Unit 7 · Topic 7.6 end-of-topic test
Suggested time: about 47 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.
A team studying rhinos has four pieces of evidence: rhino fossils in a series of rock layers; a map of where the living species live; the amino-acid sequence of one protein in each species; and a tally of the sequence differences between each pair. The five kinds of evidence for evolution are geographical (where organisms live), geological (rocks and their fossils), physical (how fast unstable atoms break down), biochemical (DNA and protein sequences) and mathematical (models and counts).
Which kind of evidence is missing from the team’s set?
Here are four finds from one museum drawer.
Which find is a fossil of an extant species?
A cliff shows four undisturbed rock layers, numbered 1 at the bottom. The table lists the fossil shells found in each layer.
Which statement do the layers support?
A rock’s isotope has been decaying since the rock formed, for exactly two half-lives.
What percentage of the isotope the rock held when it formed has decayed?
A buried wooden peg holds 80 carbon-14 atoms for every 320 that its tree’s wood held when the tree died. Carbon-14’s half-life is 5,730 years.
How old is the peg?
A lab can measure the carbon-14 left in one specimen. Four specimens are on the bench, each with an age already estimated from the rock it was found in.
Which specimen can carbon-14 date?
A fossil lies in a layer of sandstone between two layers of volcanic rock. A slow isotope in each volcanic layer gives its age, as the table shows. The layers are undisturbed.
How old is the fossil?
Wingless grasshoppers live on three island groups, as the table shows. Biologists propose that each island group’s grasshoppers descend from colonists that came from its nearest mainland.
Which result from comparing the grasshoppers’ protein sequences would best support the proposal?
Fossils of one line of descent lie in four layers of mudstone between two layers of volcanic rock. The table gives each layer’s rock, the mean hind leg length of the fossils in it, and the age measured for the volcanic layers.
Which conclusion do the data support?
The table gives the bones of the forelimb in four animals, from the shoulder outward, and what each forelimb does.
What is the shared sequence of bones evidence of?
Here are four pairs of parts, with the job each pair does and the plan underneath.
Which pair are analogous structures?
Here are four parts of four animals.
Which part is a vestigial structure?
A lab reads the same 10 amino acids of one protein in a trout, a salmon, a pike and a carp. The table shows the four readings; the trout is the reference species.
At how many positions does the pike differ from the trout?
A lab compares one protein in four gecko species and counts the positions at which each pair differ, as the table shows.
Which pair of species shares the most recent common ancestor?
A lab compares one stretch of DNA in four populations of frogs and counts the positions at which each pair differ, as the table shows. A biologist proposes that the river frogs and the highland frogs share a more recent common ancestor with each other than either shares with the woodland frogs.
Which comparison in the table is the strongest evidence for the proposal?
Suppose biologists study a group of legless lizards. Fossils in the group’s line, 20 million years old, have small hind legs. Living members have tiny leg bones inside the body, joined to no leg. The living members’ protein sequences are closest to those of a lizard with full legs. Each line of evidence supports an ancestor with legs.
Why do the three lines agreeing make the case for a legged ancestor stronger than any one line alone?
A layer of volcanic rock held 4,000 atoms of a slow isotope for every 500 it holds now. The isotope’s half-life is 1.25 billion years.
How old is the rock?
A lab reads the same 10 bases of one gene in five newts. The table shows the five readings; the marsh newt is the reference species.
Which newt shares the most recent common ancestor with the marsh newt?
(a) Identify the number of positions at which the lake snail’s sequence differs from the marsh snail’s. (1 point)
A full-credit answer: Positions 1, 3, 5, 7, 8, 9, 10 and 12 hold different amino acids in the lake snail and the marsh snail.
That is 8 positions.
Check the box for each point your answer earns
Common slip: Counting the positions that match (4) instead of the positions that differ.
(b) Determine which of the other three snails shares the most recent common ancestor with the marsh snail. (1 point)
A full-credit answer: The pond snail differs from the marsh snail at 2 positions, the river snail at 5 and the lake snail at 8.
Fewer differences mean a more recent common ancestor.
So the pond snail shares the most recent common ancestor with the marsh snail.
Check the box for each point your answer earns
(c) A student says: “The lake snail differs from the marsh snail at more positions than the river snail does, so the lake snail’s DNA must be mutating faster than the river snail’s.” Evaluate the student’s claim. (1 point)
A full-credit answer: The claim is not supported.
After two lines split, each line collects its own mutations.
So more differences most likely mean a longer time since the lake snail’s line split from the marsh snail’s, not a faster rate of mutation.
Check the box for each point your answer earns
Common slip: Reading a count of differences as a rate: the count grows with time apart as well as with the rate.
(d) The lab then reads the same 12 amino acids in a sea snail. The sea snail’s line split from the ancestor of all four freshwater snails before any of the four lines split from one another. Predict how the sea snail’s number of differences from the marsh snail compares with the other three counts. (1 point)
A full-credit answer: The sea snail’s line has been apart from the marsh snail’s for longer than any of the three freshwater lines.
Differences build up with time apart.
So the sea snail’s count is larger than 8, the largest of the three.
Check the box for each point your answer earns
(a) Identify the kind of structure the skink’s leg stubs are. (1 point)
A full-credit answer: The stubs are a vestigial structure: a reduced part that does little or none of the job it does in the skink’s relatives.
Check the box for each point your answer earns
(b) Explain what the skink’s leg stubs are evidence of. (1 point)
A full-credit answer: A reduced part that does little or none of its job is a vestigial structure.
A vestigial structure is what is left of a part that was full-sized and working in an ancestor.
So the stubs are evidence of an ancestor of the skink that had full, working legs.
Check the box for each point your answer earns
(c) A fossil lizard 15 million years old is found in the skink’s line. Predict what this fossil’s legs should look like. (1 point)
A full-credit answer: The fossil is in the skink’s line, between the walking ancestor and the living skink.
A fossil that sits between an older form and a later form has traits partway between the two: a transitional fossil.
So its legs should be partway between a walking relative’s full legs and the skink’s stubs.
Check the box for each point your answer earns
(d) A student claims that the skink and the amphibian are close relatives, because their smooth, long bodies look so alike. Evaluate the claim. (1 point)
A full-credit answer: The claim is not supported.
The skink’s body holds leg bones built on the limb’s plan; the amphibian has no leg bones at all.
So the two bodies have different plans underneath.
A likeness on the outside with different plans underneath is analogous: it came from similar pressures in the same sand, not from a recent common ancestor.
Check the box for each point your answer earns