Download Evolution by Natural Selection: A Comprehensive Overview and more Lecture notes Theory of Evolution in PDF only on Docsity!
Natural selection explains how populations become well suited to their environments over time. The shape and coloration of leafy sea dragons (a fish closely related to seahorses) are heritable traits that help them to hide from predators.
Evolutionary Patt
E
rns and Proc
E
ss
E
s
unit
T
his chapter is about one of the great ideas in science: the theory of evolution by natural selection, formulated independently by Charles Darwin and Alfred Russel Wallace. The theory explains how populations —individuals of the same species that live in the same area at the same time—have come to be adapted to environments ranging from arctic tundra to tropical wet forest. It revealed one of the five key attributes of life: Populations of organisms evolve. In other words, the heritable characteris- tics of populations change over time (Chapter 1). Evolution by natural selection is one of the best supported and most important theories in the history of scientific research. But like most scientific breakthroughs, this one did not come easily. When Darwin
Evolution by Natural
22 Selection
by reviewing by asking by applying
with regard to
keeping in mind
In this chapter you will learn that Evolution is one of the most important ideas in modern biology
What is the evidence for evolution? Evolution in action: two case studies
The process of evolution by natural selection (^) 22.
Common myths about natural selection and adaptation
The pattern of evolution: species have changed and are related (^) 22.
The rise of evolutionary thought
This chapter is part of the Big Picture. See how on pages 516–517.
436 unit 4 Evolutionary Patterns and Processes
shops for manufacturing goods by hand with huge, mechanized assembly lines. A scientific revolution, in contrast, overturns an existing idea about how nature works and replaces it with a radically differ- ent idea. Revolutionary scientific theories include Copernicus’s theory of the Sun as the center of our solar system, Newton’s laws of motion and theory of gravitation, the germ theory of dis- ease, the theory of plate tectonics, and Einstein’s general the- ory of relativity. These theories are the foundation of modern science. The advance of the theory of evolution by natural section rep- resented a profound scientific revolution. The idea that Darwin and Wallace overturned—that species were supernaturally, not naturally, created—had dominated thinking about the nature of organisms in Western civilization for over 2000 years.
Plato and Typological Thinking The Greek philosopher Plato claimed that every organism was an example of a perfect essence, or type, created by God, and that these types were unchanging (Figure 22.1a). Plato acknowledged that individuals of a species sometimes varied slightly from one another, but that these were just trivial deviations around a “perfect essence.” Today, philosophers and biologists refer to ideas like this as typological thinking. Typological thinking also occurs in the Bible’s book of Genesis and in the creation stories of many other religions, where a divine being creates each type of organism.
Aristotle and the Scale of Nature Not long after Plato developed his ideas, Aristotle ordered the types of organisms known at the time into a linear scheme called the great chain of being, or scale of nature, where “scale” means a ladder or stairway (Figure 22.1b). Aristotle proposed that species were fixed types organized into a sequence based on increased size and complexity. The scale started with minerals and lower plants at the bottom, then rose through higher plants, lower and
published his theory in 1859 in a book called On the Origin of Species by Means of Natural Selection , it unleashed a firestorm of protest throughout Europe. At that time, the leading explanation for the diversity of organisms was an idea called special creation. Special creation held that (1) all species are independent, in the sense of being unrelated to each other; (2) life on Earth is young—perhaps just 6000 years old; and (3) species are immu- table, or incapable of change. These beliefs were explained by the instantaneous and independent creation of living organisms by a supernatural being. Darwin’s theory was radically different. How did it differ? In everyday English, the word “theory” suggests a thoughtful guess, but a scientific theory is an explanation for a broad class of ob- servations that is widely supported by overwhelming evidence. (For help with problematic words, see BioSkills 17.) Scientific theories usually have two components: a pattern and a process.
1. The pattern component is a statement that summarizes a series of observations about the natural world. The pattern component is about facts—about how things are in nature. 2. The process component is a mechanism that produces that pat- tern or set of observations. This chapter begins with an overview of the development of evolutionary thought, and then examines the pattern and pro- cess components of the theory of evolution by natural selection.
22.1 The Rise of Evolutionary
Thought
People often describe the theory of evolution by natural selection as revolutionary. Revolutions overturn things—they replace an existing entity with something new and often radically differ- ent. A political revolution removes the ruling class or group and replaces it with another. The industrial revolution replaced small
Lower
Higher
Lower
Higher
(a) Plato: Typological thinking
(b) Aristotle: Typological thinking
(d) Darwin and Wallace: Change through time
(c) Lamarck: Change through time
Figure 22.1 Models of the Diversity of Life Have Changed through Time. visual models are helpful for comparing ideas. the models shown here include only five living species, for simplicity. Each model is explained in the text.
438 unit 4 Evolutionary Patterns and Processes
formed 3.4–3.8 billion years ago. Data from relative and absolute dating techniques agree: Life on Earth is ancient. A great deal of time has gone by for change to occur.
Extinction Changes the Species Present over Time In the early
nineteenth century, researchers began discovering fossil bones, leaves, and shells that were unlike structures from any known ani- mal or plant. At first, many scientists insisted that living examples of these species would be found in unexplored regions of the globe. But as research continued and the number and diversity of fossil collections grew, the argument became less and less plausible. The issue was finally settled in 1812 when Baron Georges Cuvier published a detailed analysis of several extinct species — that is, species that no longer exist. Cuvier intentionally focused on the fossils of large terrestrial animals such as mammoths, mastodons, giant armadillos, giant deer, and giant sloths. Unlike many plants or marine animals, he reasoned, it is very unlikely that large, distinctive terrestrial animals such as these would remain undiscovered if they were still alive (Figure 22.3). Cuvier’s overwhelming evidence convinced scientists of the fact of extinction. Darwin interpreted extinct forms as evidence that species are not static, immutable entities, unchanged since the moment of spe- cial creation. He reasoned that if species have gone extinct, then the array of species living on Earth has changed through time.
Hutton’s ideas were popularized by Darwin’s close friend, the geologist Charles Lyell. Sedimentary rocks, along with rocks derived from episodic lava flows, form with younger lay- ers deposited on top of older layers. Lyell and others used this information to place fossils in a younger-to-older sequence, based on the fossils’ relative position in layers of sedimentary rock (Figure 22.2). As the scientists observed similarities in rocks and fossils at different sites, they began to create a geologic time scale: a sequence of named intervals called eons, eras, and periods that represented the major events in Earth history (see Chapter 25). The geologic time scale was a relative one, however. The absolute age of Earth was still unknown. After Marie Curie’s discovery of radioactivity in the late 1800s, researchers realized that radioactive decay—the steady rate at which unstable “parent” atoms are converted into more stable “daughter” atoms—furnished a way to assign absolute ages, in years, to the relative ages in the geologic time scale. Radioactive decay functions as a “natural clock.” For example, the half-life of uranium-238 is about 4.5 billion years, which means that 50 percent of uranium-238 atoms will decay to lead-206 atoms dur- ing this time. Knowing the half-life, geologists can use the ratio of uranium to lead in a rock sample to infer the age of the sample. According to data from radiometric dating, Earth is about 4. billion years old, and the earliest signs of life appear in rocks that
Older rock layers
Younger rock layers
Tracks from a mammal- like reptile
Fern
Trilobite
Older fossils
Younger fossils
~510 mya
~280 mya
~275 mya
Figure 22.2 Sedimentary Rocks Reveal the Vastness of Geologic Time. the relative ages of sedimentary rocks are used to determine the relative ages of fossil organisms because younger layers are deposited on top of older ones. the deepest rock layer in the Grand canyon is over a billion years old, and the top layer is 270 million years old.
chaPtEr 22 Evolution by Natural Selection 439
(see Figure 22.4). Over a period of about 25 million years, the fins of species similar to today’s lungfish transitioned into limbs similar to those found in today’s amphibians, reptiles, and mammals—a group called the tetrapods (literally, “four-footed”). These observations support the hypothesis that an ancestral lungfish-like species first used stout, lobed fins to navigate in shal- low aquatic habitats. Then, over many generations, some indi- viduals acquired traits that allowed them to move onto land. Their descendants (who inherited these traits) further evolved, becoming more and more like today’s tetrapods in appearance and lifestyle. Note that such evolutionary transitions are not goal oriented or pur- poseful. Rather, some individuals with favorable traits managed to survive and reproduce in the new environment, resulting in change in the population over time (explained further in Section 22.3).
Recent analyses of the fossil record suggest that over 99 per- cent of all the species that have ever lived are now extinct. The data also indicate that species have gone extinct continuously throughout Earth’s history—not just in one or even a few cata- strophic events (see Chapter 25).
Transitional Features Link Older and Younger Species Long
before Darwin published his theory, researchers reported strik- ing resemblances between the fossils found in the rocks under- lying certain regions and the living species succeeding them in the same geographic areas, such as the extinct giant sloths of South America and the sloths that occur there today. The pat- tern was so widespread that it became known as the “law of succession.” Darwin pointed out that this pattern provided strong evidence in favor of the hypothesis that species had changed through time. He proposed that the extinct forms and living forms were related—that they represented ancestors and descendants. As the fossil record expanded, researchers discovered species with characteristics that broadened the scope of the law of succes- sion. A transitional feature is a trait in a fossil species that is inter- mediate between those of ancestral (older) and derived (younger) species. For example, intensive work over the past several decades has yielded fossils that document a gradual change over time from aquatic animals that had fins to terrestrial animals that had limbs
Figure 22.3 Evidence of Extinction. this 19th century drawing depicts cuvier’s fossil evidence. scientists agreed that the sloth, like other giant fossil vertebrates, was too large and unique to be overlooked if it were alive; it must have gone extinct.
Eusthenopteron (~385 mya)
Tiktaalik (~375 mya)
Acanthostega (~365 mya)
Tulerpeton (~362 mya)
Fin rays
Distal elementsUlna and radiusHumerus
Figure 22.4 Transitional Features during the Evolution of the Tetrapod Limb. Fossil species similar to today’s lungfish and tetrapods have fin and limb bones that are transitional features. Eusthenopteron was aquatic; Tulerpeton was probably semiaquatic (mya = million years ago). contrast how the transitions shown above would fit into lamarck’s early model of evolution (Figure 22.1c) versus darwin and Wallace’s model of evolution (Figure 22.1d).
chaPtEr 22 Evolution by Natural Selection 441
Commonanc estor
(a) Pattern: Although the Galápagos mockingbirds are extremely similar, distinct species are found on different islands.
(b) Recent data support Darwin’s hypothesis that the Galápagos mockingbirds share a common ancestor.
Nesomimus macdonaldi
Nesomimus melanotis
Nesomimus parvulus
Nesomimus trifasciatus
Galápagos islands
An ancestral population colonized the islands. Over time, the population diversified into distinct species on different islands
Isabela
Floreana Española
San Cristobal
Genovesa Marchena
Santiago
Santa Cruz N. trifasciatus (Floreana)
N. parvulus (Isabela)
N. parvulus (Marchena)
N. macdonaldi (Española)
N. melanotis (San Cristobal)
Mockingbird species from the Caribbean and Gulf of Mexico
N. parvulus (Santiago)
N. parvulus (Santa Cruz)
N. parvulus (Genovesa)
Common ancestor
Islands where species are found
50 km
Figure 22.6 Close Relationships among Island Forms Argue for Shared Ancestry.
Darwin realized that this pattern—puzzling when examined as a product of special creation—made perfect sense when inter- preted in the context of evolution, or descent with modification. The mockingbirds were similar, he proposed, because they had descended from the same common ancestor. That is, instead of being created independently, mockingbird populations that colo- nized different islands had changed through time and formed new species (Figure 22.6b). Recent analyses of DNA sequences in these mockingbirds support Darwin’s hypothesis. Researchers have used the DNA sequence comparisons to place the mockingbirds on a phylogenetic tree —a branching diagram that describes the ancestor–descendant relationships among species or other taxa (see Chapter 25). As Figure 22.6b shows, the Galápagos mockingbirds are each other’s closest living relatives. As Darwin predicted, they share a single common ancestor. (For help with reading phylogenetic trees, see BioSkills 13.)
Similar Species Share Homologies Translated literally, homol-
ogy means “the study of likeness.” When biologists first began to study the anatomy of humans and other vertebrates, they were struck by the remarkable similarity of their skeletons, muscles, and organs. But because the biologists who did these early stud- ies were advocates of special creation, they could not explain why striking similarities existed among certain organisms but not others.
Today, biologists recognize that homology is a similarity that exists in species due to common ancestry. Human hair and dog fur are homologous. Humans have hair and dogs have hair (fur) because they share a common ancestor—an early mammal species—that had hair (see Making Models 22.1).
Making Models 22.1 Tips on Drawing Phylogenetic Trees Phylogenetic trees can help you understand the concept of homology (see BioSkills 13 and Chapter 25). You can use a tick mark on a branch to symbolize the origin of a new trait. All the organisms to the right of that tick mark are assumed to possess that trait. Compare the meaning of two trees below:
“Hair is homologous in dogs and humans”
Dogs
Humans
Lizards
Humans
Dogs
Lizards “Hair is not homologous in dogs and humans”
Hair
Hair
Hair
MODEL draw a dot on the node (branching point) of each tree that represents the most recent common ancestor of dogs and humans, and indicate if it had hair. Which tree is correct? To see this model in action, go to https://goo.gl/TfHLCq
442 unit 4 Evolutionary Patterns and Processes
Level Example Genetic homology Similarity in DNA, RNA, or amino acid sequences due to inheritance from a common ancestor
Amino acid sequences from a portion of the Aniridia gene product found in humans are 90 percent identical to those found in the Drosophila eyeless gene product.
Gene: Amino acid sequence (single-letter abbreviations): Aniridia (Human) eyeless (Fruit fly)
LQRNRTSFT QE QI EA LEKEFERTHYPDVFARERLA A KI D LPEARIQVWFSNRRAKWRREE LQRNRTSFT ND QI DS LEKEFERTHYPDVFARERLA G KI G LPEARIQVWFSNRRAKWRREE
Chick Human House cat
Pharyngeal pouch
Tail Tail Tail
Human Horse Bird Bat Seal
Humerus
Radius and ulna
Carpals Metacarpals Phalanges
Developmental homology Similarity in embryonic form or developmental processes due to inheritance from a common ancestor
The early embryonic stages of a chick, a human, and a cat show a strong resemblance and are the product of similar developmental processes. Structural homology Similarity in adult organismal structures due to inheritance from a common ancestor
Even though their function varies, all vertebrate limbs are modifications of the same number and arrangement of bones. (These limbs are not drawn to scale.)
(For a key to the single-letter abbreviations used for the amino acids, see Figure 3.2.)
SUMMARY (^) Table 22.1 Three Levels of Homology
Homology can be recognized and studied at three levels, sum- marized in Table 22.1:
1. Genetic homology occurs in DNA nucleotide sequences, RNA nucleotide sequences, or amino acid sequences. Perhaps the most fundamental of all homologies is the genetic code. With a few minor exceptions, all organisms use the same rules for transferring the information coded in DNA into proteins (see Chapter 16). For example, the eyeless gene in fruit flies and the Aniridia gene in humans are so similar that their protein products are 90 percent identical in amino acid sequence. Both genes act in determining where eyes will develop—even though fruit flies have a compound eye with many lenses and humans have a camera eye with a single lens. 2. Developmental homology is observed in embryos. For example, early chick, human, and cat embryos have tails and structures called pharyngeal pouches that are a product
of similar developmental processes. Later, these pouches are lost in all three species, and tails are lost in humans. But in fish, the pharyngeal pouches stay intact and give rise to func- tioning gills in adults. To explain this observation, biologists hypothesize that pharyngeal pouches and tails exist in chicks, humans, and cats because they existed in the fishlike species that was the common ancestor of today’s vertebrates. Pha- ryngeal pouches are a vestigial trait in chicks, humans, and cats; embryonic tails are a vestigial trait in humans.
3. Structural homology is a similarity in adult morphology , or form. A classic example is the common structural plan observed in the limbs of vertebrates. In Darwin’s own words, “What could be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should
444 unit 4 Evolutionary Patterns and Processes
As you evaluate the evidence supporting the pattern compo- nent of the theory of evolution, though, it’s important to recog- nize that no single observation or experiment instantly “proved” the fact of evolution. Rather, data from many different sources are much more consistent with evolution than with special creation. Descent with modification is a successful and powerful scientific theory because it explains observations—such as vestigial traits and the close similarities among species on neighboring islands— that special creation does not. What about the process component of the theory of evolution by natural selection? If hippos and whales were not created inde- pendently, how did they come to be?
- A phylogeny of the fossil cetaceans, estimated on the basis of similarities and differences in morphological traits other than limbs and overall body shape, indicates that a gradual transition occurred between terrestrial forms and aquatic, whale-like forms (Figure 22.8).
- Relative dating, based on the positions of sedimentary rocks where the fossils were found, agrees with the order of species indicated in the phylogeny.
- Absolute dating, based on analyses of radioactive atoms in rocks in or near the layers where the fossils were found, also agrees with the order of species indicated in the phylogeny.
- A phylogeny of living whales and dolphins, estimated from similarities and differences in DNA sequences, indicates that hippos—which spend much of their time in shallow water— are the closest living relative of cetaceans. This observation supports the hypothesis that cetaceans and hippos shared a common ancestor that was semiaquatic.
- Vestigial hip and hindlimb bones occur in some whales as adults, and vestigial hindlimb buds—outgrowths where legs form in other mammals—occur in some dolphin embryos. The general message here is that many independent lines of evidence converge on the same conclusion: Whales gradually evolved from a terrestrial ancestor about 50 million years ago.
Indohyus (~50–47 mya)
Ambulocetus (~48 mya)
Rodhocetus (~47 mya)
Delphinapterus (extant)
Dorudon (~40 mya)
10 cm
50 cm
50 cm
50 cm
50 cm
Hoofed, semiaquatic
Modern whales
Hoofed, semiaquatic
Semiaquatic, foot-powered swimmer
Fully aquatic, tail-powered swimmer
Vestigial hindlimb
Common ancestor of cetaceans
CETACEANS
©2007 Nature Publishing Group
Figure 22.8 Data on Evolution from Independent Sources Are Consistent. this phylogeny of fossil cetaceans is consistent with data from relative dating, absolute dating, and phylogenies estimated from molecular traits in living species—all agree that whales evolved from terrestrial ancestors that also gave rise to hippos.
check YoUR UndeRStAnding
If you understand that...
- Species are not static, but change through time.
- Species are related by common ancestry.
You should be able to...
- determine what kind of evidence would support the hypothesis that birds evolved from dinosaurs.
- Explain why the dna sequences of chimpanzees and humans are about 96 percent similar. Answers are available in Appendix A.
chaPtEr 22 Evolution by Natural Selection 445
Malthus’s studies of human populations in England and else- where led him to a startling conclusion: Since many more indi- viduals are born than can survive, a “struggle for existence” occurs as people compete for food and places to live. Darwin combined his observations of artificial selection with this notion of “struggle for existence” in natural populations, which he knew—from his countless studies—contained varia- tion. From this synthesis arose his concept of natural selection. Although both Darwin and Wallace arrived at the same idea, Darwin’s name is more closely associated with natural selection because of his extensive evidence for it in On the Origin of Species.
Darwin’s Four Postulates Darwin distilled the process of evolution by natural selection into four simple postulates (criteria) that form a logical sequence (see Making Models 22.2):
1. Variation exists among individual organisms that make up a population, such as variation in size and shape. 2. Some of the trait differences are heritable, meaning that they are passed on to offspring. For example, parent flowers with long petals tend to have offspring with long petals. 3. Survival and reproductive success is highly variable. Many more offspring are produced than can possibly survive. Thus, only some individuals in each generation survive long enough to produce offspring, and among the individuals that produce offspring, some will produce more than others. 4. The subset of individuals that survive best and produce the most offspring is not a random sample of the popu- lation. Instead, individuals with certain heritable traits are more likely to survive and reproduce. Altogether, natural selection occurs when individuals with certain heritable traits produce more surviving offspring than do individuals without those traits. Thus, the frequency of the selected traits increases from one generation to the next. Biolo- gists now know that traits are determined by alleles, particular versions of genes (see Chapter 14). Thus, the outcome of evolu- tion by natural selection is a change in allele frequencies in a population over time (see Chapter 23).
22.3 The Process of Evolution:
How Does Natural Selection Work?
Darwin’s greatest contribution did not lie in recognizing the fact of evolution. Lamarck and other researchers, including Darwin’s own grandfather Erasmus Darwin, had proposed evolution long before Charles Darwin began his work. Instead, Darwin’s crucial insight lay in recognizing a process, called natural selection, that could explain the pattern of descent with modification.
Darwin’s Inspiration
How did Darwin arrive at his insight? To begin, Darwin had spent decades exploring and documenting the diversity of plants and animals, both around the globe and in his native England. So he had a wealth of data on variation within and among species, and he viewed this variation in the context of the ancient and chang- ing Earth as popularized by his geologist friend, Charles Lyell. All of this careful work gave Darwin an especially strong foun- dation in the pattern of evolution. To make sense of the process of evolution, Darwin turned in part to pigeon breeding—a model system that would be easier to study and manipulate than popu- lations in the wild. Pigeon breeding was popular in England at the time, and in Darwin’s words, “The diversity of the breeds is some- thing astonishing” (see Figure 22.9). Darwin crossbred pigeons and observed how characteristics were passed on to offspring. He could choose certain individuals with desirable traits to reproduce, thus manipulating the compo- sition of the population by a process called artificial selection. It was clear to Darwin and other breeders that the diverse varieties were all descended from the wild rock pigeons. Another influence on Darwin was the publication of a book by Thomas Robert Malthus, An Essay on the Principle of Popula- tion , which inspired heated discussion in England at the time.
Figure 22.9 Diversity of Pigeon Breeds in Captivity. darwin used the breeding of pigeons as a model system to study how the characteristics of populations can change over time.
Making Models 22.2 Tips on Drawing Darwin’s Postulates Drawing models can help you understand and remember abstract ideas such as Darwin’s postulates. Simple drawings can be very effective, so focus on the ideas rather than trying to make lifelike illustrations. Consider this example:
Postulate 2: Some trait differences are heritable
MODEL using the flower example shown here, make a model of darwin’s first postulate.
To see this model in action, go to https://goo.gl/cYtAHn
chaPtEr 22 Evolution by Natural Selection 447
3. Was there variation in survival and reproductive suc- cess? The answer is yes. Only a tiny fraction of M. tuberculosis cells in the patient survived the first round of antibiotics long enough to reproduce. Most cells died and left no or almost no offspring. 4. Were survival and reproduction nonrandom? The answer is yes. When rifampin was present, certain cells— those with the drug-resistant allele—had higher reproductive success than cells with the normal allele.
of the RNA polymerase (from a serine to a leucine at the 153rd amino acid)—and a change in its shape. This shape change proved critical. Rifampin, the antibiotic that was being used to treat the patient, works by binding to the RNA polymerase of M. tuberculosis and interfering with tran- scription. Bacterial cells with the C S T mutation continue to produce offspring efficiently even in the presence of the drug, because the drug cannot bind efficiently to the mutant RNA polymerase. These results suggest the steps that led to this patient’s death (Figure 22.10).
1. Completely by chance, one or a few of the bacterial cells present in the patient before the onset of drug therapy happened to have the rpoB allele with the C S T mutation. Under normal condi- tions, mutant forms of RNA polymerase do not work as well as the more common form, so cells with the C S T mutation would not produce many offspring and would stay at low frequency— even while the overall population grew to the point of inducing symptoms that sent the young man to the hospital. 2. Therapy with rifampin began. In response, cells in the pop- ulation with normal RNA polymerase began to grow much more slowly or to die outright. As a result, the overall bacte- rial population declined in size so drastically that the patient appeared to be cured—his symptoms began to disappear. 3. Cells with the C S T mutation continued to increase in number after therapy ended. Eventually the M. tuberculosis population regained its former abundance, and the patient’s symptoms reappeared. 4. Drug-resistant M. tuberculosis cells now dominated the popu- lation, so the second round of rifampin therapy failed. If you understand how antibiotic resistance evolves, you should be able to explain (1) why the relapse in step 3 occurred, and (2) whether a family member or health-care worker who got TB from this patient at step 3 or step 4 would respond to drug therapy.
Testing Darwin’s Postulates Does the sequence of events illus-
trated in Figure 22.10 indicate that evolution by natural selection occurred? One way of answering this question is to review Darwin’s four postulates and test whether each one was verified:
1. Did variation exist in the population? The answer is yes. Due to mutation, both resistant and nonresistant strains of TB were present before administration of the drug. Most M. tuberculosis populations, in fact, exhibit variation for the trait; studies on cultured M. tuberculosis show that a muta- tion conferring resistance to rifampin is present in one out of every 10^7 to 10^8 cells. 2. Was this variation heritable? The answer is yes. The researchers showed that the variation in the phenotypes of the two strains—from drug susceptibility to drug resistance— was due to variation in their genotypes. Because the mutant rpoB gene is passed on to daughter cells when a Mycobacterium replicates, the allele and the phenotype it produces—drug resistance—are passed on to offspring.
PROCESS: EVOLUTION OF DRUG RESISTANCE
M. tuberculosis in lung tissue
Even though the population is drastically reduced, most of the remaining cells are resistant to the drug
normal rpoB gene
Mutant C T mutation cell
- Large population of M. tuberculosis bacteria in patient’s lungs makes him sick. A few of the bacteria cells happen to have a point mutation in the rpoB gene.
- Drug therapy begins killing most M. tuberculosis. Patient seems cured and drug therapy is ended.
- The mutant cells proliferate, resulting in another major infection of the lungs. The patient becomes sick again.
- A second round of drug therapy begins but is ineffective on the drug- resistant bacteria. The patient dies.
Figure 22.10 Alleles That Confer Drug Resistance Increase in Frequency when Drugs Are Used.
448 unit 4 Evolutionary Patterns and Processes
Further, resistance to a wide variety of insecticides, fungi- cides, antiviral drugs, and herbicides has evolved in hundreds of insects, fungi, bacteria, viruses, and plants. In every case, evolution has occurred because the original population includ- ed individuals with the heritable ability to resist some chemi- cal compound. As the susceptible individuals died from the pesticide, herbicide, or drug, the alleles that confer resistance increased in frequency.
Case Study 2: Why Do Beak Sizes and Shapes Vary in Galápagos Finches? Human use of antibiotics has clearly influenced the evolution of some bacterial species. Have biologists also observed evolu- tion caused by natural environmental changes? The answer is certainly yes. As an example, consider research led by Peter and Rosemary Grant. For over four decades, these biologists have been investigating changes in beak size, beak shape, and body size that have occurred in finches native to the Galápagos Islands. Because the island of Daphne Major of the Galápagos is so small—about the size of 80 football fields (see tiny dot between Santiago and Santa Cruz in Figure 22.6)—the Grants’ team has been able to catch, weigh, and measure all the medium ground finches in the island’s population (Figure 22.12) and mark each one with a unique combination of colored leg bands. The medium ground finch makes its living by eating seeds. Finches crack seeds with their beaks. Early studies of the finch population established that beak size and shape and body size vary among individuals, and that beak morphology and body size are heritable. More specifically, parents with particularly deep beaks tend to have offspring with deep beaks. Large parents also tend to have large off- spring. Beak size and shape and body size are traits with heri- table variation.
Selection during Drought Conditions Not long after the team
began to study the finch population, a dramatic selection event occurred. In the annual wet season of 1977, Daphne Major received just 24 millimeters (mm) of rain instead of the 130 mm that normally falls. During the drought, few plants were able to produce seeds, and 84 percent (about 660 individuals) of the medium ground finch population disappeared. Two observations support the hypothesis that most or all of these individuals died of starvation:
1. The researchers found 38 dead birds, and all were emaciated. 2. None of the missing individuals were spotted on nearby islands, and none reappeared once the drought had ended and food supplies returned to normal. The research team realized that the die-off was a natural experiment. Instead of comparing groups created by direct manipulation under controlled conditions, natural experiments allow researchers to compare treatment groups created by an unplanned change in conditions. In this case, the Grants’ team could test whether natural selection occurred by comparing the population before and after the drought.
Percentage of ICU patients with
S. aureus
infections
resistant to vancomycin
Year
1975 1980 1985 1990 1995 2000 2005
Vancomycin use begins here
Vancomycin- resistant S. aureus
30
20
10
0
Figure 22.11 Trends in Infections Due to Antibiotic-Resistant Bacteria. the line indicates changes in the percentage of infections caused by the bacterium Staphylococcus aureus, acquired in the intensive care unit (icu) of hospitals in the united states, that are resistant to the antibiotic vancomycin. DATA: Centers for Disease Control, 2004.
M. tuberculosis individuals with the mutant rpoB allele had higher fitness in an environment where rifampin was present. The mutant allele produces a protein that is an adaptation when the cell’s environment contains the antibiotic. This study verified all four postulates and confirmed that evolution by natural selection had occurred. The M. tuberculosis population evolved because the mutant rpoB allele increased in frequency. It is critical to note, however, that the individual cells them- selves did not evolve—they could not mutate their genes in order to survive the antibiotics. When natural selection occurred, the individual bacterial cells simply survived or died, or produced more or fewer offspring. This is a fundamentally important point: Natural selection acts on individuals, because individuals expe- rience differential reproductive success. But only populations evolve. Allele frequencies change in populations, not in individu- als. Understanding evolution by natural selection requires popu- lation thinking.
Drug Resistance: A Widespread Problem The events reviewed
for this single patient have occurred many times in other pa- tients. Recent surveys indicate that drug-resistant strains now account for about 10 percent of the M. tuberculosis –causing infec- tions throughout the world. Unfortunately, the emergence of drug resistance in TB is far from unusual. Resistance to a wide variety of antibiotics is sky- rocketing globally due to the escalating use of antibiotics in appli- cations ranging from toothpaste to livestock feed. Consider the prevalence in hospitals of the bacterium Staphylococcus aureus that are resistant to the antibiotic vancomycin (Figure 22.11). Most of these S. aureus cells are also resistant to methicillin and other antibiotics—a phenomenon known as multidrug resistance. In some cases, physicians have no effective antibiotics available to treat these infections. The incidence of antibiotic resistance has risen so quickly that the World Health Organization recently announced that the world could soon enter a “post-antibiotic era.”
450 unit 4 Evolutionary Patterns and Processes
- As in chickens, there is a strong correlation between the amount of Bmp4 expression when beaks are developing in young Galápagos finches and the depth of adult beaks (Figure 22.15).
- Further, when the researchers experimentally increased Bmp expression in young chickens, they found that beaks got deeper than normal. This is evidence of a causal relationship between Bmp4 expression and beak depth, not just a correlation. The Grants have recently collaborated with a team of geneticists to sequence the genomes of 120 birds representing all of the finch species on the Galápagos. Among other insights, this research has pinpointed another gene that appears to play an important role in the regulation of beak shape, ALX1. The frequencies of different alleles of ALX1 are highly correlated with beak shape. This same gene is important in the development of human facial structures.
Beak size and depth are only two of the many characteris- tics that the Grants documented over time. Figure 22.14 shows changes that have occurred in three characteristics—average body size, beak size, and beak shape—over 35 years. Long-term studies like this are powerful because they have succeeded in documenting natural selection in response to changes in the environment. The take-home message from the data is that most traits are not inherently “good” or “bad”—the adaptive value of a trait depends on context, and context can change over time. CAUTION Evaluate this statement in the context of the finch data: “Bigger is always better.” How do the error bars (see BioSkills 3 ) affect your evaluation?
Which Genes Are under Selection? Characteristics like body
size and beak shape are polygenic, meaning that many genes— each one exerting a relatively small effect—influence the trait (see Chapter 14). Because many genes are involved, it can be dif- ficult for researchers to know exactly which alleles are changing in frequency when polygenic traits evolve. To explore which of the medium ground finch’s genes might be under selection, researchers in Clifford Tabin’s lab began study- ing beak development in an array of Galápagos finch species. Spe- cifically, they looked for variation in the pattern of expression of cell–cell signals that had already been identified as important in the development of chicken beaks. The hope was that homologous genes might affect beak development in finches. The researchers struck gold when they carried out in situ hybridization (a tech- nique featured in Chapter 21) showing where a cell–cell signal gene called Bmp4 is expressed.
Geospiza fortis
Geospiza magnirostris
Shallow adult beak
Deep adult beak
2 mm 2 mm
Lower Bmp4 expression (little yellow) in embryo’s beak
Higher Bmp4 expression (bright yellow) in embryo’s beak
Figure 22.15 Changes in Bmp4 Expression Change Beak Depth. these micrographs are in situ hybridizations showing the location and extent of Bmp4 expression in young Geospiza fortis and G. magnirostris. in these and four other species that were investigated, the amount of Bmp4 protein produced correlates with the depth of the adult beak.
check YoUR UndeRStAnding
If you understand that...
- If individuals with certain alleles produce the most offspring in a population, then those alleles increase in frequency over time. Evolution results from this process of natural selection on heritable variation.
You should be able to...
- apply darwin’s four postulates to the Galápagos finch case study to verify that natural selection has occurred.
- MODEL use simple sketches of finches with different beak depths to illustrate how darwin’s four postulates apply in the finch case study. Answers are available in Appendix A.
Body size
Large
Small
Beak size
Large
Small
Beak shape Pointed
Blunt
1970 1980 1990 Year
2000
Figure 22.14 Body Size, Beak Size, and Beak Shape in Finches Fluctuated over a 35-Year Interval. Each of the dependent vari- ables is a mathematical composite of three measurements. Body size is calculated from body mass, wing length, and leg length. Beak size and shape are calculated from beak depth, length, and width. DATA: Grant, P. R., and B. R. Grant. 2002. Science 296: 707–711; Grant, P. R., and B. R. Grant.
- Science 313: 224–226.
chaPtEr 22 Evolution by Natural Selection 451
hemoglobin-carrying red blood cells. Your body does not nor- mally produce more red blood cells than it needs, because viscous (thick) blood can cause a disease—chronic mountain sickness— that can lead to heart failure. The increase in red blood cells is an example of what biologists call acclimatization —a change in an individual’s phenotype that occurs in response to a change in natural environmental conditions. (When this process occurs in study organisms in a laboratory, it is called acclimation .) Phenotypic changes due to acclimatization
22.5 Debunking Common Myths
about Natural Selection and Adaptation
Evolution by natural selection is a simple process—just the logi- cal outcome of some straightforward postulates. However, natural selection and population thinking are relatively new concepts in the scope of human culture. After more than two thousand years of belief in typological thinking, the scale of nature, and goal-oriented evolution (Section 22.1), a great deal of cultural inertia has fos- tered myths about evolution by natural selection (see BioSkills 17). Let’s consider three of the most common types of misconceptions about natural selection, summarized in Table 22.3.
Natural Selection Does Not Change Individuals
Perhaps the most important point to clarify about natural selec- tion is that during the process, individuals do not change—only the population does. During the drought, the beaks of individual finches did not become deeper. Rather, the average beak depth in the population increased over time because deep-beaked individu- als survived and produced more offspring than shallow-beaked individuals did. Natural selection acted on individuals, but the evo- lutionary change occurred in the characteristics of the population. In the same way, individual M. tuberculosis cells did not change when rifampin was introduced to their environment. Each of these bacterial cells had the same RNA polymerase allele throughout its life. But because the mutant allele increased in fre- quency in the population over time, the average characteristics of the bacterial population changed.
Natural Selection Is Not “Lamarckian” Inheritance There is a
sharp contrast between evolution by natural selection and evo- lution by the inheritance of acquired characters—the hypothesis promoted by Jean-Baptiste de Lamarck. If you recall, Lamarck proposed that (1) individuals change in response to challenges posed by the environment (such as giraffes stretching their necks to reach leaves high in the treetops), and (2) the changed traits are then passed on to offspring. The key claim is that the impor- tant evolutionary changes occur in individuals. In contrast, Darwin realized that individuals do not change when they are selected. Instead, they simply produce more or fewer offspring than other individuals do. When this happens, alleles found in the selected individuals become more or less fre- quent in the population. Darwin was correct: There is no mechanism that makes it possible for natural selection to edit the nucleotide sequence of an allele inside an individual. An individual’s heritable charac- teristics don’t change when natural selection occurs. Natural selection sorts existing variants—it doesn’t change them.
Acclimatization Is Not Adaptation The issue of change in indi-
viduals is tricky because individuals often do change in response to changes in the environment. For example, if you were to travel to the Tibetan Plateau in Asia, your body would experience oxy- gen deprivation due to the low partial pressure of oxygen at high elevations (see Chapter 42). As a result, your body would pro- duce more of the oxygen-carrying pigment hemoglobin and more
Misconception Example “Evolutionary change occurs in organisms”
CORRECTION:
- Natural selection just sorts existing variants in organisms; it doesn’t change them
- Evolutionary change occurs only in populations
- Acclimatization ≠ adaptation
“Evolution is goal directed”
CORRECTION:
- Adaptations do not occur because organisms want or need them
- Mutation, the source of new alleles, occurs by chance
- Evolution is not progressive
- Loss of traits can be adaptive
- There is no such thing as a higher or lower organism
“Evolution perfects organisms”
CORRECTION:
- Not all traits are adaptive
- Some traits cannot be optimized due to fitness trade-offs
- Some traits are limited by genetic, historical, or environmental constraints
Roses cannot grow thorns on purpose to deter herbivores
Humans lack the ability to grow wings, even though flight could be an adaptive trait
Selection does not cause neck length to increase in individual giraffes, only in populations
SUMMARY Table 22. Common Misconceptions, Corrected
chaPtEr 22 Evolution by Natural Selection 453
Genetic correlations are not the only genetic constraint on adaptation. Lack of genetic variation is also important. Consider that salamanders have the ability to regrow severed limbs. Some eels and sharks can sense electric fields. Birds can see ultraviolet light. Even though these traits would possibly confer increased reproductive success in humans, they do not exist—because humans lack the necessary genes.
Traits Are Historically Constrained In addition to being con-
strained by fitness trade-offs, genetic correlations, and lack of genetic variation, adaptations are constrained by history. The reason is simple: All traits evolve from previous traits. Natural selection can change the function of structures dra- matically over time. For example, the tiny incus, malleus, and stapes bones found in your middle ear evolved from bones that were either part of the jaw or supported the jaw in the ances- tors of mammals. These bones now function in transmitting and amplifying sound from your outer ear to your inner ear. Biolo- gists routinely interpret these bones as adaptations that improve your ability to hear airborne sounds. But are the bones a “perfect” solution to the problem of transmitting sound from the outside of the ear to the inside? The answer is no. They are a good solution, given an important historical constraint. Other vertebrates have different structures involved in transmitting sound to the ear. In at least some cases, those structures may be more efficient than our incus, malleus, and stapes.
Traits Are Environmentally Constrained Natural selection often
occurs in the context of a changing environment. Recall that the amount of rain fluctuates on Daphne Major in the Galápagos Islands, causing dramatic changes in vegetation and seed avail- ability over time. Thus, a beak shape that is adaptive in one season may not be adaptive in the next season, preventing the “perfection” of beak shape in the finch population (see Figure 22.14). In addition to fluctuating over time, the abiotic and biotic fea- tures of the environment can change over the geographic range of the population. For example, one of the most notorious invasive plants in North America, purple loosestrife, has spread from the southern United States into Canada over a distance of 1000 km in less than a century. In the south, where the growing season is long, it is adaptive for purple loosestrife to grow large and then flower late. But in the north, it is more adaptive for purple loose- strife to grow less and flower early. No growth rate or flowering time is optimal for all environments. Some environmental events are so catastrophic that organisms are wiped out regardless of which adaptations they have. These chance events, such as volcanic eruptions, asteroid impacts, and deforestation by humans, can cause a change in the average traits of a population that are random with respect to fitness. To summarize, not all traits are adaptive, and even adaptive traits are constrained by genetic, historical, and environmental factors. In addition, natural selection is not the only process that causes evolutionary change. Three other processes—genetic drift, gene flow, and mutation—change allele frequencies over time (see Chapter 23). Compared with natural selection, these processes have very different consequences. (You can see the Big Picture of how natural selection relates to other evolutionary processes on pages 516–517.)
Natural Selection Does Not Lead to Perfection
Although organisms are often exquisitely adapted to their envi- ronment, adaptation is far from perfect. A long list of circum- stances limits the effectiveness of natural selection; only a few of the most important ones are discussed here.
Traits Are Not Always Adaptive Vestigial traits such as the
human coccyx (tailbone) and the tiny hindlimbs of early whales do not increase the fitness of individuals with those traits. The structures are not adaptive. They exist because they were present in the ancestral population. Vestigial traits are not the only types of structures with no or reduced function. Perhaps the best example of nonadaptive traits involves evolutionary changes in DNA sequences. A mutation may change a base in the third position of a codon without changing the amino acid sequence of the protein encoded by that gene. Changes such as these are said to be silent. They occur due to the redundancy of the genetic code (see Chapter 16). Silent changes in DNA sequences are extremely common. But because they don’t change the phenotype, they can’t be acted on by natural selection and are not adaptive.
Fitness Trade-offs Exist In everyday English, the term “trade-off”
refers to a compromise between competing goals. It is difficult to design a car that is both large and fuel efficient, a bicycle that is both rugged and light, or a plane that is both steady and maneuverable. In nature, selection occurs in the context of fitness trade-offs. A fitness trade-off (or simply trade-off ) is a compromise between two traits that cannot be optimized simultaneously. In medium ground finches, for example, there is a trade-off between having a beak that is large and deep enough to crack open tough seeds and one that is small and shallow enough to extract tiny seeds. Simi- larly, there is a trade-off between bright coloration to attract mates and cryptic coloration to hide from predators. Many types of trade-offs occur due to energetic constraints, because every individual has a restricted amount of time and energy available—meaning that its resources are limited. For example, there is a trade-off between the size of eggs or seeds that an individual makes and the number of offspring it can produce (Chapter 51) and a trade-off between investing energy in repro- duction versus immune function (Chapter 39). The message of these findings is simple: Because selection acts on many traits at once, every adaptation is a compromise.
Traits Are Genetically Constrained The studies on Bmp4 expres-
sion in developing finch beaks (see Figure 22.15) produced an interesting observation—increased Bmp4 expression resulted in beaks that were not only deeper but also wider in their side-to-side dimension. This common pattern is due to genetic correlation. Genetic correlations occur because of pleiotropy, in which a single gene affects multiple traits (see Chapter 14). In this case, selec- tion on a gene for one trait (increased beak depth) caused a cor- related, though not necessarily adaptive, increase in another trait (beak width). As it turns out, a finch beak that is deep but narrow from side to side would be more effective at twisting open tough Tribulus fruits, but this phenotype was never produced due to genetic constraints.
454 unit 4 Evolutionary Patterns and Processes
22.5 Debunking Common Myths about Natural Selection and Adaptation
- Natural selection acts on individuals, but evolutionary change occurs in populations. Nonheritable changes that occur in individ- uals due to acclimatization or acclimation are not adaptations and do not result in evolution.
- Evolution is not goal directed and does not lead to perfection. Mutations occur by chance, not because organisms “want” or “need” them to survive. There is no such thing as a higher or lower organism.
- Not all traits are adaptive, and even adaptive traits are limited by fitness trade-offs and genetic, historical, and environmental constraints.
Answers are available in Appendix A
teSt YoUR knoWLedge
1. True or false? Some traits are considered vestigial because they existed long ago. 2. CAUTION Why does the presence of extinct forms and transitional features in the fossil record support the pattern component of the theory of evolution by natural selection? a. It supports the hypothesis that individuals change over time. b. It supports the hypothesis that weaker species are eliminated by natural selection. c. It supports the hypothesis that species evolve to become more complex and better adapted over time. d. It supports the hypothesis that species change over time. 3. Traits that are derived from a common ancestor, like the bones of human arms and bird wings, are said to be __________. 4. CAUTION How can evolutionary fitness be estimated? a. Document how long individuals survive. b. Count the number of healthy, fertile offspring produced. c. Determine which individuals are strongest. d. Determine which phenotype is the most common.
teSt YoUR UndeRStAnding
5. CAUTION According to data presented in this chapter, which one of the following statements is correct? a. When individuals change in response to challenges from the environment, their altered traits are passed on to offspring. b. Species are created independently of each other and do not change over time. c. Populations—not individuals—change when natural selection occurs. d. The traits of populations become more perfect over time. 6. Some biologists summarize evolution by natural selection with the phrase “mutation proposes, selection disposes.” Mutation is a process that creates heritable variation. Explain what the phrase means.
22.1 The Rise of Evolutionary Thought
- Plato, Aristotle, and several religious texts present species as unchanging types. This view is called typological thinking.
- Lamarck proposed a theory of evolution—that species are not static but change through time. He proposed that evolution occurs by the inheritance of acquired characteristics.
- Darwin and Wallace proposed that evolution occurs by natu- ral selection. This was the beginning of population thinking, whereby variation among individuals is the key to understand- ing evolution.
22.2 The Pattern of Evolution: Have Species
Changed, and Are They Related?
- Evidence that species change through time includes data on (1) Earth’s age and the fact of extinction; (2) the resemblance of modern to fossil forms in the same area; (3) transitional features in fossils; (4) the presence of vestigial traits; and (5) change in contem- porary populations.
- The consensus that species are related by common ancestry is sup- ported by data on (1) the geographic proximity of closely related species; (2) the existence of genetic, developmental, and structural homologies; and (3) the contemporary formation of new species from preexisting species.
- Evidence for evolution is internally consistent, meaning that data from several independent sources are mutually reinforcing.
22.3 The Process of Evolution: How Does
Natural Selection Work?
- Darwin developed four postulates that outline the process of evo- lution by natural selection. These postulates can be summarized by the following statement: Heritable variation leads to differen- tial reproductive success.
- Alleles or traits that increase the reproductive success of an individual are said to increase the individual’s fitness. A trait that leads to higher fitness, relative to individuals without the trait, is an adaptation. If a particular allele increases fitness and leads to adaptation, the allele will increase in frequency in the population.
22.4 Evolution in Action: Recent Research
on Natural Selection
- Selection by drugs on the TB bacterium and changes in the size and shape of finch beaks in the Galápagos as a result of seed availabil- ity are well-studied examples of natural selection.
- Both examples demonstrate that evolution can be observed and measured. Evolution by natural selection has been confirmed by a wide variety of studies and is considered to be a central organizing principle of biology.
CHAPTER 22 ReVieW For media, go to MasteringBiology
