CHAPTER 14
How Biological Diversity
Evolves
Why This Chapter Matters
1. Recognizing the types of life on Earth and relationships between them helps us
understand the reasons for the similarities and differences between organisms.
2. Life on Earth has a long and dynamic history. Understanding how the earth has changed
helps us understand the processes occurring on Earth today.
Chapter Objectives
Biology and Society: The Sixth Mass Extinction
1. Explain why scientists think we may be in the middle of the sixth mass extinction in the
last 600 million years.
Macroevolution and the Diversity of Life
2. Describe the processes included in the study of macroevolution. Distinguish between
speciation and nonbranching evolution.
The Origin of Species
3. Define the biological species concept. Explain its limitations when applied to all types
of living organisms.
4. Describe the different types of prezygotic and postzygotic reproductive barriers, noting
examples of each.
5. Distinguish between allopatric and sympatric speciation, noting examples of each.
6. Compare the punctuated equilibria model to the graduated model of evolution. Relate a
“sudden” geological appearance to our “human” sense of time.
The Evolution of Biological Novelty
7. Explain the concept of exaptation and describe several examples.
8. Explain how paedomorphosis can create new species. Describe some of the
paedomorphic traits of humans.
9. Explain how duplications and mutations of homeotic genes can result in major
evolutionary change.
Earth History and Macroevolution
10. List, in order, the four distinct ages in the history of life on Earth. Explain how the
boundaries of each division are defined.
11. Explain how analyzing sedimentary rock layers and radiometric dating help us
understand the fossil history of life on Earth.
12. Explain how the formation and breakup of the supercontinent Pangaea affected the
evolution of life on Earth.
13. Explain how an extraterrestrial object could have caused the mass extinction 65 million
years ago. Explain how mass extinctions can be seen as times of great opportunity.
Classifying the Diversity of Life
14. Define and distinguish between taxonomy and systematics. Explain how the binomial
system is used to identify species. Finally, list the levels of taxonomic groups in order,
beginning with species and ending at domain.
15. Explain how homologies are used to create classification systems. Distinguish between
homologous and analogous structures. Explain how convergent evolution can make
interpreting homologies difficult.
16. Define cladistics and describe how it is used to create classification systems.
17. Distinguish between the two-kingdom, five-kingdom, and three-domain systems of
classification. Explain why classification systems are revised.
Evolution Connection: Rise of the Mammals
18. Explain how the extinction of most of the dinosaurs affected mammalian evolution.
Key Terms
allopatric speciation
analogy
binomial
biological species concept
clade
cladistics
class
convergent evolution
domain
evo-devo
family
genus
geologic time scale
kingdom
macroevolution
order
paedomorphosis
phylum
phylogenetic tree
postzygotic barriers
prezygotic barriers
punctuated equilibria
radiometric dating
reproductive barrier
speciation
species
sympatric speciation
systematics
taxonomy
three-domain system
Word Roots
allo = other; patric = country (allopatric: the type of speciation in which a new species
forms by geographical isolation)
bi = two; nom = name (binomial: a two-part name used to identify a species)
con = together (convergent evolution: the type of evolution in which unrelated organisms
evolve structures with similar functions)
ex = beyond (exaptation: structures that evolve in one context and later become adapted for
other functions)
macro = large (macroevolution: major biological changes evident in the fossil record)
paedo = child; morphosis = shaping (paedomorphosis: the retention in the adult of features
that were juvenile in its ancestors)
post = after (postzygotic barriers: the type of barriers that prevent development of a zygote
that is a hybrid between species)
pre = before (prezygotic barriers: the type of barriers that prevent the fertilization of the
eggs of different species)
sym = together; patri = country (sympatric: the type of speciation in which a new species
forms in the same geographical region)
Student Media
Activities
Overview of Macroevolution
Polyploid Plants
Allometric Growth
A Scrolling Geologic Record
Classification Schemes
Biology Labs On-Line
EvolutionLab
MP3 Tutors
Speciation
Process of Science
How Do New Species Arise by Genetic Isolation?
How Is Phylogeny Determined By Comparing Proteins?
Videos
Discovery Channel Video: Charles Darwin
Discovery Channel Video: Mass Extinction
Albatross Courtship Ritual
Blue-footed Boobies Courtship Ritual
Giraffe Courtship Ritual
Grand Canyon
Lava Flow
Volcanic Eruption
Galápagos Islands Overview
Relevant Current Issues in Biology Articles
Current Issues in Biology, volume 1 (ISBN 0-8053-7507-4)
On the Termination of Species
Current Issues in Biology, volume 2 (ISBN 0-8053-7108-7)
The Mammals that Conquered the Seas
Current Issues in Biology, volume 4 (ISBN 0-8053-3566-8)
Sustaining the Variety of Life
Current Issues in Biology, volume 6 (ISBN 0-321-59849-0)
Traces of a Distant Past
Relevant Songs to Play in Class
“I Got a Name,” Jim Croce
Chapter Guide to Teaching Resources
Macroevolution and the Diversity of Life
Student Misconceptions and Concerns
1. Students might not realize that evolutionary change includes (a) linear events, in which
a species changes over time, and (b) branching events, which produce new species and
diversity. Some students expect that when new species evolve, they replace their ancestors.
Questions such as “If we came from chimps, why are there still chimps alive today?” reveal
this, and other, misunderstandings.
Teaching Tips
1. Before lecturing about speciation, consider a short writing assignment. Have students
work individually or in small groups, without the benefit of books, to define a species.
2. Students might not realize that entire categories of animals have come and gone. Onceabundant trilobites and nonavian dinosaurs are now extinct. Birds and mammals are
relatively recent additions. Such examples of macroevolution are apparent in the fossil
record.
The Origin of Species
Student Misconceptions and Concerns
1. Students may think that species evolve because of need. However, need has no role in
biological evolution. Biological diversity exists and the environment selects. Evolution is
not deliberate. It is reactive. Species do not deliberately change. There is no plan. As
teachers, we must be careful in how we express evolution to best reflect this process. This
use of the passive voice in our descriptions of evolution better reflects the nature of this
fundamental process.
2. The concept of “sudden” in geological terms is likely misunderstood. Events such as
major floods, earthquakes, or asteroid impacts, which might be so rare as occurring every
1,000 years, are actually common in geological terms. Students might not realize that 1,000
such events would be expected to occur in a million years.
3. Students might not have considered how species are “naturally” kept separate and
unique. Instead, students are more likely to consider species as fixed entities, especially the
species to which they belong. As instructors of biology, it can become increasingly difficult
to empathize with this perspective. To help ease students into the topic, consider pointing
out that species of life do not reflect an even spectrum of diversity. Instead, there are many
clear groups of related organisms (fungi, flowers, owls, sharks, beetles, butterflies, and
frogs, for example). Ask students to consider why such clumping exists. Is it in any way due
to the same reason that a particular human family is distinct from other families? Can this
grouping of kinds be related to shared common ancestors?
Teaching Tips
1. Identify or have your students find several commonly recognized and related species of
plants or animals in your area and find out what reproductive barriers keep these species
from interbreeding. Local examples help to bring a point home.
2. The isolation of a few individuals from a parent population may result from a
catastrophic weather or geological event. Ask your students to think back to news footage of
torrential rains, massive debris rocketed down a river, and the struggles of animals to haul
themselves onto these rafts. Better yet, show them a short news clip of such events.
Dramatic weather and geological events may be rare in our lifetimes, but are frequent
enough to play a role in speciation.
3. Challenge your students to explain why the field of paleontology has largely been
concerned with macroevolution. The broader perspective of evolutionary change studied by
paleontologists rarely permits an examination of change within a species.
4. The silvery salamander, Ambystoma platineum, is a triploid, all-female species living in
parts of the midwestern United States. It is believed to have formed by the hybridization of
two related species thousands of years ago. It is an unusual example of sympatric speciation
in animals. The following website is a good starting point to learn more about this species:
www.inhs.uiuc.edu/cbd/herpdist/species/am_platine.html.
5. Have your students think of analogous examples of punctuated equilibrium in our
culture. Perhaps such an example is the switch from vinyl records to compact discs, with the
brief transitional form of 8 tracks (do you remember these?). Between the years 1900–2000,
there were long periods of stasis (vinyl records) and a relatively short period of transition to
the CDs (who knows how long they will last as MP3 files take over). Debating the validity
of analogies can be instructive as students articulate the biological principles and use them
to test the accuracy of the analogies.
The Evolution of Biological Novelty
Student Misconceptions and Concerns
1. Students are unlikely to appreciate the life history of salamanders and the similarity of a
larval salamander to the axolotl form. Students might therefore best understand what
happens during salamander paedomorphosis by considering more commonly understood life
stages. Ask students to imagine what would result if a caterpillar reproduced and never
turned into a winged adult butterfly. Then relate the caterpillar stage to the early larval form
of salamanders.
2. Clear examples of evolutionary remodeling include the many variations of the pattern of
bones in the vertebrate forelimb. Bat wings, bird wings, penguin flippers, the arms of apes,
and the digging forelimbs of moles all show how the ancestral pattern was revised as new
functions evolved. Descent with modification is a powerful explanation of such diversity.
Teaching Tips
1. Another way to think about evolutionary remodeling is to make an analogy to
remodeling a home. A remodeled home retains many of the “ancestral” traits—perhaps the
same plumbing and electrical system. But, where there once was a wall might now be an
opening into an enlarged family room, or a window to the outside. Evolution can work like
the TV show This Old House!
2. When discussing exaptations, have students consider the many new uses for common
household items if they were to have them in a survival situation, stranded on an island or
lost in the woods. A handkerchief, a screwdriver, and a pair of pliers might take on new
functions in this different context.
Earth History and Macroevolution
Student Misconceptions and Concerns
1. Most of us are unable to comprehend the vast lengths of time considered by geologists.
Exercises and examples can increase this comprehension. Consider the number of seconds
in a year (60 × 60 × 24 × 365.25 = 31,557,600) or how much money you could spend each
day if you spent $1 million a year ($1,000,000/365 = $2,739.73/day).
2. Students also need to be reminded that 1 billion is 1,000 million. Many students (and
too many politicians) easily confuse million and billion without realizing the scale of the
error.
Teaching Tips
1. The sequence but not absolute ages are revealed by the stratifications in sedimentary
rocks. The authors make an analogy to peeling the layers of wallpaper from the walls of a
very old house that has been inhabited by many owners. By the sequence we can tell which
layers are older, but not the absolute ages of each layer. Cutting into a layered cake is
another analogy. Although we do not know the absolute age of each layer, we know the
sequence in which the layers were placed.
2. Consider this analogy to help students understand the biological consequences of
speciation as continents drifted apart. As your students left high school, and entered the
workforce or continued their education, their high school social groups drifted apart. Your
students, in their new circumstances, adapted and changed in different ways, separate from
the members of their old social groups. A note of caution: This analogy to individual social
changes is not an example of biological evolution, which occurs over generations.
3. The consequences of an asteroid impact, large or small, illustrates the role of random
forces influencing evolution. Like throwing a dart at a spinning globe, where the asteroid
hits and what continents and life are most greatly affected can change greatly if the impact
is delayed by a few hours. An asteroid delayed by 12 hours, in what might be a journey of
millions or billions of years, will land on the opposite side of the earth.
Classifying the Diversity of Life
Student Misconceptions and Concerns
1. Students can become frustrated by the changing state of systematics. Perhaps some
comfort can be offered by noting that this is true about many active areas of science. For
example, scientists continue to learn more and revise advice regarding the causes, treatment,
and prevention of heart disease and cancer.
2. Students might express concern over the need to learn scientific names, when common
names already seem sufficient. Depending on where you live, find some examples of
common organisms with more than one common name. Fishermen are famous for the
various names they assign to the same species, depending on the geographic region where
they fish. Have your students imagine the problems of using common names when
communicating with someone in another language. Clearly, there are advantages to
scientific names!
3. Students may struggle with many aspects of phylogenetic trees. (A) Students may not
realize that each node/branch can be rotated to rearrange the groups, without changing the
nature of the relationships. For example, in Figure 14.23, the position of the beaver and
kangaroo can be reversed without changing any relationships represented in the
phylogenetic tree. (B) The length of each branch is not meaningful and is not intended to be
proportional to time. (C) The spacing between groups is not meaningful. The same
phylogenetic tree squeezed onto a page or stretched wide does not denote some degree of
divergence between the groups.
Teaching Tips
1. The authors note that our hierarchical classification system is analogous to sorting mail
first by zip code, then by street, house number, and finally members of the household.
2. Although Linnaeus recognized a hierarchical structure in the natural world, he had no
natural explanation for the occurrence of such groups. One might wonder why all life does
not blend evenly from one form to another. One of Darwin’s greatest insights was to
understand that these clusters reflect similarities due to shared ancestry: that life is grouped
into family trees. Further, Darwin proposed a natural mechanism for the formation of new
species and the generation of this diversity.
3. Molecular systematics and cladistics are combining to further test and modify
classification schemes. This is another opportunity to discuss an important aspect of the
scientific method. Scientists value multiple lines of evidence testing the same hypothesis.
Most of us do this in our lives. Even our court systems value different lines of evidence all
pointing toward the guilt or innocence of a suspect.
4. Phylogenetic trees are tentative hypotheses. As new data are collected, the hypotheses
are modified or outright rejected. Students should be cautioned to understand the tentative
nature of these systems.
Answers to End-of-Chapter Questions
The Process of Science
11. Suggested answer: You could start by observing the mice in their native habitat. Do the
mice naturally interbreed, or is the river a true reproductive barrier between the two groups?
Observation of even one pair of interbreeding mice could negate the biological species
concept definition of separate species. In the lab, experiments that artificially place the two
groups in close proximity could be used to determine whether other barriers to interbreeding
exist. One problem with such an effort is that the artificial habitat of the lab might
compromise the experiment, since breeding habits might be altered under such conditions.
Alternatively, genetic analysis of the two groups could provide an indication of whether the
two gene pools are mixed or separate.
Biology and Society
12. Some issues and questions to consider: Whereas previous mass extinctions have
resulted from catastrophic events, such as asteroid collisions or volcanism, this mass
extinction is the result of human-caused environmental alteration. The rate of this mass
extinction appears to be much greater than that of the others. Just because life bounces back,
does that mean we will? Do we have any ethical responsibility to minimize our impact on
other species?
Additional Critical Thinking Questions
The Process of Science
1. Imagine that certain fruit flies exhibit mating behavior that separates them from another
group of fruit flies: one group mates and lays eggs only on apple trees, and the other group
mates and lays eggs only on pear trees. How could you design an experiment to test if these
are different species or still the same species?
Suggested answer: Set up an experiment in the laboratory in which members from both
populations are mixed and mating frequencies are calculated. If a reproductive barrier is
present, you should see more apple females mating with apple males and the same for pear
females and pear males even though they encounter each other in the experimental
conditions. If speciation has truly occurred, we would expect little interbreeding. However,
this experiment cannot rule out the impact of artificial conditions.
2. Cultivated American cotton plants have 52 chromosomes (2n 5 52). In each cell, there
are 13 pairs of large chromosomes and 13 pairs of smaller chromosomes. Old World cotton
plants have 26 chromosomes (2n 5 26), all large. Wild American cotton plants have 26
chromosomes (2n 5 26), all small. Propose a testable hypothesis to explain how cultivated
American cotton probably originated.
Suggested answer: A broad hypothesis would be that cultivated American cotton arose
from a sequence of hybridization, meiotic failure, and self-fertilization.
We can divide this broad statement into at least three testable hypotheses.
Hypothesis 1: The first step in the origin of cultivated American cotton was
hybridization between a wild American cotton plant (with 13 pairs of small chromosomes)
and an Old World cotton plant (with 13 pairs of large chromosomes). If this hypothesis is
correct, we would predict that the hybrid offspring would have 13 small chromosomes and
13 large chromosomes.
Hypothesis 2: The second step in the origin of cultivated American cotton was a failure
of meiosis in the hybrid offspring. If this hypothesis is true, we would expect the gametes
resulting from meiotic failure to each have 13 large chromosomes and 13 small
chromosomes.
Hypothesis 3: The third step in the origin of cultivated American cotton was selffertilization of the gametes resulting from meiotic failure. If this hypothesis is true, we
would expect the outcome of self-fertilization to be a polyploid plant with 52
chromosomes—13 pairs of large ones and 13 pairs of small ones. This is the genetic
makeup of cultivated American cotton.
3. Scientists have calculated that a particular piece of rock contained about 12 grams (g) of
radioactive potassium-40 when it was formed. It now contains 3 g of potassium-40. The
half-life of potassium-40 is 1.3 billion years. About how old is the rock?
Suggested answer: The rock is about 2.6 billion years old. (If the half-life of potassium40 is 1.3 billion years, there would be 6 g left after 1.3 billion years and 3 g left after another
1.3 billion years.)
Biology and Society
4. Your friend says that humans have evolved from apes. How would you respond to that
statement using the information provided in this chapter?
Some issues and questions to consider: Humans and apes share a common ancestor, but
we make up distinct lineages that result from an accumulation of separate mutations that led
to the human populations and the great ape populations that exist today. We certainly share
common features (going back to the common ancestors) but have also evolved quite
differently from the time of the split from the ancestral organism. The statement is
analogous to saying that you are the children of your cousins. Instead, you and your cousins
share common ancestors—most recently your grandparents.
5. The red wolf, Canis rufus, formerly widespread in the southeastern and southcentral
United States, nearly became extinct in the late 1970s. Saved by a captive breeding program
under the authority of the Endangered Species Act (ESA), it has been reintroduced in areas
such as the Great Smoky Mountains National Park. Recent genetic evidence indicates that
the red wolf may not be a separate species, but a hybrid of the coyote, Canis latrans, and the
gray wolf, Canis lupus. Though the original intent of the ESA was to protect all endangered
groups—whether species, subspecies, or hybrids—the costs may be prohibitive. What
criteria should be applied if we must decide which organisms to protect? Are there reasons
to preserve hybrids, subspecies, or local populations of species when the species as a whole
is not at risk?
Some issues and questions to consider: The rationale behind protecting all endangered
groups is the desire to preserve genetic diversity. Each species, subspecies, and hybrid
group may represent a unique mix of genes. Studies of the degree of genetic distinctiveness
of a subspecies or hybrid group may help decision makers if cost is an issue. If the species
as a whole is not at risk, it seems appropriate to determine how distinctive the gene pool of a
subspecies or hybrid group is before assigning it a lower priority for saving. Questions for
society in general to address are, What is the value of any particular species and its
genetically distinct subgroups? In addition, how far are we willing (should we be willing) to
go to preserve a genetically distinct group of organisms? How should the costs of preserving
genetic diversity compare with the costs of other public projects?