Answers to Mastering Concepts Questions
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. It is divided into eras and stretches as far back as Earth has
existed.
2. What types of information provide the clues that scientists use in investigating
evolutionary relationships?
Fossils, biogeographical studies, comparative anatomy, and molecular techniques such as
sequencing of proteins and DNA provide the evidence that scientists use in investigating
evolutionary relationships.
13.2
1. 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.
2. Distinguish between relative and absolute dating of fossils.
Relative dating of fossils is based upon the position of the rock stratum in which a fossil
is found. It allows a paleontologist to decide whether one fossil is older or younger than
another. Absolute dating assigns a range of dates that tell when a fossil formed with a
direct testing method, such as carbon dating.
3. 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 do biogeographical observations help biologists interpret evolutionary history?
The locations of fossils and living organisms help biologists understand where and when
different groups originated and diversified. In addition, by comparing species on isolated
islands with related species from a nearby mainland, biogeographical observations help
biologists to interpret the evolutionary history of species on isolated islands.
13.4
1. What can homologies reveal about evolution?
Homologous genes, chromosomes, and anatomical structures in related organisms reveal
common ancestry.
2. What is a vestigial structure? What are some examples of vestigial structures in
humans and other animals?
A vestigial structure is nonfunctional, but it is homologous to a functional structure in
related organisms. The human appendix, the muscles that erect our body hair, the tails of
human embryos, and the muscles that allow us to wiggle our ears are vestigial structures.
The leg bones of snakes and of whales also are also vestigial structures.
3. What is convergent evolution?
Convergent evolution is the result of similar selective pressures on unrelated organisms.
The result of convergent evolution is adaptations with similar functions but different
evolutionary origins.
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
shared developmental genes inherited from a shared common ancestor.
2. Why are evolutionary biologists interested in how genes influence development?
Evolutionary biologists are interested in how homeotic genes influence development
because they shed light on the genetic changes that are involved in the evolution of new
body parts and of new species 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
unrelated 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. How can molecular clocks help determine when two species diverged from a common
ancestor?
A molecular clock uses the known mutation rate for a gene to estimate how long ago two
species diverged from a common ancestor.
13.7
1. How might the loss of limbs enhance the fitness of a burrowing animal such as a
snake?
It is easier to move about in small, tight spaces if legs are not sticking out the side of the
organism. Easier movement may mean enhanced ability to acquire food, which may
translate to greater reproductive success.
2. Why was Najash a significant discovery?
It was the first fossil snake with legs, a pelvis, and a sacrum. The sacrum is present in
other tetrapods, so the discovery of Najash provided strong evidence that snakes
originated on land, not in water
Answers to Write It Out Questions
1. What types of information are used to determine how species are related to one another
by descent from shared ancestors? Give an example of how multiple types of evidence
can support one another.
Paleontology, the study of fossil remains or other clues of past life, is used to make
hypotheses about extinct organisms. The study of biogeography allows scientists to
determine how and when species diverged in the context of continental drift and other
large-scale events. Anatomical and developmental studies reveal the presence of
homologous structures. Molecular techniques, including protein and DNA sequencing,
have further enabled scientists to understand relationships among species that have
descended from a common ancestor.
2. Why is the fossil record useful, even if it doesn’t represent every type of organism that
ever lived?
Although not every species is represented in the fossil record, it does reveal a sample of
species that lived in the past and are now extinct. In addition, the fossil record allows
scientists to better understand evolutionary relationships among species and why some
species were adapted to a given environment. The fossil also record gives clues as to
what the environmental conditions were like at a particular time, which are important in
determining what other organisms could have existed during the time period being
studied.
3. 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.
4. How do molecular sequences provide different information than relative and absolute
dating?
Relative dating and absolute dating provide the ages of fossils; however, molecular
sequences provide information about approximate dates for speciation events or the
appearance of mutations in a population.
5. How have geological events such as continental movements influenced the history of
life on Earth?
Continental movements provide opportunities for speciation. 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 soon 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.
6. 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. If structures that are deemed homologous are
actually analogous, or vice versa, it may lead to mistaken conclusions about evolutionary
relationships.
7. How did the discovery of homeotic genes help launch the new field called “evo-devo”?
Homeotic genes revealed that a single mutation can lead to enormous changes in body
form and organization. Homeotic genes therefore help explain how a few small mutations
in key developmental genes can have profound phenotypic effects. These new
phenotypes are subject to natural selection and may lead to the development of new
species, providing a link between evolution and development.
8. 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 bring about new body plans, which could have a
drastic effect on evolution.
9. 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.
10. 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.
11. Search the internet to learn about evidence suggesting that birds and dinosaurs are
closely related. If scientists could extract DNA from dinosaur fossils, how could they use
the sequence to learn more about the origin of birds?
Besides using the DNA to confirm relationships within the clade, it could also be useful
in learning more about specific events in reptile evolution. For example, the dinosaur
fossil DNA could reveal a particular gene that mutated and gave a selective advantage to
birds.
12. Why are molecular clocks useful, and what are their limitations?
If biologists know the mutation rate for any gene, plus the number of differences in the
DNA sequences for that gene in two species, they can use the DNA as a molecular clock
to estimate the time when the organism diverged from a common ancestor. One limitation
is that not all DNA sequences mutate at the same rate.
13. Genetic anthropology combines the study of DNA with physical evidence such as
fossils to reveal the history of the human species. Use the Internet to research the goals
and methods of the Genographic Project, HapMap, or the Human Genome Diversity
Project. What are the benefits of these projects? What ethical issues arise from this type
of research?
The Genographic Project is trying to map the migration patterns of humans from our last
common ancestral population to today. It uses DNA samples from hundreds of thousands
of volunteers around the world and analyzes it with sophisticated software programs.
The HapMap project is looking at genetic similarities and differences among people
around the world to look for DNA sequences of genetic diseases. The DNA samples are
donated anonymously from four populations. The Human Genome Diversity Project
collects cell lines from individuals around the world, documenting the genetic diversity in
the human population and making the DNA available to researchers. Benefits could
include better understanding of many diseases, how different populations got to where
they are today geographically, and relationships among population groups. Ethical issues
could include misuse of individual’s DNA information, stigmatizing groups based on
genetic information, or interference with religious or cultural explanations of origins.
Answers to Pull It Together Questions
1. What are some examples of each line of evidence for evolution?
Fossils: Fossils of bivalve shells that do not exist today show us that there were once
species with different morphology than there are today. Radiometric dating could
indicate the age of the fossils relative to the age of other animals.
Biogeography: The similarity of species in geographical regions that were once
connected, like in Chile and New Zealand, suggests that those species were once part of a
united population and have now changed enough to be categorically distinct.
Anatomy: The anatomical similarity of butterflies and moths suggest they share a
common ancestor. Their wing structures are homologous. So, moths and butterflies may
have evolved from a common ancestral species.
Development: The early stages of embryonic development in mammals are very similar;
an early embryonic mouse looks very similar to the early stage of a human. This
supports the notion that mammals are all derived from a common ancestor.
Molecular: The genes involved in development are strongly conserved in mammals,
meaning they share very similar sequences.
2. Add the following terms to this concept map: homologous structures, vestigial
structures, homeotic genes, and molecular clock.
“Anatomy” connects with “reveals common evolutionary origin via” to “Homologous
structures”. “Vestigial structures” connects with “are functionless examples of” to
“Homologous structures”. “Homeotic genes” connects with “profoundly influence” to
both “Anatomy” and “Development”. “DNA” connects with “mutates at a fairly constant
rate, providing scientists with a” to “Molecular clock”.