14 EVOLUTION AND NATURAL SELECTION
CHAPTER OUTLINE
Evolution (p. 286)
14.1 Darwin’s Voyage on HMS Beagle (p. 286; Figs. 14.1, 14.2, 14.3)
A. English naturalist, Charles Darwin (1809–1882) was the first to propose natural selection as a
mechanism of evolution in On the Origin of Species by Means of Natural Selection.
B. In Darwin’s time, most people believed that species were specially created once and
remained unchanged through time.
1. The views of Darwin put him at odds with most people of his time.
C. One of the most influential events in Darwin’s life was his five-year journey as ship’s
naturalist aboard the HMS Beagle.
1. During this voyage around the coasts of South America, Darwin observed tropical
forests, fossils of extinct mammals in Patagonia, and related but distinct species on the
Galápagos Islands.
14.2 Darwin’s Evidence (p. 287; Figs. 14.4, 14.5)
A. The fossils and patterns of life that Darwin observed on his voyage led to his conclusion that
evolution had occurred.
B. The writings of geologist Charles Lyell (1797–1875) were highly influential to Darwin during
his voyage.
1. Lyell believed, unlike most people of his day, that the earth was extremely old.
C. What Darwin Saw
1. Fossils of extinct armadillos were similar in form to living species.
2. On the Galápagos Islands, Darwin saw several species of finches that differed slightly.
3. Darwin saw that plants and animals on these islands resembled those on the mainland,
but were distinctly different.
14.3 The Theory of Natural Selection (p. 289; Figs. 14.6, 14.7, 14.8)
A. Darwin and Malthus
1. Mathematician Thomas Malthus (1798) wrote Essay on the Principle of Population in
which he pointed out that human populations tend to increase geometrically while food
supplies increase arithmetically.
2. However, populations remain fairly constant year after year because death limits
population size.
3. Malthus’s ideas provided the key that was needed for Darwin to develop his hypothesis
that evolution occurs by natural selection.
B. Natural Selection
1. Darwin now saw that each population could produce enough offspring to outstrip its
food supply, but only a limited number survived to reproduce.
2. This led Darwin to the idea of “survival of the fittest” in which only those organisms
that were well-adapted survived long enough to reproduce.
3. The traits of organisms that survive to produce more offspring will be more common in
future generations.
4. Darwin’s theory provides a simple and direct explanation for biological diversity.
C. Darwin Drafts His Argument
1. Darwin wrote a draft of his ideas in 1842, then turned to other research for sixteen years.
D. Wallace Has the Same Idea
1. English naturalist Alfred Russel Wallace (1823–1913) wrote an essay about his own
ideas on evolution by natural selection from his observations in Malaysia.
2. Darwin and Wallace gave a joint presentation. Darwin then expanded his 1842
manuscript.
E. Publication of Darwin’s Theory
1. Darwin’s book appeared in 1859 and began a controversy about the origin of humans.
2. After the 1860s, Darwin’s ideas were widely accepted in the intellectual community of
Great Britain.
Darwin’s Finches: Evolution in Action (p. 291)
The Beaks of Darwin’s Finches (p. 291; Figs. 14.9, 14.10, 14.11)
A. Darwin’s finches from the Galapagos Islands are a classic example of evolution by natural
selection.
B. The Importance of the Beak
1. Beak shape of this group of 14 species of finches indicated a correspondence between
shape and food source.
C. Checking to See if Darwin Was Right
1. David Lack set out to test Darwin’s hypothesis in 1938 and observed many different
species of finches eating the same seeds.
D. A Closer Look
1. In 1973, the Grants of Princeton University discovered a relationship between beak
shape, seed size, and climatic conditions which indicated that beak size was adjusted to
the food supply and was passed on from one generation to the next.
E. Support for Darwin
1. Natural selection does seem to be operating to adjust the beak to its food supply.
14.5
How Natural Selection Produces Diversity (p. 293; Fig. 14.12)
A. Darwin’s finches, all derived from one similar mainland species, exhibit adaptive radiation
on the Galápagos Islands in the absence of competition.
B. Four groups of finches have been recognized from these islands: ground finches, tree
finches, warbler finches, and a vegetarian finch.
The Theory of Evolution (p. 294)
14.6
The Evidence for Evolution (p. 294; Figs. 14.13, 14.14, 14.15, 14.16, 14.17, 14.18, 14.19)
A. The Fossil Record
1. The most direct evidence of macroevolution is found in the fossil record.
2. Fossils are the preserved remains, traces, or tracks of once-living creatures.
3. A fossil can be dated by measuring the rate of decay of certain radioisotopes contained in
the rock.
4. Using Fossils to Test the Theory of Evolution
a. When fossils are lined up according to their age, they often provide evidence of
successive evolutionary change.
5. The Fossil Record Confirms Evolution’s Key Prediction
a. Many examples serve to illustrate a record of successive change and are some of the
strongest lines of evidence of evolution.
b. Today the fossil record is very complete and few gaps exist.
c. Among the vertebrates, fossils have been found linking all the major groups.
B. The Anatomical Record
1. Many diverse organisms go through the same early stages of embryologic development,
which is evidence for evolutionary relatedness.
2. In vertebrates, homologous structures can be seen from the study of anatomy.
3. Vertebrate forelimbs have diverged to perform different functions, but consist of the
same bone structure, indicating a common ancestry.
4. Sometimes analogous structures are found in animals that have evolved the same solution
to a problem, although they did not share a common ancestry.
5. Vestigial organs—which served a function in an ancestor but have no function in the
modern counterpart (such as the appendix that has no function in humans but functions as
a reservoir for cellulose bacteria in apes)—are also anatomical evidence for evolution.
C. The Molecular Record
1. The evolutionary past is also evident at the molecular level.
14.4
2.
14.7
Since the record of evolutionary change is linked to changes in DNA, organisms that are
more distantly related will have accumulated a greater number of genetic changes in
DNA.
3. When analyzing nucleotide sequences for the gene encoding the protein cytochrome c,
biologists can construct a molecular clock showing the relatedness of organisms based on
how many nucleotide sequences they are away from each other.
4. Not all proteins evolve at the same rate: cytochrome c and hemoglobin have changed at
relatively constant rates, but other proteins, like the fibrinopeptides, evolve considerably
faster.
Evolution’s Critics (p. 299; Figs. 14.20)
A. History of the Controversy
1. While initially attacked by clergyman, Darwin’s book and ideas were accepted in the
scientific community by the turn of the century.
2. The Fundamentalist Movement of the late 1920’s through the early 1960’s banned the
topics of Darwin and evolution from textbooks.
3. In the early 1960s, scientific advances gave evolution a new emphasis, and Darwin’s
ideas were back in most textbooks.
4. The Scientific Creationism Movement beginning in 1964 seeks to establish Biblical
creation as a scientific theory.
5. Local control of education has resulted in only 22 states that mandate the teaching of
natural selection, and four states which do not mention evolution at all.
6. Intelligent Design is the most recent concept being proposed as an alternative explanation
for natural selection.
B. Arguments Advanced by Darwin’s Critics
1. Critics have raised a variety of objections to Darwin’s theory, all of which can be either
explained by biologists or do not necessarily refute evolutionary theory.
C. The Irreducible Complexity Fallacy
1. Irreducible complexity is an idea that states that individual parts of a complex,
interconnected process cannot evolve independently.
2. However, using the clotting cascade as an example, each protein at each step in the
cascade can indeed be acted upon by natural selection, but evolution has acted on the
system as a whole; the parts have evolved together.
How Populations Evolve (p. 303)
14.8
Genetic Change Within Populations: The Hardy-Weinberg Rule (p. 303; Fig. 14.21)
A. Population genetics is the study of the properties of genes in populations.
B. Hardy-Weinberg Equilibrium
1. The proportion of alternative forms of a gene, or alleles, in a population can be calculated
and the allele frequencies determined.
2. The equations of Hardy-Weinberg equilibrium can then be used to predict the frequencies
of genotypes in future populations.
3. The symbol p denotes the frequency of the dominant allele, and q stands for the
frequency of the recessive allele.
4. By definition, p + q = 1.
5. By expanding the binomial, (p + q)2 = p2 + 2pq + q2 = 1, where p2 is the proportion of
homozygous dominant individuals in the population, 2pq indicates the proportion of
heterozygotes, and q2 is the proportion of homozygous recessives.
C. Hardy-Weinberg Assumptions
1. According to Hardy and Weinberg, gene frequencies do not change when the size of the
population is large, when mating occurs at random, when natural selection is not
occurring, and while there are no mutations or migration.
D. Hardy-Weinberg: A Null Hypothesis
1. When genotypic frequencies over several generations do not match those predicted by the
Hardy-Weinberg equation, it is likely that some force is acting on the population to
change the frequency of alleles.
14.9
E. Case Study: Cystic Fibrosis in Humans
1. Currently, the frequency of the allele for cystic fibrosis in the United States is very close
to projections based on the Hardy-Weinberg equation.
Agents of Evolution (p. 305; Figs. 14.22, 14.23, 14.24)
A. Five factors alter the proportions of homozygotes and heterozygotes enough to produce
significant deviations from the proportions predicted by the Hardy-Weinberg rule.
B. Mutation
1. Genetic mutations, or alterations in DNA nucleotide sequences, are rare but are the
ultimate source of genetic variation.
C. Migration
1. Migration from the movement of individuals into or out of the population can alter the
genetic composition of a population.
D. Genetic Drift
1. In small populations, by random chance alone, it is possible for the allele frequencies to
change from one generation to the next.
2. Such a phenomenon is termed genetic drift.
3. The founder effect occurs when a few individuals are separated from the rest and give
rise, over time, to a new population; this effect often occurs on islands.
4. The founder effect is somewhat similar to the bottleneck effect in which a few members
of a population of species are all that are left to give rise to the next generations of that
species.
5. The bottleneck effect limits the genetic diversity of a population.
E. Nonrandom Mating
1. Nonrandom mating and inbreeding (mating with relatives) also lead to changes in gene
frequencies from one generation to the next.
F. Selection
1. Selection, whether artificial selection by humans or natural selection, operates to select
certain fit phenotypes, which are able to leave more offspring and thus pass on their
genes to successive generations.
G. Forms of Selection
1. In stabilizing selection, individuals toward the middle of a range of phenotypes are
selected.
2. In disruptive selection, both extremes of a phenotype are favored and individuals in the
middle of the range of phenotypes are selected against.
3. Directional selection favors a phenotype at one extreme or the other of an array of
phenotypes.
Adaptation Within Populations (p. 310)
14.10
14.11
Sickle-Cell Anemia (p. 310; Figs. 14.25, 14.26, 14.27)
A. Sickle-cell anemia is a hereditary disease in which the homozygous condition is often lethal.
B. The Puzzle: Why So Common?
1. In central Africa, one in 100 people is homozygous for the disorder and develops sicklecell anemia.
2. Why is the sickle-cell allele so common in Africa?
C. The Answer: Stabilizing Selection
1. It turns out that individuals who are heterozygous for the sickle-cell allele are resistant to
the malarial parasite that otherwise kills the person with normal hemoglobin; those with
sickle-cell anemia perish due to this genetic abnormality.
2. Sickle-cell anemia in humans is an example of stabilizing selection in which the middle
phenotype, in this case the heterozygote with sickle-cell trait but not anemia, is more
adapted to an environment that hosts the malarial parasite.
Peppered Moths and Industrial Melanism (p. 312; Fig. 14.28, 14.29)
A. Selection for Melanism
B. Industrial Melanism
C. Selection Against Melanism
D. Reconsidering the Target of Natural Selection
14.12
E. Natural Selection in Mice
Selection on Color in Guppies (p. 314; Figs. 14.30, 14.31)
A. Guppies Live in Different Environments
1. Guppies living in high-predation pools exhibit drab coloration and have a small adult size.
2. In absence of predators, male guppies are larger and exhibit gaudy colors.
B. The Experiments
1. A controlled experiment was conducted by John Endler in a laboratory to test the response
to differences in strength of predation.
2. The results established that predation can lead to rapid evolutionary change.
3. A field experiment was conducted by Endler that revealed that natural selection can lead
to rapid evolutionary change.
How Species Form (p. 317)
14.13
14.14
The Biological Species Concept (p. 317; Table 14.1)
A. According to Darwin's ideas, species form slowly over time as microevolutionary changes
accumulate and give rise to macroevolution, or the formation of new species.
B. A species is defined as a group of potentially or actually interbreeding organisms and that is
reproductively isolated from other groups in nature.
C. What causes reproductive isolation?
1. Two kinds of barriers act to isolate species: prezygotic and postzygotic isolation
mechanisms.
Isolating Mechanisms (p. 318; Figs. 14.32, 14.33)
A. Prezygotic isolating mechanisms lead to reproductive isolation by preventing the formation of
hybrid zygotes.
B. Prezygotic isolating mechanisms include geographical isolation, ecological isolation,
temporal isolation, behavioral isolation, mechanical isolation, and prevention of gamete
fusion.
C. Postzygotic isolating mechanisms prevent the proper functioning of hybrid zygotes, and they
include the improper development of hybrids, the failure of the hybrids to become established
in either parental habitat, or sterility of hybrid adults.
KEY TERMS









evolution (p. 286)
species (p. 286)
natural selection (p. 289)
On the Origin of Species (p. 286) If possible, get a copy of this book from the school library and
show it to the students during lecture. Read a few excerpts to help your students understand the
depth of Darwin’s ideas.
 Galápagos Islands (p. 286)
 Artificial selection (p.290)
 adaptive radiation (p. 293) Evidence from island biogeography and adaptive radiation are among
the most convincing arguments for evolution as a result of natural selection. Point out any local
examples from your own ecosystem.
 fossil (p. 294)
homologous structure (p. 296) These are derived from a common ancestry.
analogous structure (p. 296) These result from similar environmental pressure but have separate
evolutionary histories.
convergent evolution (p. 297)
molecular clock (p. 298) In some proteins, changes in nucleotide sequences appear to occur at a
constant rate, providing a “molecular clock.”
population genetics (p. 304) The study of the properties of genes in populations.














allele frequency (p. 304) Allele frequency refers to the proportion of alleles of a particular type in a
population.
Hardy-Weinberg equilibrium (p. 304)
mutation (p. 306)
genetic drift (p.306)
founder effect (p. 306)
bottleneck effect (p. 307)
stabilizing selection (p. 308) Human infant birth weight is a good example of this type of selection.
disruptive selection (p. 309)
directional selection (p. 309)
sickle-cell anemia (p. 310)
heterozygote advantage (p. 311)
industrial melanism (p. 312)
biological species concept (p. 317)
reproductive isolating mechanisms (p. 317)
LECTURE SUGGESTIONS AND ENRICHMENT TIPS
1.
2.
3.
4.
Lead a discussion with your students involving their cultural or religious beliefs about the origin of life
on earth. Ask several students how they think they, as humans, got to be here. Then describe
creationism and naturalist evolution as two distant ends of the spectrum and mention that there is room
for intermediate ideas. By introducing evolution in this manner, any students who fear discussing
evolution will relax and feel more open to the subject. Be sure to mention that evolution, as a scientific
theory, is the only one of these alternatives that can be tested scientifically.
Discuss examples of artificial selection with students. If they can see how artificial selection operates,
natural selection becomes easy to understand. Darwin used artificial selection as a model to illustrate
natural selection. Describe how dog breeds, for example, are the result of thousands of years of
artificial selection, with a wolf as the original animal about 14,000 years ago. Other examples include
the variety of foods of the cabbage family (Brussels sprouts, kohlrabi, Chinese cabbage) that have all
been developed from a single species of Brassica.
Fossils. Demonstrate what a fossil is by making an imprint of a leaf in some sand or soil. Describe how
scientists study imprints of soft structures. Then show students a number of examples of fossils,
describing what they are and, if possible, their ages. Discuss how the age of fossils is determined and
describe other types of organisms that lived at the same time as the fossils they are viewing.
Changing Gene Frequencies. Divide students into groups and pass out 10 red and 10 black jellybeans
to each group (any pair of color combinations will do). With a population of 20, they can easily see
that the dominant gene has a frequency of 0.5, the same as the frequency of the recessive phenotype.
Devise a scenario to change gene frequencies in the succeeding generations. For example, a certain
type of predator (jelly bean jackal) prefers red jellybeans to black, and as long as they are fairly
abundant in the population, the jackal will take the time to seek out, chase down, and consume the
reds. Remove 5 red jellybeans and replace them with 5 black ones (keep the total population at 20,
assuming that's the carrying capacity for the environment). Show how gene frequencies are changed. In
the next generation, remove 3 more red jellybeans and replace them with black ones, again illustrating
the change. Now, with few red jellybeans to be found, the jellybean jackal must alter his eating habits a
bit and select equally from blacks and reds. Have students remove 10 jellybeans at random and then
calculate the gene frequencies of the remaining group of 10. You should see differences among groups
as to gene frequencies, illustrating both what can happen by random chance as well as what can happen
through time as gene frequencies are affected by natural selection.
CRITICAL THINKING QUESTIONS
1.
2.
3.
4.
How might it be possible to reestablish the diversity of cichlid fishes in Lake Victoria? Or, is it
possible? Discuss your ideas either way. Then, devise a plan that might help the lake to recover to its
historical condition.
Are the recent outbreaks of Ebola, AIDS, and tuberculosis that are especially prevalent in crowded,
economically poor conditions examples of how nature regulates population size? How could the idea
of “survival of the most fit” apply here?
Devise a scenario where two populations become reproductively isolated.
Explain how the bottleneck effect, such as is now experienced in living cheetahs, can make a species
more susceptible to disease.