Mastering Concepts
13.1
1. What is the geologic timescale?
The geological timescale is the record of Earth’s history based on geological formations
and major biological events.
2. What types of information provide the clues that scientists use in investigating
evolutionary relationships?
Comparisons among living organisms, fossils, biogeographical studies, comparisons of
embryonic development patterns, and molecular sequence data provide the evidence that
scientists use in investigating evolutionary relationships.
13.2
1. What are some of the ways that fossils form?
Fossils form by compression, petrifaction, impression, the formation of casts, and when
organisms or parts of organisms are trapped in sediments, oozing sap, or tar.
2. Why will the fossil record always be incomplete?
The fossil record will always be incomplete because most organisms never fossilize; softbodied organisms, for example, seldom leave fossils, and organisms that decompose or
are eaten by scavengers will not leave fossils. Erosion destroys many fossils, and some
fossils are buried in places that are unlikely to be sampled by fossil hunters, such as
extremely deep sediments beneath ocean bottoms.
3. Distinguish between relative and absolute dating of fossils.
Relative dating of fossils is based upon the position of the rock layer in which a fossil is
found. It allows a paleontologist to decide whether one fossil is older or younger than
another. Absolute dating techniques, including radiometric dating, assign a range of
dates that tell when a fossil formed,
4. How does radiometric dating work?
Radiometric dating analyzes the amounts of isotopes in a sample. For instance, carbon
dating measures the amount of an isotope of carbon (14C) in a sample. 14C has a half-life
of 5730 years. When an organism is alive, 14C is taken into its tissues at a constant
proportion. When the organism dies, 14C is no longer taken in, but the 14C in the dead
tissues continues to decay. The proportion of 14C to 12C therefore gradually declines. So,
by measuring the amount of 14C in a sample, the age of the sample can be estimated.
13.3
1. How have the positions of Earth’s continents changed over the past 200 million
years?
Over the past 200 million years, Earth’s continents have drifted apart from a central,
global landmass to their present positions as separate entities. They are continuing to
move in different directions, riding on tectonic plates.
2. How does biogeography provide evidence for evolution?
Biogeography is the study of the distribution of species across the planet. Studying
biogeography helps explain some observations that might otherwise be puzzling. For
example, plate tectonics explains why fossils of marine species appear on mountaintops,
why fossils of the same extinct species are distributed across multiple continents, why
Australia and New Zealand have more native marsupial species than other continents,
and why island species are typically closely related to those on the nearest mainland.
13.4
1. What can homologous structures reveal about evolution?
Homologous anatomical structures reveal how natural selection can mold the same basic
parts into structures with different functions.
2. What is a vestigial structure? What are some examples of vestigial structures in
humans and other animals?
A vestigial structure is nonfunctional but homologous to functional structures in related
organisms. The muscles that cause human hairs to stand on end, the tails of human
embryos, and the muscles that allow us to wiggle our ears are vestigial structures. The leg
bones of certain snakes and of whales also are also vestigial structures.
3. What is convergent evolution?
Convergent evolution produces similar adaptations in unrelated organisms, thanks to
similar selective pressures in different locations on Earth’s surface.
13.5
1. How does the study of embryonic development reveal clues to a shared evolutionary
history?
Similar physical traits are the result of similar developmental processes, which indicate
developmental genes inherited from a common ancestor.
2. Why are evolutionary biologists interested in how genes influence development?
Evolutionary biologists are interested in how genes influence development because they
shed light on the genetic changes that are involved in evolution of new phenotypes from
an ancestral type.
13.6
1. How does analysis of DNA and proteins support other evidence for evolution?
Molecular analysis supports evidence for evolution because it is highly unlikely that two
related species would evolve identical DNA or protein sequences. Biochemical
similarities derive from a common ancestor; differences arise from mutations that
occurred after two species diverged from a common ancestor. Thus, the more similar the
DNA or protein sequences, the closer the evolutionary relationship.
2. What is an advantage of using mtDNA instead of nuclear DNA in tracing evolution?
The biggest advantage is that each cell contains many mitochondria, and each
mitochondrion contains multiple DNA molecules. As a result, some intact mtDNA may
still be present in a sample even if the nuclear DNA is too degraded to be analyzed.
3. What is the basis of using a molecular clock to determine when two species diverged
from a common ancestor?
A molecular clock uses the known mutation rate for a gene to determine how long ago
two species diverged from a common ancestor.
13.7
1. How might the ability to crawl on land for short periods have enhanced the
reproductive fitness of Tiktaalik?
The ability to crawl on land for short periods of time would have allowed Tiktaalik to
exploit new food resources without competition, and this would have increased fitness.
2. How might the loss of hind limbs enhance the reproductive fitness of a burrowing
animal such as Najash?
Najash would be able to burrow more efficiently without legs, enabling it to conserve
energy and find food more easily.
Write It Out
1. Explain the significance of the geologic timescale in the context of evolution.
The geologic timescale functions as a sort of “calendar” by which scientists mark the
most important events in the history of life.
2. What types of information are used to hypothesize how species are related to one
another by descent from a shared ancestor? Give an example of how multiple types of
evidence can support one another.
The evidence includes observations of living organisms, fossils, biogeography,
comparisons of embryonic structures, and molecular sequences. Human evolution
provides one example: anatomical similarities reveal that humans are primates, fossils
and biogeography reveal that humans originated in Africa along with other ape species,
and similarities in DNA/protein sequences support the hypothesis that chimpanzees are
the closest relatives to humans.
3. Describe six types of fossils and how they form. What present environmental
conditions might preserve today’s organisms to form the fossils of the future?
A compression fossil forms when the decomposed remains of plants and other organisms
are preserved by fine sediment that eventually forms sedimentary rock. Petrifaction
occurs when an organism dies, is buried, and is eventually replaced by minerals. An
impression fossil forms when an organism presses against soft sediment, which then
hardens after the organism decays, leaving an outline of the organism. A cast is an imprint
that is later filled with mud that hardens into rock, creating a rocky replica of the ancient
organism. In intact preservation, a whole organism is trapped in tree resin or tar.
Today’s organisms would be likely to form the fossils of the future if their bodies have
hard parts (such as bones, shells, or teeth) and if they are not decomposed or eaten after
death. A landslide or catastrophic flood might preserve large numbers of fossils of today’s
organisms, provided the sediments are not later eroded away.
4. Why are transitional fossils especially useful for understanding evolutionary
relationships? Why might some transitional fossils never be found?
Transitional fossils are useful because they provide direct evidence of common ancestry,
even for tracing evolutionary events that happened long ago. Some transitional fossils
might never be found because the organisms were soft-bodied, the organisms
decomposed or were eaten before they fossilized, natural processes destroyed the fossils
before they could be found, or the fossils are inaccessible.
5. Index fossils represent organisms that were widespread but lived during relatively
short periods of time. How are index fossils useful in relative dating?
Since the index fossil lived for a short period, if one is found in association with a fossil
of unknown date, the index fossil can provide a narrow range of relative dates for the
unknown fossil.
6. How was the discovery of Wallace’s line evidence for the predictive power of
evolution?
Wallace’s line is an imaginary line that divides the organisms of Australia from those of
southeast Asia. Wallace noticed differences between organisms on each side of this line,
suggesting that the regions must have been separated for a long time. The deep trench
separating the islands was later discovered.
7. How have geological events such as continental movements and the emergence of new
volcanic islands influenced the history of life on Earth?
Long ago, all the continents were joined as one supercontinent known as Pangaea.
However, species were split up when the continents separated. Once-uniform populations
diverged as selective pressures changed. Similarly, as organisms colonized isolated
volcanic islands, the new selective pressures selected for new adaptations, which
eventually led to the formation of new species.
8. Why is it important for evolutionary biologists to be able to distinguish between
homologous and analogous anatomical structures?
If two structures are termed homologous then there are similarities between them that
reflect a common ancestry. Analogous structures appear superficially similar, but the
structures have evolved independently.
9. Suppose that plants in the San Francisco Bay area and in southern Chile share a
common seed dispersal method. Scientists determine that the evolutionary divergence of
these plants happened long before this seed dispersal method arose in each plant. What
term relates the seed dispersal method of the San Francisco Bay plant to the seed
dispersal method of the southern Chile plant? Explain your answer.
The seed dispersal methods are analogous. They provide an example of convergent
evolution because the seed dispersal method originated independently after the two plant
species shared a common ancestor,
10. Explain why vestigial structures are important to evolutionary biologists.
Vestigial structures do not have a function, yet they are homologous to functional
structures in related species. They are important because they provide clues to the
ancestry of the organism.
11. Many species look similar as embryos. What causes them to appear different as
adults? Why does the study of development give insights into evolutionary relationships?
Species that look similar as embryos may appear different as adults because different
parts may or may not develop, or they may develop to a different extent. The study of
development is important because it may reveal clues to common descent that are not
evident in adult bodies.
12. Suggest a type of genetic change that could have a drastic effect on the evolution of a
species.
A mutation in a homeotic gene could produce a totally new trait, such as the loss of
limbs. The new phenotype may increase fitness, which could have a drastic effect on the
evolution of the species.
13. How do biologists use sequences of proteins and genes to infer evolutionary
relationships?
All species use the same four DNA nucleotides and the same 20 amino acids, and many
genes and proteins only have minor differences from one species to another. It’s unlikely
that two unrelated species would evolve precisely the same DNA and protein sequences.
Therefore, it is logical to surmise that the greater the molecular similarities, the more
closely related the species.
14. Some genes are more alike between human and chimp than other genes are from
person to person. Does this mean that chimps are humans or that humans with different
alleles are different species? What other explanation fits the facts?
No. Although chimps and humans share a common ancestor (and therefore share many
genes), they are reproductively isolated from one another so they are different species.
Genes that are very similar between humans and chimps may be the most important
genes (that is, mutations in those genes may be fatal, slowing their rate of change).
Genes that vary within a population may not be essential to survival or may have mutated
after the two species diverged from the common ancestor.
15. Evolutionary biologists often try to assign an approximate date when two organisms
last shared a common ancestor. Why do you think that molecular evidence often yields an
earlier date than fossil evidence?
Molecular evidence would be apparent from approximately the point of mutation.
Physical evidence in the form of a fossil would have to rely on expression of the mutation
in an organism that died under just the right circumstances to be fossilized. Since this is a
rare event, the likelihood of its happening right after the mutation’s appearance is slim.
Pull It Together
1. Write a phrase to connect fossils and biogeography and a separate phrase to connect
development and life’s molecules.
“Fossils” could connect with the phrase “locations are part of the study of” to
“Biogeography.” “Development” could connect with the phrase “reflects the expression
of DNA, which is one of” to “Life’s molecules.”
2. Add the following terms to this concept map: homologous structures, vestigial
structures, homeotic genes, and molecular clock.
“Homologous structures” leads with “appear in” to “Anatomy” and to “Life’s
molecules.” “Vestigial structures” leads with “are clues to common ancestry in” to
“Anatomy.” “Homeotic genes” leads with “strongly influence” to “Development.”
“Molecular clock” leads with “uses known mutation rates in” to “DNA.”