CHAPTER
13
How Populations Evolve
Lecture Outline
I. Introduction
1. In the 1960s, the World Health Organization (WHO) launched a campaign to eradicate malaria.
2. It focused on killing the mosquitos that carry the parasite from person to person by massive spraying of the pesticide DDT.
3. Early success was followed by rebounding mosquito populations that had evolved resistance to the pesticide DDT.
4. Today, malaria causes more than a million deaths and 250 million cases of illness each year.
5. At the same time that DDT was being celebrated in the war against malaria, a drug called chloroquine was hailed as a miracle cure.
a. But its effectiveness diminished as the parasite evolved resistance to the drug.
b.In some regions chloroquine is now powerless against the disease.
6. At present, the most effective antimalarial drug is artemisinin, a compound extracted from a plant used in traditional Chinese medicine.
a. But the effectiveness of this drug will eventually succumb to the power of evolution, too.
b.Medical experts have found cases of malaria in Southeast Asia that do not respond to artemisinin.
7. An understanding of evolution informs all of biology, from exploring life’s molecules to analyzing ecosystems.
II. Darwin’s Theory of Evolution
A.13.1 A sea voyage helped Darwin frame his theory of evolution
1. Charles Darwin is best known for his book On the Origin of Species by Means of Natural Selection, commonly referred to as The Origin of Species, which launched the era of evolutionary biology.
2. Darwin’s early career gave no hint of his future fame.
a. He enrolled in but left medical school.
b.Then he entered Cambridge University to become a clergyman.
3. The cultural and scientific context of his time instilled Darwin with a conventional view of Earth and its life.
a. Most scientists accepted the views of the Greek philosopher Aristotle, who generally held that species are fixed, permanent forms that do not evolve.
b.Judeo-Christian culture taught a literal interpretation of the biblical book of Genesis, that each form of life is individually created in its present-day form.
d.Thus, the idea that all living species came into being relatively recently and are unchanging in form dominated the intellectual climate of the Western world at the time.
4. At the age of 22, Darwin took a position on HMS Beagle, a survey ship preparing for a long expedition to chart poorly known stretches of the South American coast.
5. As the ship’s naturalist (field biologist), Darwin
a. spent most of his time on shore collecting thousands of specimens of fossils and living plants and animals and
b.kept detailed journals of his observations.
6. Many of Darwin’s observations indicated that geographic proximity is a better predictor of relationships among organisms than similarity of environment.
7. Darwin was particularly intrigued by the geographic distribution of organisms on the Galápagos Islands, including
a. marine iguanas and
b.giant tortoises.
8. While on his voyage, Darwin was strongly influenced by the newly published
Principles of Geology by Scottish geologist Charles Lyell.
a. The book presented the case for an ancient Earth sculpted over millions of years by gradual geographic processes that continue today.
9. By the time Darwin returned to Great Britain five years after Beagle first set sail, he had begun to seriously doubt that Earth and all its living organisms had been specially created only a few thousand years earlier.
10. As he reflected on his observations, analyzed his collections, and discussed his work with colleagues, he concluded that the evidence was better explained by the hypothe-sis that present-day species are the descendants of ancient ancestors that they still resemble in some ways.
11. He hypothesized that as the descendants of a remote ancestor spread into various habitats over millions and millions of years, they accumulated diverse modifications, or adaptations, that fit them to specific ways of life in their environment.
12. By the early 1840s, Darwin had composed a long essay describing the major features of his theory of evolution by natural selection.
13. But he delayed publishing his essay, continued to compile evidence in support of his hypothesis, and finally released his essay to the scientific community when learning of the work of another British naturalist, Alfred Wallace, who had a nearly identical hypothesis.
14. The next year, Darwin published The Origin of Species, a book that supported his hypothesis with immaculate logic and hundreds of pages of evidence drawn from observations and experiments in biology, geology, and paleontology.
15. The hypothesis of evolution generated predictions that have been tested and verified by more than 150 years of research.
16. Consequently, scientists regard Darwin’s concept of evolution by means of natural selection as theory, a widely accepted explanatory idea that
a. is broader in scope than a hypothesis,
b.generates new hypothesis, and
c. is supported by a large body of evidence.
B. 13.2 The study of fossils provides strong evidence for evolution
1. Fossils
a. are the imprints or remains of organisms that lived in the past,
b.document differences between past and present organisms, and
c. reveal that many species have become extinct.
2. For example, the fossilized skull of one of our early relatives, Homo erectus, represents someone who lived 1.5 million years ago in Africa.
3. Some fossils are not the actual remnants of organisms.
4. The 375-million-year-old fossils in Figure 13.2B are casts of ammonites, shelled marine animals related to the present-day nautilus.
5. The sequence in which fossils appear within strata, layers of sedimentary rocks, is a historical record of life on Earth.
6. Paleontologists (scientists who study fossils) sometimes gain access to very old fossils when erosion carves through upper (younger) strata, revealing deeper (older) strata that had been buried.
7. The fossil record is the chronicle of evolution over millions of years of geologic time engraved in the order in which fossils appear in rock strata.
8. The fossil record is incomplete because
a. many of Earth’s organisms did not live in areas that favor fossilization,
b.fossils that did not form were in rocks later distorted or destroyed by geologic processes, and
c. not all fossils that have been preserved are accessible to paleontologists.
C.13.3 SCIENTIFIC THINKING: Fossils of transitional forms support Darwin’s theory of evolution
1. InThe Origin of Species, Darwin predicted the existence of fossils of transitional forms linking very different groups of organisms.
2. For example, if his hypothesis that whales evolved from land-dwelling mammals was correct, then fossils should show a series of changes in a lineage of mammals adapted to a fully aquatic habitat.
3. Many fossils link early extinct species with species living today.
a. A series of fossils traces the gradual modification of jaws and teeth in the evolution of mammals from a reptilian ancestor.
b.A series of fossils documents the evolution of whales from a group of land mammals.
4. Thousands of fossil discoveries have since shed light on the evolutionary origins of many groups of plants and animals, including
a. the transition of fish to amphibian,
b.the origin of birds from a lineage of dinosaurs, and
5. Whales are cetaceans, a group that also includes dolphins and porpoises.
a. They have forelimbs in the form of flippers but lack hind limbs.
b.If cetaceans evolved from four-legged land animals, then transitional forms should have reduced hind limb and pelvic bones.
6. Beginning in the late 1970s, paleontologists unearthed an extraordinary series of transitional fossils in Pakistan and Egypt.
7. The new fossil discoveries were consistent with the earlier hypothesis, and
paleontologists became more firmly convinced that whales did indeed arise from a wolf-like carnivore.
8. But molecular biologists
a. found a close relationship between whales and hippopotamuses and
b.hypothesized that whales and hippos were both descendants of a cloven-hoofed ancestor.
9. Paleontologists were taken aback by the contradictory results, but openness to new evidence is a hallmark of science.
10. Two fossils discovered in 2001 provided the answer.
a. Both Pakicetus and Rodhocetus had the distinctive ankle bone of a cloven-hoofed mammal.
b.Thus, as it is often the case in science, scientists are becoming more certain about the evolutionary origin of whales as mounting evidence from different lines of inquiry converge.
D.13.4Homologies provide strong evidence for evolution
1. Evolution is a process of descent with modification.
a. Characteristics present in an ancestral organism are altered over time by natural selection as its descendants face different environmental conditions.
b.Evolution is a remodeling process.
c. Related species can have characteristics that have an underlying similarity yet function differently.
d.Similarity resulting from common ancestry is known as homology.
2. Darwin cited the anatomical similarities among vertebrate forelimbs as evidence of common ancestry.
3. As Figure 13.4A shows, the same skeletal elements make up the forelimbs of humans cats, whales, and bats, but the functions of these forelimbs differ.
4. Biologists call such anatomical similarities in different organism’s homologous structures.
5. Molecular comparisons between diverse organisms have allowed biologists to develop hypotheses about the evolutionary divergence of major branches on the tree of life.
6. Darwin’s boldest hypothesis was that all life-forms are related. Molecular biology pro-vides strong evidence of this chain.
a. All forms of life use the same genetic language of DNA and RNA.
b.The genetic code—how RNA triplets are translated into amino acids—is essentially universal.
7. An understanding of homology helps explain why
a. early stages of development in different animal species reveal similarities not visible in adult organisms and
b.at some point in their development, all vertebrate embryos have
i. a tail posterior to the anus, and
8. Some of the most interesting homologies are “leftover” structures that are of marginal or perhaps no importance to the organism.
9. These vestigial structures are remnants of features that served important functions in the organism’s ancestors.
E. 13.5 Homologies indicate patterns of descent that can be shown on an evolutionary tree
1. Darwin was the first to view history of life as a tree, with multiple branches from a common ancestral trunk to the descendant species at the tips of the twigs.
2. Today, biologists represent these patterns of descent with an evolutionary tree, although today they often turn the trees sideways.
3. Homologous structures can be used to determine the branching sequence of an evolutionary tree.
4. These homologies can include
a. anatomical structure and/or
b.molecular structure.
F. 13.6Darwin proposed natural selection as the mechanism of evolution
1. Darwin’s greatest contribution to biology was his explanation of how life evolves.
2. Because he thought that species formed gradually over long periods, he knew that he would not be able to study the evolution of new species by direct observation.
3. But insights into how incremental change occurs could be seen in examples of
artificial selection, in which humans have modified species through selective breeding.
4. Darwin knew that individuals in natural populations have small but measurable differences.
5. But what forces in nature played the role of the breeder, choosing which individuals became the breeding stock for the next generation?
6. Darwin found inspiration in an essay written by economist Thomas Malthus, who contended that much of human suffering—disease, famine, and war—was the consequence of human populations, increasing faster that food supplies and other resources.
7. Darwin deduced that the production of more individuals than the limited resources can support leads to a struggle for existence, with only some offspring surviving in each generation.
8. The essence of natural selection is this unequal reproduction.
a. Individuals whose traits better enable them to obtain food or escape predators or tolerate physical conditions will reproduce more successfully, passing these adaptive traits to their offspring.
9. Darwin reasoned that if artificial selection can bring about so much change in a relatively short period, then natural selection could modify species considerably over hundreds or thousands of generations.
10. It is important to emphasize three key points about evolution by natural selection.
a. Although natural selection occurs through interactions between individual organisms and the environment, individuals do not evolve. Rather, it is the population, the group of organisms, that evolves over time.
b.Natural selection can amplify or diminish only heritable traits.
G.13.7 Scientists can observe natural selection in action
1. Biologists have documented evolutionary change in thousands of scientific studies.
2. Peter and Rosemary Grant have worked on Darwin’s finches in the Galápagos for over 30 years. They found that
a. in dry years, large strong beaks are favored because all seeds are in short supply and birds must eat more larger seeds, but
b.in wet years, small seeds are more abundant and small beaks are favored.
3. Another example of natural selection in action is the evolution of pesticide resistance in insects.
a. A relatively small amount of poison initially kills most of the insects, but subsequent applications are less and less effective.
b.The few survivors are individuals that are genetically resistant, carrying an allele that somehow enables them to survive the chemical attack.
c. When these resistant insects reproduce, the percentage of the population resistant to the pesticide increases.
4. These examples of evolutionary adaptation highlight two important points about natural selection.
a. Natural selection is more of an editing process than a creative mechanism.
b.Natural selection is contingent on time and place, favoring those heritable traits in a varying population that fit the current, local environment.
III. The Evolution of Populations
A.13.8 Mutation and sexual reproduction produce the genetic variation that makes evolution possible
1. Organisms typically show individual variation.
2. However, in The Origin of Species, Darwin could not explain
a. the cause of variation among individuals or
b.how variations were passed from parents to offspring.
3. Just a few years after the publication of The Origin of Species, Gregor Mendel wrote a groundbreaking paper on inheritance in pea plants.
4. Although the significance of Mendel’s work was not recognized during his or Darwin’s lifetime, its rediscovery in 1900 set the stage for understanding the genetic differences on which evolution is based.
5. Each person has a unique genome, reflected in individual phenotypic variations, such as appearance and other traits.
6. Indeed, individual variation occurs in all species.
7. Mutations are
a. changes in the nucleotide sequence of DNA and
b.the ultimate source of new alleles.
8. Thus, mutation is the ultimate source of the genetic variation that serves as raw material for evolution.
9. A change as small as a single nucleotide in a protein-coding gene can have significant effect on phenotype, as in sickle-cell disease.
10. On rare occasions, however, a mutated allele may actually improve the adaptation of an individual to its environment and enhance its reproductive success.
12. Chromosomal duplication is an important source of genetic variation.
a. If a segment of DNA can persist over the generations, mutations may accumulate in the duplicate copies without affecting the function of the original gene, eventually leading to new genes with novel functions.
b.For example, the remote ancestors of mammals carried a single gene for detecting odors that has since been duplicated repeatedly. As a result, mice have about 1,300 different olfactory receptor genes.
c. In organisms that reproduce sexually, most of the genetic variation in a population results from the unique combination of alleles that each individual inherits.
13. Fresh assortments of existing alleles arise every generation from three random components of sexual reproduction:
a. crossing over,
b.independent orientation of homologous chromosomes at metaphase I of meiosis, and
c. random fertilization.
B. 13.9Evolution occurs within populations
1. A population isa group of individuals of the same species that live in the same area and interbreed.
2. We can measure evolution as a change in the prevalence of certain heritable traits in a population over a span of generations.
3. Different populations of the same species may be geographically isolated from each other to such an extent that an exchange of genetic material never occurs, or occurs only rarely.
4. Such isolation is common in populations confined to different lakes.
5. A gene pool consists of all copies of every type of allele, at every locus, in all members of the population.
6. Microevolution is
a. change in the relative frequencies of alleles in a gene pool over a number of generations and
b.evolution occurring on its smallest scale.
C.13.10 The Hardy-Weinberg equation can test whether a population is evolving
1. To understand how microevolution works, let’s first examine a simple population in which evolution is not occurring and thus the gene pool is not changing.
2. Consider an imaginary population of iguanas with individuals that differ in foot webbing.
3. Let’s assume that
a. foot webbing is controlled by a single gene and
b.the allele for nonwebbed feet (W) is completely dominant to the allele for webbed feet (w).
4. The shuffling of alleles that accompanies sexual reproduction does not alter the genetic makeup of the population.
a. No matter how many times alleles are segregated into different gametes, and united in different combinations by fertilization, the frequency of each allele in the gene pool will remain constant unless other factors are operating.
b.This equilibrium is the Hardy-Weinberg principle, named for the two scientists who derived it independently in 1908.
5. To test the Hardy-Weinberg principle, let’s look at two generations of our imaginary iguana population.
7. From these genotype frequencies, we can calculate the frequency of each allele in the population.
8. Figure 13.10C shows a Punnett square that uses these gamete allele frequencies and the rule of multiplication to calculate the frequencies of the three genotypes in the next generation.
9. Because the genotype frequencies are the same as in the parent population, the allele frequencies p and q are also the same.
10. Thus, the gene pool of this population is in a state of equilibrium—Hardy-Weinberg equilibrium.
11. If a population is in Hardy-Weinberg equilibrium, allele and genotype frequencies will remain constant, generation after generation.
12. The Hardy-Weinberg principle tells us that something other than the reshuffling processes of sexual reproduction is required to change allele frequencies in a population.
13. For a population to be in Hardy-Weinberg equilibrium, it must satisfy five main conditions. There must be
a. a very large population,
b.no gene flow between populations,
c. no mutations,
d.random mating, and
e. no natural selection.
14. Rarely are all five conditions met in real populations and thus allele and genotype frequencies often do change.
15. The Hardy-Weinberg equation can be used to test whether evolution is occurring in a population.
D.13.11 CONNECTION: The Hardy-Weinberg equation is useful in public health science
1. Public health scientists use the Hardy-Weinberg equation to estimate how many people carry alleles for certain inherited diseases.
2. One out of 10,000 babies born in the United States has phenylketonuria (PKU), an inherited inability to break down the amino acid phenylalanine.
3. The health problems associated with PKU can be prevented by strict adherence to a diet that limits the intake of phenylalanine.
4. PKU is a recessive allele.
5. The frequency of the recessive allele for PKU in the population, q, equals the square root of 0.0001, or 0.01.
a. The frequency of the dominant allele would equal 1 – q, or 0.99.
b.The frequency of carriers = 2pq = 2 0.99 0.01 = 0.0198 = 1.98% of the U.S. population.
c. Thus, the equation tells us that about 2% (actually 1.98%) of the U.S. population are carriers of the PKU allele.
IV. Mechanisms of Microevolution
A.13.12 Natural selection, genetic drift, and gene flow can cause microevolution
1. If the five conditions for the Hardy-Weinberg equilibrium are not met in a population, the allele frequencies may change. However,
a. mutations are rare and random and have little effect on the gene pool, and
2. The three main causes of evolutionary change are
a. natural selection,
b.genetic drift, and
c. gene flow.
3. 1—Natural selection
a. If individuals differ in their survival and reproductive success, natural selection will alter allele frequencies.
b.Consider the imaginary iguana population. Individuals with webbed feet
(genotype ww) might survive better and produce more offspring because they are more efficient at swimming and catching food than individuals that lack webbed feet.
c. Genetic equilibrium would be disturbed as a frequency of the w allele increased in the gene pool from one generation to the next.
4. 2—Genetic drift
a. In a process called genetic drift, chance events can cause allele frequencies to fluctuate unpredictably from one generation to the next.
b.The smaller the population, the more impact genetic drift is likely to have.
c. Catastrophes such as hurricanes floods or fires may kill large numbers of
individuals, leaving a small surviving population that is unlikely to have the same genetic makeup as the original population.
d.The bottleneck effect leads to a loss of genetic diversity when a population is greatly reduced.
e. Analogous to shaking just a few marbles through a bottleneck, certain alleles may be present at higher frequency in the surviving population than in the original population, others may be present at lower frequency, and some (orange marbles) may not be present at all.
f. After a population is drastically reduced, genetic drift may continue for many generations until the population is again large enough for fluctuations due to chance to have less of an impact.
g. One reason it is important to understand the bottleneck effect is that human
activities such as overhunting and habitat destruction may create severe bottlenecks for other species.
h. Examples of species affected by bottlenecks include the endangered Florida panther, the African cheetah, and the greater prairie chicken.
i. Genetic drift is also likely when a few individuals colonize an island or other new habitat, producing what is called the founder effect.
j. The smaller the group, the less likely the genetic makeup of the colonists will represent the gene pool of the larger population they left.
k. The founder effect explains the relatively high frequency of certain inherited disorders among some human populations established by small numbers of colonists.
5. 3—Gene flow
a. Allele frequencies in a population can also change as a result of gene flow, where a population may gain or lose alleles when fertile individuals move into or out of a population or when gametes (such as plant pollen) are transferred between populations.
c. To counteract the lack of genetic diversity in the remaining Illinois greater prairie chickens, researchers added 271 birds from neighboring states to the Illinois populations, which successfully introduced new alleles.
d. This strategy worked. New alleles entered the population, and the egg-hatching rate improved to more than 90%.
B. 13.13 Natural selection is the only mechanism that consistently leads to adaptive evolution
1. Genetic drift, gene flow, and mutations could each result in microevolution, but only by chance could these events improve a population’s fit to its environment.
2. In natural selection only the genetic variation produced by mutation and sexual reproduction results from random events.
a. The process of natural selection, in which some individuals are more likely than others to survive and reproduce, is not random.
b.Because of this sorting, only natural selection consistently leads to adaptive evolution.
3. The adaptations of organisms include many striking examples.
4. Consider some of the features that make the blue-footed booby suited to its home on the Galápagos Islands.
a. The bird’s large, webbed feet make great flippers, propelling the bird through the water at high speeds.
b.Its body and bill are streamlined, minimizing friction as it dives from heights up to 24 m (over 75 feet) into the shallow water below.
c. To pull out of this high-speed dive once it hits the water, the booby uses its large tail as a brake.
5. Let’s take a closer look at natural selection.
6. The commonly used phrases “struggle for existence” and “survival of the fittest” are misleading if we take them to mean direct competition between individuals.
Reproductive success is generally more subtle and passive.
7. Relative fitness is the contribution an individual makes to the gene pool of the next generation relative to the contribution of other individuals.
C.13.14 Natural selection can alter variation in a population in three ways
1. Natural selection can affect the distribution of phenotypes in a population.
a. Stabilizing selection favors intermediate phenotypes.
b. Directional selection shifts the overall makeup of the population by acting against individuals at one of the phenotypic extremes.
c. Disruptive selection typically occurs when environmental conditions vary in a way that favors individuals at both ends of the phenotypic range over individuals with intermediate phenotypes.
D.13.15 Sexual selection may lead to phenotypic differences between males and females
1. Sexual selection is a form of natural selection in which individuals with certain characteristics are more likely than other individuals to obtain mates.
2. In many animal species, males and females may have secondary sexual characteristics, noticeable differences not directly associated with reproduction or survival, called sex-ual dimorphism.
3. In some species, individuals compete directly with members of the same sex for mates.
5. Contests may involve physical combat but are more often ritualized displays.
6. In the more common type of sexual selection, called intersexual selection (between sexes) mate choice, individuals of one sex (usually females) are choosy in selecting their mates.
7. What is the advantage to females of being choosy?
8. One hypothesis is that females prefer male traits that are correlated with “good genes.”
a. In several bird species, research has shown that traits preferred by females, such as bright beaks or long tails, are related to overall male health.
b.The “good genes” hypothesis was also tested in gray tree frogs. Female frogs prefer to mate with males that give long mating calls.
E. 13.16 EVOLUTION CONNECTION: The evolution of drug-resistant microorganisms is a serious public health concern
1. Antibiotics are drugs that kill infectious microorganisms.
2. During the 1950s, some medical experts even thought the age of human infectious diseases would soon be over.
3. Why didn’t that optimistic forecast come true?
a. Answer: It did not take into account the force of evolution.
4. In the same way that pesticides select for resistant insects, antibiotics select for resistant bacteria.
5. Again we see both the random and nonrandom aspects of natural selection: the ran-dom genetic mutations in bacteria and the nonranran-dom selective effects as the environ-ment favors the antibiotic-resistant phenotype.
6. In what ways do we contribute to the problem of antibiotic resistance?
a. Doctors may overprescribe antibiotics.
b.Patients may misuse prescribed antibiotics by prematurely stopping the medication because they feel better.
c. Livestock producers add antibiotics to animal feed as a growth promoter and to pre-vent illness.
7. Each year in the United States nearly 100,000 people die from infections they contract in the hospital, often because the bacteria are resistant to multiple antibiotics.
8. A formidable “superbug” known as MRSA (methicillin-resistant Staphylococcus aureus) can cause “flesh-eating disease” and potentially fatal systemic (whole-body) infections.
F. 13.17 Diploidy and balancing selection preserve genetic variation
1. What prevents natural selection from eliminating all variation as it selects against unfavorable genotypes?
2. Why aren’t less adaptive alleles eliminated as the “best” alleles are passed to the next generation?
3. It turns out that the tendency for natural selection to reduce variation in a population is countered by mechanisms that maintain variation.
4. In some cases, genetic variation is preserved rather than reduced by natural selection.
5. Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population.
7. Frequency-dependent selection is a type of balancing selection that maintains two different phenotypes in a population.
a. In this case, selection acts against either phenotypic form it if becomes too common in the population.
b.An example is a scale-eating fish in Lake Tanganyika, Africa, which attacks other fish from behind, darting in to remove a few scales from the side of its prey.
G.13.18 Natural selection cannot fashion perfect organisms
1. The evolution of organisms is constrained.
a. Selection can act only on existing variations. New, advantageous alleles do not arise on demand.
b.Evolution is limited by historical constraints. Evolution co-opts existing structures and adapts them to new situations.
c. Adaptations are often compromises. The same structure often performs many functions.
