ACGT) sequences. In fact, the more closely related rwo species are, the more similar their DNA sequences tend to be. This is true not only for DNA sequences that code for proteins, but even for sequences that have no known function. If each species on Earth had originated independently, would we expect to see similar noncoding DNA in related species? DNA similarity suggests that DNA did not originate independently in each species but was inherited from a common ancestor and then modified during evolution.

6. Patterns of development. Related species develop in similar ways. If each species on Earth had originated independently, we wouldn't expect these similarities in development. For example, even though humans have no tails, we go through a tailed stage, just like other vertebrates (Figure 17.33).

7. Hierarchical organization of living thin gr. Darwin's theory of evolution explains Earth's diversity of species as originating through numerous speciation events. If this is the case, then we expect living things to be organized into hierarchical sers of "nested groups"- that is, "groups within groups." Each living species should have fewer traits in common with more distant rela- tives, and more traits in common with species that it split off from more recently. This is in fact how living things on Earth are organized. Humans, for example, share a backbone with other vertebrates such as fishes, amphib- ians, reptiles, and mammals; they share four limbs with terrestrial vertebrates such as amphibians, reptiles, and mammals but not with fish, which are more distantly related; they share a waterproof skin with reptiles and mammals

CHAPTER 17 THE EVOLUTION OF LIFE 505

Turtle

FIGURE 17.33

Mouse Human

Related species go through similar stages in their development. The human embryo goes through a tailed stage just like the other vertebrates, even though humans don't have tails.

Chick

but not with amphibians, which are more distancly related; and they share the trait of nursing their young with milk with other mammals but not with the more distancly related reptiles. Living things flt into a hierarchical organization, as predicted by evolution. We do not see traits scattered across living things. For example, we do not see a backbone in vertebrates plus some worms and some insects and some snails. The characteristics that organisms have make sense based on their evolutionary history and relationships.

8. Biogeography. Biogeography is the study of how species are distributed on Earth. Biogeography is consistent with evolution: It supports rhe idea chat organisms evolved in a certain place and then left descendants in the places where they were able to spread. Biogeography does not support the idea that organisms were specially designed to flt into a specific type of habitat and then distributed where these habitats occur on Earth. For example, even though the Arctic and Antarctic have similar environments, they are occu- pied by entirely different species (Figure 17.34). The same is true for New World tropical forests and Old World tropical forests.

What biogeography does show is that the ranges of many species are bounded by geographic barriers such as oceans or mountain ranges. For example, many organisms are restricted to a single continent. In addition, closely related species tend to be found close together, suggesting that they evolved in one place and then spread. For example, all of Darwin's finches

FIGURE 17.34 The Arctic and Antarctic, which have similar habitats, are occupied by very different spe· des. Polar bears are found in the Arctic but not the Antarctic. Penguins arc found in the Antarctic but not the Arct ic.

Pig

506 PART THREE BIOLOGY

FIGURE 17.35 Why arc terrestrial vertebrates rare or absent from islands, whereas flying species are common? This is rhc: Hawaiian hoary bat, chc only mammal found on Hawaii prior to human colonization of the islands.

Explain how fossils provide evidence of evolution.

arc found in or near the Galapagos, and all the honeycreepers are found in Hawaii. Similarly, island species are usually most closely related to species found on the closest mainland. Islands also tend to have fewer species than an equally sized area of the mainland, and many island species are endemic, meaning they are found nowhere else on Earth. Finally, islands tend to be occupied by many flying animals but few terrestrial ones (Figure 17.35). All these points suggest that organisms were not dispersed purposefully around Earth, but instead evolved in one place and then left descendants where they were able to spread.

CHECK YOURSELF Why Is the fact that many species found on Islands resemble species found on the nearest mainland evidence for evolution?

CHECK YOUR ANSWER This pattern suggests that island species evolved when some mainland indi- viduals colonized the island and then evolved in isolation, rather than that species were distributed purposefully around the Earth.

Integrated Science 17C EARTH SCIENCE

Fossils: Earth's Tangible Evidence of Evolution

EXPLAIN TH 15 Why do fossil whales have legs?

E volution has left a record in Earth's rocks-fossils. Because we can dare fossils from the age of the rock formations they belong to, we can follow the evolution of certain groups of organisms over time. For example, fos-

sil whales show that whales are descended from hoofed mammals. Fossil whales also tell us how many key whale traits evolved. In Figure 17.36a, we can see how, over time, whale nostrils moved from the front of the skull to the top of the skull, forming a blowhole. Fossil whales also show how whales lost their hind legs as they became more and more adapted to an aquatic existence. The oldest whale fossils, such as the 50-million-year-old Ambulocetus, have large hind legs that were used both on land and for swimming (Figure 17.36b). Ambulocetus also has small hooves on its front legs, providing clear evidence that whales are descended from hoofed mammals. Rhodocetus, a 46-million-year-old fossil whale, shows reduced hind legs-these are not attached to the backbone and so could not have supported much weight. Rhodocetus also shows prominent tail muscles that would have been effective for swimming. In the 40-million~year~old Dorodon, hind limbs are present, bur they are tiny: Dorodon was clearly a fully aquatic spe- cies. In modern whales, there is no evidence of hind limbs on the outside of the body, although tiny remnants of che pelvis and sometimes femurs remain inside the body.