Mastering Concepts
12.1
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, molecular techniques such as sequencing of proteins and DNA, and
biogeographical studies provide the evidence that scientists use in investigating
evolutionary relationships.
12.2
1. What are some of the ways that fossils form?
Fossils form by compression, impression, the formation of casts, petrifaction, 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 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.
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.
12.3
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?
By comparing species on isolated islands with related species from nearby mainlands,
biogeographical observations help biologists to interpret the evolutionary history of
species on isolated islands.
12.4
1. What can homologies reveal about evolution?
Homologous genes, chromosomes, and anatomical structures in related organisms reveal
common ancestry and evolutionary relationships between species.
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 human appendix, human muscles that erect hairs, 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 in
similar (but not the same) environments, such that they show similar adaptations.
12.5
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 evolution of new
species from an ancestral type.
12.6
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 the basis of using a molecular clock to determine when two species diverged
from a common ancestor?
The known mutation rate for a gene is the basis of using DNA as a molecular clock to
determine how long ago two species diverged from a common ancestor.
3. What is an advantage of using mtDNA instead of nuclear DNA in tracing evolution?
Using mitochondrial DNA instead of nuclear DNA in tracing evolution allows
investigators to trace mothers and children. The molecular clock of mitochondrial DNA
is faster than the nuclear DNA clock, so it is useful in tracing recent evolutionary events.
12.7
1. What hypothesis are the Grants testing, and what observations are they using to test
it?
The Grants are testing the hypothesis that short-term, extreme, environmental changes
can lead to observable changes in heritable physical traits. They observed the two main
finch species (ground and cactus) on the island of Daphne Major and compared beak size
and seed size after the 1977 drought and the 1983 El Niño rainy season.
2. Would careful examination of finch fossils or DNA be a better way to address the
questions of short-term evolution that the Grants are studying?
DNA would be better. Fossils are rare and might not reveal enough pattern detail over
such a short period; however DNA, especially mtDNA with its faster clock, could reveal
a lot.
Write It Out
1. What types of information are used to hypothesize 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. Molecular techniques, including protein and DNA
sequencing, have further enabled scientists to understand relationships among species
that have descended from a common ancestor. Anatomical and developmental studies
reveal the presence of homologous structures, and the study of biogeography allows
scientists to determine how and when species diverged in the context of continental drift
and other large-scale events.
2. 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?
Compression fossil: decomposed remains of plants and other organisms are preserved.
For example, a leaf sinks, fine sediment covers leaf, and sedimentary rock forms.
Impression fossil: an organism presses against soft sediment, which then hardens after
the organism decays, leaving an outline of the organism.
Cast: if an imprint is later filled with mud that hardens into rock, the resulting cast is a
rocky replica of the ancient organism.
Petrifaction: an organism dies, decays, and is buried. Water containing dissolved minerals
seeps through, and the organic matter that is replaced by minerals “turns to stone.”
Intact preservation: sticky tree resin entraps plants and animals, hardening them in amber
tombs.
Trace fossils: feces or animal tracks may be preserved as evidence of an organism’s
movements or behavior.
Organisms do not always leave fossils when they die. Their bodies may be too soft to be
preserved, or they may be decomposed or eaten after death. In addition, even fossils that
do form may be destroyed by erosion, or they may form in areas where no one is likely to
discover them.
3. 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.
4. 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.
5. Why can’t scientists use potassium–argon dating on a preserved woolly mammoth that
died 20,000 years ago?
The half-life of 40K is 1.3 billion years, far too long to date fossils from an animal that
died only 20,000 years ago. Potassium-argon dating is appropriate for fossils 300,000
years or older.
6. 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 dates for the appearance of mutations in a
population and dates for speciation events.
7. How have geological events such as continental movements and the emergence of new
volcanic islands influenced the history of life on Earth?
Continental movements and the emergence of new volcanic islands provide opportunities
for allopatric speciation. Long ago, all the continents were joined as one super-continent
known as Pangaea. However, species were split up when the continents separated. Onceuniform 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.
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. Provide examples of homologous and analogous structures other than those mentioned
in this chapter.
The wings of a bird and of a fly are analogous; both function in flight, but the embryonic
origin is different. The wing of a bird and the foreleg of a frog are homologous structures;
their embryonic origin is similar.
10. The giant anteater lives in South America, the scaly anteater lives in tropical parts of
Asia and Africa, and the spiny anteater lives in Australia. The three animals are not at all
closely related, but they resemble each other closely. They have long, sticky tongues that
they use to eat ants, no teeth, large salivary glands, and long, bald snouts. Which
phenomenon do these animals illustrate?
These animals illustrate convergent evolution since their physical traits closely resemble
each other without a common ancestor connection. They have evolved similar
adaptations in response to similar environmental selective pressures.
11. How did the discovery of homeotic genes help launch the new field of “evo-devo”?
Homeotic genes help explain how a few small mutations in key developmental genes can
have profound phenotypic effects, and how evolutionary events can occur in short
periods of time.
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 bring about new body plans, which could have a
drastic effect on evolution.
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. Scientists have extracted flu viruses from people who died in the 1918 influenza
epidemic. How could genetic sequences from these viruses be useful today?
Genetic sequences from each of these flu viruses would provide a mutation history that
could help scientists understand the evolutionary history of the flu virus and how to better
combat it.
15. Why is the DNA sequence of one gene a less accurate indicator of the evolutionary
relationship between two species than a comparison of large portions of the two
genomes?
One gene may or may not mutate rapidly. Comparing large portions of two genomes
gives a more general view of the overall rate of evolutionary divergence between the
species.
16. 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.
17. 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.
18. Birds normally lack teeth, but scientists have discovered a mutant chicken embryo
with a mouth full of teeth resembling those of alligators. They subsequently found that a
mutation in one gene was responsible for the toothy bird. How does this finding provide
evidence for evolution?
The finding shows that chickens and alligators share a similar gene and common
ancestry, but that one key mutation to the tooth gene occurred after birds split off from
the other reptiles.
19. 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.
20. 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.
21. 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? Can you see any ethical issues arising
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 between different population groups.
Ethical issues could include misuse of individual’s DNA information, stigmatizing
groups based on genetic information discovered, or interference with established
religious or cultural explanations of origins.
22. Search the Internet for a copy of the Declaration of Independence. Look for words
that were spelled differently in 1776 compared to today’s spelling. How are changes in
language similar to evolutionary changes? How are they different?
Like changes to DNA, changes to spelling and language are random and come from a
change to an original template. Spelling changes accumulate in the language as mistakes
are made in copying information. While many alternative spellings may appear, not all
are selected for and remain in the language. One could also argue that American English
evolved from British English in a manner similar to a founder effect. It is also possible to
trace the history of a population through the changes to its language. However, unlike
evolutionary changes, changes to spelling and language have more latitude. Spellings
can drastically change (e.g. “logique” to “logic”) yet still leave the word recognizable and
serving the same function.
Pull It Together
1. What are some examples of each line of evidence for evolution?
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 that the
fossil is 200 million years old..
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.
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.
The early stages of embryonic development by 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.. This is further supported
by the genetics involved in coordinating those early embryonic events. The genes
involved in these processes 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.
“DNA” leads with “mutates at a fairly constant rate, providing scientists with a” to
“Molecular clock”. “Anatomy” and “Development” lead with “are determined largely
by” to “Homeotic genes”. “Homologous structures” leads with “develop despite species'
differing” to “Biogeography”. “Vestigial structures” leads with “are functionless
remnants not yet removed by” to “Evolution”.