Mastering Concepts
14.1
1. What is the relationship between macroevolution and microevolution?
Macroevolutionary events are large-scale evolutionary changes, such as the evolution or
extinction of a species, that typically occur over long periods of time. Small-scale
microevolutionary events occur over shorter time periods, such as the changes in allele
frequencies that occur each generation within a species. Over many generations,
microevolutionary events can lead to macroevolution.
2. How does the biological species concept differ from Linnaeus’s definition?
The modern definition of a species recognizes the potential for interbreeding, whereas
Linnaeus’ definition relied on external appearance alone. Linnaeus thought that each
species was specially created and that species never changed or disappeared; modern
taxonomists recognize the importance of evolution.
3. What are some of the challenges in defining species?
Some of the challenges in defining species include: the biological species concept cannot
be applied to asexual or extinct species; some species that never encounter one another in
nature will interbreed in captivity and produce fertile offspring; some species are not
absolutely reproductively isolated; the use of DNA sequences to define species leads to
arbitrary lines that may or may not accurately reflect reproductive isolation.
14.2
1. How do reproductive barriers lead to speciation?
Speciation requires reproductive isolation, which keeps gene pools separate.
Reproductive barriers create this reproductive isolation.
2. Research and describe examples of each of the eight modes of reproductive isolation.
Alternatively, make up your own examples.
Answers will vary; the examples should illustrate ecological (habitat) isolation, temporal
isolation, behavioral isolation, mechanical isolation, gametic isolation, hybrid inviability,
hybrid sterility, and hybrid breakdown.
14.3
1. Distinguish among allopatric, parapatric, and sympatric speciation, and provide
examples of each.
In allopatric speciation, a geographic barrier separates two populations. One example is
the dry land that separates the pools where different species of desert pupfish live.
Parapatric speciation occurs along a border between the ranges of two species. The little
greenbul birds living in the transitional zones between rainforest and grassland are an
example. Sympatric speciation occurs within the overlapping ranges of two species. For
example, new polyploid plant species can arise from, and be reproductively isolated
from, their diploid ancestors.
2. How can polyploidy contribute to sympatric speciation?
Polyploidy results in plants with extra sets of chromosomes. Sympatric speciation occurs
if plants with extra chromosomes cannot interbreed with the original plant population,
even if the plants share the same habitat.
3. Why is it sometimes difficult to determine whether speciation is allopatric, parapatric,
or sympatric?
The difficulty arises from several factors. First, the three patterns represent points along
a continuum, from total isolation to continuous intermingling. Second, it is often difficult
to recognize what constitutes a barrier to a species. Finally, the size of the barrier needed
varies with the range over which the gametes spread.
14.4
1. Describe the theories of gradualism and punctuated equilibrium. How can the fossil
record support both?
Gradualism proposes that the rate of evolutionary change is slow and steady, with many
intermediate forms. Punctuated equilibrium proposes that evolutionary change occurs in
discontinuous, fast bursts that interrupt long periods in which a species does not change.
The fossil record can support both models because of the great variety in species and
habitats worldwide, and because the pace of environmental changes over time.
2. What are three ways that adaptive radiation can occur?
A few individuals may colonize a new habitat that simultaneously selects for different
phenotypes, eventually leading to reproductive isolation; a new adaptation may
contribute to reproductive success in a wide variety of environments; after a catastrophe
wipes out many species, the survivors diversify as they exploit the changed environment.
14.5
1. What factors can cause or hasten extinction?
Factors that can hasten extinction include habitat loss, the introduction of new diseases or
new predators, rapid environmental change, and small population size (which is
associated with low genetic diversity and high rates of inbreeding).
2. Distinguish between background extinction and mass extinctions.
Background extinctions continually occur at a steady rate of 0.1 to 1.0 species per year
per million species. Mass extinctions occur only rarely and kill off many species over a
short period of time.
3. How have humans influenced extinctions?
Humans have influenced extinctions by destroying and polluting habitats, introducing
alien species that compete with native species, and harvesting species faster than they can
reproduce.
14.6
1. Describe the taxonomic hierarchy.
The taxonomic hierarchy consists of nested groups based on similarities from the level of
the species (the least inclusive category) up to the three domains (the most inclusive
category). The order of the hierarchy is domain, kingdom, phylum, class, order, family,
genus, species.
2. What is the advantage of a cladistics approach over a more traditional approach to
phylogeny?
A traditional approach to taxonomy may be based on superficial similarities that do not
reflect evolutionary relationships. Cladistics generates phylogenetic trees based on
common ancestry and shared derived characters.
3. Distinguish between ancestral and derived characters.
An ancestral character is one that was present in the last common ancestor, and a derived
character arose after the taxon split off from the ancestral group.
4. What sorts of evidence do biologists use in a cladistics analysis?
Biologists use ancestral traits (shared with the common ancestor) and derived traits
(features that are different from those in the common ancestor) in cladistics analysis.
5. How is a cladogram constructed?
A researcher begins by selecting the most relevant traits (ancestral and derived) and
entering the character states for each taxon into a chart. Taxa with that share the most
derived characters are hypothesized to be most closely related and are therefore grouped
together. Researchers attempt to create the most parsimonious tree possible, which
requires the fewest evolutionary steps to construct.
6. How do paraphyletic and polyphyletic groups differ from monophyletic groups?
A monophyletic group includes an ancestor and all of its descendants. A paraphyletic
group includes some but not all of the descendants of a common ancestor. Polyphyletic
groups exclude the common ancestor from the group.
14.7
1. What hypothesis were the investigators testing, and why did they choose these two
plant species as experimental subjects?
The hypothesis was that a single change in a flower’s color can create a reproductive
barrier within a plant species by attracting a new pollinator. The two species of
monkeyflower were chosen because they are closely related but have different pollinators
(bumblebees and hummingbirds).
2. If biologists could mutate the gene that controls carotenoid concentration, they could
generate pairs of plants that differ only in that one gene, improving on the 97% similarity
they achieved by backcrossing hybrids with wild-type plants. Explain why this
improvement would help the researchers test their hypothesis.
The 3% difference represents a variable that is not controlled for; perhaps something in
that 3% difference accounts for the observed differences between the wild-type and
experimental groups. Changing just one gene would enable the researchers to standardize
all variables other than flower color.
Write It Out
1. What type of reproductive barrier applies to each of these scenarios?
a. Water buffalo and cattle can mate, but the embryos die early in development.
b. Scientists try to mate two species of dragonfly that inhabit the same pond at the same
time of day. However, females never allow males of the other species to mate with them.
c. One species of reed warbler is active in the upper parts of the canopy while another
species of reed warbler is active in the lower canopy. Both species are active during the
day.
d. Scientists mate two parrots from different populations to see if speciation has occurred.
The parrots mate over and over again, but the male’s sperm never fertilizes the female’s
egg.
(a) Hybrid inviability; (b) behavioral isolation; (c) habitat isolation; (d) gametic isolation
2. How does natural selection predict a gradualistic mode of evolution? Does the
presence of fossils that are consistent with punctuated equilibrium mean that natural
selection does not occur?
Natural selection predicts gradual changes within a species, based upon unequal
reproductive success in a changing environment. Punctuated equilibrium does not counter
the concept of natural selection; it just means that natural selection occurs more rapidly at
some times than at others.
3. Why do species become extinct? Choose a species that has recently become extinct
and describe some possible evolutionary consequences to other species that interacted
with that species before its extinction.
Species can become extinct when too few individuals have adaptations that enable them
to survive and reproduce in a changing environment. (Species choices and consequences
will vary.)
4. Figure 19.3 summarizes the hypothesized evolutionary relationships among living
plants. Are the bryophytes monophyletic, polyphyletic, or paraphyletic? Explain your
answer.
Bryophytes are a paraphyletic group because the rest of the plants, which are also
descendants of the bryophytes’s last common ancestor, are not included.
5. On figure 14.19, circle a monophyletic group, a paraphyletic group, and a polyphyletic
group. Describe the qualities that define how each group is classified.
Answers will vary. A monophyletic group would be any group that includes a common
ancestor and all of its descendants, such as the group that includes Protarchaeopteryx,
Archaeopteryx, modern birds, and their most recent common ancestor. A paraphyletic
group would include a common ancestor and some, but not all, of its descendants; an
example would be a group consisting only of Coelophysis, Allosaurus, and their common
ancestor, but not the other animals in the cladogram. A polyphyletic group would exclude
the common ancestor of the members of the group. A group consisting only of
Protarchaeopteryx and modern birds would be polyphyletic.
Pull It Together
1. How do reproductive barriers relate to the biological species concept?
Reproductive barriers separate gene pools within a population. If two genetic lines
remain isolated for generations, differences will arise. A species therefore forms when
part of a population cannot breed with the rest of the population.
2. Add fertilization and offspring to the concept map.
“Prezygotic” could connect with the phrase “barriers prevent” to “Fertilization.”
“Postzygotic” could connect with the phrase “barriers prevent the production of fertile” to
“Offspring.”
3. Draw pictures of allopatric, parapatric, and sympatric speciation.
Answers will vary, but species relationships should resemble those in figure 14.5.
4. Add gradualism and punctuated equilibrium to this concept map.
“Speciation” could connect with the phrase “occurring in small steps is called” to
“Gradualism”. “Speciation” could connect with the phrase “occurring in bursts is called”
to “Punctuated equilibrium.”
5. How do species become extinct?
A species becomes extinct when its individuals die faster than they reproduce. Forces that
drive species extinct include overhunting, habitat destruction, predation, disease, and
catastrophes.