MS-LS4-2: Using Anatomy to Infer Evolutionary Relationships
In plain English: MS-LS4-2 is the NGSS middle school performance expectation that asks students to explain anatomical similarities and differences among modern organisms, and between modern and fossil organisms, to infer how they are related. Students compare the visible structure of bones, teeth, shells and limbs and use shared structures as evidence of common ancestry. It is usually taught in grade 6, 7 or 8 life science.
Apply scientific ideas to construct an explanation for the anatomical similarities and differences among modern organisms and between modern and fossil organisms to infer evolutionary relationships.
Clarification Statement: Emphasis is on explanations of the evolutionary relationships among organisms in terms of similarity or differences of the gross appearance of anatomical structures.
Next Generation Science Standards · Disciplinary Core Idea: Biological Evolution: Unity and Diversity · Official standard
Students compare the anatomy (body structure) of living and fossil organisms and use what they see to explain how the organisms are related. An anatomical structure is a body part such as a bone, a tooth, a shell, a limb or a wing. Organisms that share many structures built on the same plan most likely inherited them from a common ancestor, an ancestral population that both lines of descendants come from. Such structures are called homologous structures: the same basic parts, arranged the same way, even when they now do different jobs. Analogous structures do a similar job but are built in different ways, so they are not evidence of a close relationship. An evolutionary relationship is how closely two groups are related by descent, which students show on a branching diagram, a drawing in which each fork stands for a common ancestor.
The NGSS Clarification Statement says: "Emphasis is on explanations of the evolutionary relationships among organisms in terms of similarity or differences of the gross appearance of anatomical structures." Gross appearance means what can be seen with the eye or a hand lens, such as the number and shape of bones, not cells or DNA. MS-LS4-2 has no Assessment Boundary. The anatomical and fossil facts on this page are real and rounded; trait tables marked invented were written for practice. Evolution is presented as the well-supported scientific explanation it is, and students are asked to explain the evidence in their own words.
Learning Objectives
By the end of this lesson, students will be able to:
Compare the structures of modern organisms and identify which similarities are homologous and which are analogous
Compare fossil organisms with modern ones, using bones, teeth and shells, to infer which modern groups they are related to
Count shared traits in a table and use the counts to decide which organisms are most closely related
Build a simple branching diagram from anatomical evidence
Construct a written explanation that uses anatomical similarities and differences as evidence of evolutionary relationships
Prior Knowledge Required
Students should already be comfortable with:
Plants and animals have internal and external structures that support survival 4-LS1-1
Fossils give evidence about organisms that lived long ago 3-LS4-1
Fossils in rock layers show the order in which life forms appeared MS-LS4-1
Show side-by-side photos of a giraffe and a mouse. Ask students to guess, on a sticky note, how many bones each animal has in its neck.
Warm-Up Prompt
"A giraffe's neck is about 2 meters long, and a mouse's neck is only a few millimeters long. Do you think they have the same number of neck bones? Why might two animals that look so different share a hidden pattern?"
Collect guesses, then reveal the answer in the first worked example. Tell students that hidden patterns like this one are the evidence they will use all lesson.
Direct Instruction15 minutes
Similarities, differences and relationships. Define anatomy, anatomical structure, common ancestor, homologous and analogous structures, and branching diagram. A vertebra (plural vertebrae) is one of the bones of the backbone. Work through the examples. For each, ask: "Which structures are shared? Are they built the same way, or do they only do the same job? What relationship does that suggest?"
Modern organisms: a hidden pattern in mammal necks (real)
A giraffe, a human and a mouse all have 7 neck vertebrae. In the giraffe, each neck vertebra is longer than 20 cm; in the mouse, each is only a few millimeters long.
Result: Almost all mammals have 7 neck vertebrae, whatever the length of the neck. The giraffe's long neck came from lengthening each bone, not from adding bones. Sharing the same count suggests that mammals inherited it from a common ancestor.
Modern organisms: animals with jointed legs (real)
An insect has 6 jointed legs, 3 body parts and 1 pair of antennae. A spider has 8 jointed legs, 2 body parts and no antennae. A crab has 10 jointed legs, including its claws, and 2 pairs of antennae. All three have a hard outer covering, called an exoskeleton.
Result: The differences (legs, body parts, antennae) separate them into different groups. The shared jointed legs and exoskeleton, found together in no other animals, suggest that all three descend from one common ancestor. Scientists group them as arthropods.
Fossil and modern: building the turtle shell (real, Diagram 1)
A modern turtle's top shell is made of widened ribs and backbone joined to bony plates, and it has a toothless beak. A fossil about 260 million years old has very broad, flat ribs but no shell. A fossil about 220 million years old has a full belly shell, widened ribs on its back but no solid top shell (only a row of small plates along the backbone), and teeth. A fossil about 210 million years old has a complete shell and no teeth on the edges of its jaws.
Result: The fossils share turtle-like ribs and shell parts, so they are related to modern turtles. Their differences, in order of age, show the shell built up step by step: broad ribs first, then a belly shell, then a full shell, with teeth lost along the way. From 260 to 220 million years ago is 40 million years, and 220 to 210 is 10 million years.
Fossil and modern: mammal jaws and ears (real)
A modern mammal has one bone in each half of its lower jaw and three tiny bones in each middle ear. A lizard has several bones in each half of its lower jaw and one middle-ear bone. Fossils called cynodonts, about 260 to 200 million years old, have one large tooth-bearing jaw bone plus a few small bones at the back of the jaw, and teeth of different shapes (front teeth, canines and cheek teeth), like a mammal's.
Result: The cynodonts share some structures with mammals (mostly one large jaw bone, teeth of different shapes) and some with reptile-like ancestors (extra small jaw bones). This mix suggests that mammals descend from animals like cynodonts. In mammals, the two small bones that formed the old jaw joint, one at the back of the lower jaw and one on the skull, became two of the three ear bones.
Stress two points. First, related organisms are cousins, not parent and child: a modern turtle did not come from a modern lizard, but both groups share older ancestors. Second, similar shapes alone can fool us: a shark and a dolphin have the same body outline, but the bones inside tell a different story (Activity 3).
Guided Practice15 minutes
Pairs compare two living elephants with two fossil relatives, using facts from teeth, tusks and ears. A molar is a large back tooth used for grinding. Frozen mammoths found in Arctic ground preserve skin and ears; mastodons are known only from bones and teeth.
Real traits, simplified: elephants and their fossil relatives
Trait
African elephant
Asian elephant
Woolly mammoth (fossil)
American mastodon (fossil)
Molar chewing surface
ridged plates
ridged plates
ridged plates
rows of cone-shaped bumps
Shape of molar ridges
diamond-shaped loops
narrow, parallel bands
narrow, parallel bands
no ridged plates
Ear size
very large
medium
small
not preserved
Tusks (long upper front teeth)
yes
yes, mostly in males
yes
yes
For each pair of animals, count the traits that match. Skip any trait that is not preserved.
Which pair shares the most traits? Which animal shares the fewest with the others?
Tusks are found in all four. Does that trait help you decide which two are closest? Why or why not?
Draw a branching diagram and mark where each shared trait fits. Compare it with Diagram 2.
Discuss: the Asian elephant and the woolly mammoth match on 3 traits (molar surface, ridge shape, tusks); every other pair matches on 2 or fewer, and the mastodon matches the others only on tusks. A trait shared by all four only shows that they belong to one group. DNA from mammoth remains (beyond this standard) gives the same answer: mammoths are closest to Asian elephants.
Independent Practice10-15 minutes
Each student writes an explanation using the evidence card below. Pterosaurs are extinct flying reptiles known from fossils about 228 to 66 million years old.
Evidence card: three kinds of wings (real, simplified)
Animal
Bones in the wing
What forms the flying surface
Bird (modern)
upper arm bone, two forearm bones, a few short, joined finger bones
feathers
Bat (modern)
upper arm bone, two forearm bones, four very long finger bones and a short, clawed thumb
skin stretched between the long fingers
Pterosaur (fossil)
upper arm bone, two forearm bones, three small clawed fingers and one very long fourth finger
skin stretched from the long fourth finger to the body
Prompt: "All three wings have one upper arm bone and two forearm bones. Explain what this similarity shows. Then explain what the differences in the fingers and flying surfaces show about whether flight evolved once or more than once." Students cite at least two rows of the card.
Closure5 minutes
Return to the warm-up guesses about neck bones. Exit ticket: "Name one similarity that is evidence of a close relationship and one similarity that is not. Explain the difference in one sentence."
Differentiation Strategies
For Struggling Students
Give color-coded skeleton diagrams in which the upper arm bone is always one color and the two forearm bones another
Provide a sentence frame: "___ and ___ both have ___. This suggests ___ because ___."
Limit trait tables to three traits and two organisms at first, then add one organism at a time
For Advanced Students
Ask students to explain why a trait shared by every organism in a table cannot help decide which two are closest
Have students find one more pair of analogous structures in plants or animals and defend their choice
Ask students to predict what a fossil between modern turtles and their 260-million-year-old relative might look like, and in what age of rock it should be found
Assessment Guidance
What to Look For
Check that students name specific structures (bones, teeth, shell parts) rather than general looks, and that they tell homologous structures (same plan) from analogous ones (same job, different build). Look for explanations that use both similarities (evidence of shared ancestry) and differences (evidence of change). Watch for the idea that one living species turned into another, and for claims based only on size, color or habitat.
02
Classroom Activities
3 Activities
1
Chicken Wing Dissection: A Bird Wing and a Human Arm
30 minPairs
Pairs take apart a cooked chicken wing to find the bones inside and compare them with a diagram of a human arm. The goal is to see a homologous structure with their own eyes: a wing and an arm that do different jobs but are built on the same plan.
Procedure
Put on gloves. Place one cooked, cooled chicken wing on a paper plate.
Pull away the skin and muscle with fingers and a plastic knife. Look for the tough white cords (tendons) that attach muscles to bones.
Find the bones of each of the three wing sections. Count the bones in the section nearest the body and in the middle section.
Lay the bones in order next to the human arm diagram and label them: upper arm bone, two forearm bones, wrist and hand bones.
Safety: use only cooked wings, never raw chicken; no eating in the lab; students with poultry allergies work from photos. Put all waste in the trash bag, wipe tables, and wash hands with soap.
What Students Should Find
One bone in the section nearest the body, two bones side by side in the middle section (one thicker, one thinner), and a tip with small wrist bones and a few short, joined finger bones. This matches the human arm: one upper arm bone, two forearm bones, then wrist and finger bones.
Discussion Questions
How many bones did you find in the first two sections, and how does that compare with your own arm?
What is the biggest difference between the chicken's hand and yours? What job does each do?
Why is a bird wing and a human arm a better example of homologous structures than a bird wing and a butterfly wing?
Modification for Distance Learning
Students use a labeled photo set of a dissected wing, or dissect a wing from a home meal with an adult, then post a labeled photo next to a sketch of their own arm.
2
Mystery Fossil Match
25 minGroups of 3-4
Groups receive three fossil cards and four cards of modern animals. They match each fossil to its closest modern relative using only anatomy, and they write the evidence on the back of each card. One modern card is a decoy that shares a look but not a body plan.
Fossil Cards (real, rounded)
Fossil 1: a skeleton about 52 million years old with wings made of very long finger bones that held up skin, and a claw on every one of its five fingers.
Fossil 2: a car-sized animal that died out about 10,000 years ago, with a rigid dome-shaped shell made of more than a thousand small bony plates set in its skin, and the teeth and jaw of a mammal.
Fossil 3: an animal as large as an elephant, extinct for about 10,000 years, with huge curved claws and simple peg-shaped teeth that have no hard white enamel.
Modern Animal Cards (real)
Armadillo: a mammal whose skin holds a shell of small bony plates.
Tree sloth: a mammal with long curved claws and peg-shaped teeth without enamel.
Bat: a mammal with wings of skin held up by very long finger bones; only the thumb, and in some kinds one other finger, has a claw.
Tortoise: a reptile whose shell is made of widened ribs and backbone, with a toothless beak.
Discussion Questions
Fossil 2 has a shell, and so does the tortoise. Why is the armadillo the better match? Name two structures.
How is Fossil 1 like a modern bat, and how is it different? What does the difference suggest about early bats?
Fossil 3 is far larger than a tree sloth. Why is size weak evidence when deciding relationships?
Modification for Distance Learning
Post the seven cards in a shared slide. Groups draw arrows from each fossil to its match and type the evidence in a text box.
3
Same Shape, Different Ancestors: Shark, Dolphin and Ichthyosaur
25 minGroups of 3
A shark, a dolphin and an ichthyosaur (an extinct sea reptile known from fossils about 250 to 90 million years old) share a streamlined body with fins or flippers. Groups compare cards that show what is inside, then decide which similarities are homologous and which are analogous.
Comparison Cards (real, simplified)
Shark (modern fish): skeleton of cartilage, not bone; breathes with gills; the tail fin stands upright, with the backbone running into its upper part, and sweeps from side to side.
Dolphin (modern mammal): bony skeleton; breathes air through a blowhole; each flipper holds an upper arm bone, two forearm bones and finger bones; the tail flukes lie flat and move up and down.
Ichthyosaur (fossil reptile): bony skeleton with a reptile skull; breathed air; each flipper holds an upper arm bone, two short forearm bones and rows of many small finger bones; the tail fin stood upright, with the backbone bending down into its lower part. Some fossils show mothers with unborn young inside.
Tasks
Make a table with rows for skeleton material, breathing, flipper or fin bones and tail, and a column for each animal.
Circle the similarities that are only about shape. Box the ones that are about the same bones.
Write a claim: which two are more closely related to land animals with four limbs, and what is the evidence?
Discussion Questions
Why would three unrelated animals that swim fast in the sea all end up with a similar outline?
Which structure is the strongest evidence that the ichthyosaur descends from four-limbed land animals?
Is the dolphin's tail homologous with the shark's tail fin? Use the cards to explain.
Challenge Variation
Ask groups to sort a fourth card, a penguin, into the table, and explain which of its similarities to the dolphin are homologous and which are analogous.
03
Diagrams & Visual Aids
2 diagrams
Diagram 1: Fossils Along the Path to the Turtle Shell (Real Ages, Rounded)
Ages drawn to scale. A relative about 260 million years old had broad ribs but no shell; one about 220 million years old had a full belly shell, widened ribs but no solid top shell, and teeth; one about 210 million years old had a complete shell. Modern turtles have a complete shell and a toothless beak. The fossils are relatives of modern turtles, not a chain of parents and offspring.
Diagram 2: Branching Diagram for Elephants and Their Fossil Relatives
Built from the guided practice traits. Each fork stands for a common ancestor. Tusks are shared by all four, molars with ridged plates by three, and narrow parallel molar ridges only by the Asian elephant and the woolly mammoth, which makes them the closest pair.
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Homework Assignment
~30 min
MS-LS4-2 Homework: Explaining Relationships from Anatomy
Directions: Anatomical and fossil facts are real and simplified. The trait table in Problem 5 is invented for practice. For every problem, name the structures you compare and say whether each similarity is homologous or analogous when that applies.
Part 1: Modern Organisms (Problems 1-2)
Hind legs: a human leg has a thigh bone, two lower-leg bones, ankle bones and toe bones. A kangaroo's hind leg has a thigh bone, two long lower-leg bones, ankle bones and long toe bones. A frog's hind leg has a thigh bone, one lower-leg bone formed from two bones joined together, long ankle bones and toe bones. (a) List the parts all three share. (b) Describe two differences and connect each to how the animal moves. (c) Explain what the shared parts suggest about these animals' ancestry.
Flower and leaf traits (real): apple, strawberry and wild rose flowers have 5 petals, and their leaves have net-like veins. Lily and tulip flowers have their parts in sets of 3 (6 petal-like parts), and their leaves have parallel veins. (a) Sort the five plants into two groups. (b) For each pair of plants, count how many of the two traits match, and give the count for apple and strawberry and for apple and tulip. (c) Explain what the groups suggest about relationships. (d) Garden roses can have 40 or more petals. Why does a scientist compare wild roses instead?
Part 2: Fossil and Modern Organisms (Problems 3-4)
A fossil seabird about 90 million years old has the wing bones, breastbone and leg bones of a modern bird, but its jaws hold rows of small, sharp teeth. Modern birds have toothless beaks. (a) What evidence shows this fossil is related to modern birds? (b) What difference stands out, and what does it suggest about the ancestors of birds? (c) Write one sentence that uses both a similarity and a difference.
Ants in amber about 92 million years old have the narrow waist segment and the gland on the thorax (middle body part) that only ants have, but their antennae and small jaws look like a wasp's. Modern ants have long, bent antennae and large jaws. (a) List the traits the fossil shares with modern ants and with wasps. (b) Is it more closely related to ants or to wasps? Explain. (c) What does the mix of traits suggest about how ants and wasps are related?
Part 3: Building an Explanation (Problems 5-6)
Invented data: students compare 10 skull features of fossil J with three living species. Fossil J matches species 1 on 8 features, species 2 on 5 and species 3 on 3. (a) Write each match as a percent. (b) Which living species is the fossil most likely most closely related to? (c) Species 1 and species 2 match each other on 6 of the 10 features. Sketch a branching diagram for fossil J and species 1 and 2. (d) Name one more kind of evidence that would test your diagram.
A seahorse swims upright, has a head shaped like a horse's and no tail fin. Yet it has gills, fins on its back and sides, a bony skeleton and a gas-filled swim bladder. (a) Is a seahorse a fish? Write a claim. (b) Give at least three structures as evidence. (c) Explain why the horse-like head is weak evidence of a relationship to horses.
Rubric
Criterion
Full Credit (2 pts)
Partial Credit (1 pt)
Naming Structures
Names specific bones, teeth or other parts and compares them correctly
Compares general looks, or one structure is wrong
Homologous or Analogous
Uses both terms correctly and explains the difference
Uses the terms with one mix-up
Using Counts
Correct counts and percents, with work shown
One calculation error
Explanation
Claim supported by similarities and differences, with reasoning about common ancestry
Claim with evidence but little reasoning
05
Quiz: 20 Questions
Interactive, with answers
Instructions
Anatomical and fossil facts are real and simplified; trait counts marked invented were written for practice. Your score updates as you answer, and Reset quiz clears everything so you or your students can try again.
Multiple choice: pick an option to check it. Short answer: write your answer, then reveal the model answer.
0 of 20 answered · 0 correct
Question 1 of 20 · Multiple Choice
Homologous structures are best described as which of these?
Answer: B
Homologous structures share the same basic parts in the same arrangement because they were inherited from a common ancestor, even if they now do different jobs. Choice A describes analogous structures, which do the same job but can be built differently. Choices C and D have nothing to do with shared ancestry.
Question 2 of 20 · Multiple Choice
Which pair of structures is analogous?
Answer: D
Both are fins used for swimming, but a squid fin is a muscle flap with no bones, and a trout fin is held up by thin bony rays: same job, different build. Choices A, B and C are homologous: each pair has the same set of limb bones inherited from a four-limbed ancestor, even the bat wing, whose bones are a modified hand.
Question 3 of 20 · Multiple Choice
Invented data: fossil Z shares 9 of 12 skull features with living species P and 4 of 12 with living species Q. What percent of the features does Z share with P?
Answer: C
9 ÷ 12 = 0.75, or 75%. Choice A is 4 ÷ 12, the match with Q. Choice B writes the count 9 as if it were a percent. Choice D divides 12 by 9, the wrong way around.
Question 4 of 20 · Multiple Choice
Which animal's anatomy shows that it is most closely related to insects?
Answer: A
Jointed legs together with an exoskeleton (hard outer covering) are the shared structures of arthropods, the group that includes insects, spiders and centipedes. Choice B has segments but no jointed legs. Choice C has a hard covering but no jointed legs, and choice D has neither.
Question 5 of 20 · Multiple Choice
A real fossil sabertooth skull, from rocks between about 2.5 million and 10,000 years old, has upper canine teeth more than 15 cm long. Which feature is the best evidence that it is related to modern cats such as lions?
Answer: B
A short face and cheek teeth that slice like scissors are structures shared with modern cats. Choice A is a difference, not a shared feature. Choice C is a way of life that many unrelated hunters share, and choice D uses size, which says little about ancestry.
Question 6 of 20 · Multiple Choice
Invented counts of skeletal differences between four species: W-X 3, W-Y 9, W-Z 10, X-Y 8, X-Z 11, Y-Z 4. Which pair most likely shares the most recent common ancestor?
Answer: D
W and X differ in only 3 features, the fewest in the table. Choice A, Y and Z with 4 differences, is the next closest pair. Choice B has the most differences (11), which points to the most distant relationship, and choice C has 9.
Question 7 of 20 · Multiple Choice
Why are bones, teeth and shells used most often when scientists compare fossil organisms with modern ones?
Answer: A
Hard parts are what usually survive as fossils, while skin, muscles and organs usually decay. Choice B is false: soft parts change too. Choices C and D are false statements about soft parts.
Question 8 of 20 · Multiple Choice
Koalas are often called "koala bears." Koala mothers carry their young in a pouch, as kangaroo mothers do, and koala skeletons share other features with kangaroos. Bears have no pouch. What does the anatomy suggest?
Answer: C
The pouch and the shared skeleton features place koalas with kangaroos. Choice B uses fur, which all mammals share, so it cannot pick out bears. Choice A follows the nickname, not the anatomy, and choice D ignores the shared structures.
Question 9 of 20 · Multiple Choice
Invented data: fossil F shares 14 of 20 traits with living species G and 8 of 20 with living species H. How many percentage points higher is F's match with G than with H?
Answer: A
14 ÷ 20 = 70% and 8 ÷ 20 = 40%, so the difference is 70 - 40 = 30 percentage points. Choice B subtracts the trait counts (14 - 8) without making percents. Choice C divides 14 by 8, and choice D adds the two percents.
Question 10 of 20 · Multiple Choice
Which observation would be the strongest evidence that two living animals are closely related?
Answer: D
Many bones in the same arrangement are homologous structures, inherited from a common ancestor. Choices A and C describe habitat and use, which unrelated animals can share. Choice B describes outward looks, which can be analogous.
Question 11 of 20 · Multiple Choice
Real fossils of a giant beaver, extinct for about 10,000 years, show an animal about 2 m long, much larger than a modern beaver. Its skull has the same arrangement of bones as a modern beaver's and large front teeth that kept growing, like a beaver's. What do these data suggest?
Answer: B
The shared skull plan and ever-growing front teeth are homologous structures, so the fossil is a relative of modern beavers; size is a difference that can change over time. Choice A treats size as stronger evidence than structure. Choice C goes far beyond the data, and choice D is about habitat, not anatomy.
Question 12 of 20 · Multiple Choice
A student claims that two species are closely related. Which finding would most weaken the claim?
Answer: C
If the only shared traits are ones that many unrelated groups also have, such as having a backbone, they do not single out these two species as close relatives. Choices A, B and D all add evidence for the claim.
Question 13 of 20 · Multiple Choice
Which observation is evidence that birds are related to reptiles?
Answer: A
Scales on bird legs and feet are a structure shared with reptiles. Choices B and C compare flying structures that evolved separately, which are analogous as wings. Choice D is about habitat, not anatomy.
Question 14 of 20 · Multiple Choice
Real fossils of horseshoe crabs about 150 million years old look very much like living horseshoe crabs, with the same body parts and shell shape. What is the best explanation?
Answer: D
Shared structures show the relationship, and the close match shows that this group's body plan changed little over a very long time; evolution does not require every group to change quickly. Choice A rejects the age of the rocks without evidence. Choice B ignores the shared structures, and choice C is not how fossils form.
Question 15 of 20 · Short Answer
A rock hyrax is a small furry animal about the size of a rabbit. It has upper front teeth that grow into short tusks, and flat nails on its toes. Elephants have tusks that are upper front teeth, and flat nails on their feet. Explain what these similarities could suggest, and why size is not a problem for the idea.
The tusk-like upper front teeth and flat toenails are shared structures built the same way, which suggests that hyraxes and elephants inherited them from a common ancestor. Size is a difference that can change a lot over many generations, so it is weak evidence against a relationship; the giraffe and the mouse share the same neck bones despite their sizes. Full credit names both structures and explains why structure matters more than size. Scientists also use other evidence, such as DNA, to test the idea.
Question 16 of 20 · Short Answer
Real fossils about 300 million years old show a giant insect with a wingspan of about 70 cm. It had two pairs of long wings with a network of veins, large eyes and a long thin body, like a modern dragonfly, which has a wingspan of about 10 cm. Explain what the similarities and differences suggest.
The similarities (two pairs of veined wings, large eyes, long thin body) are shared structures that suggest the giant insect was a relative of modern dragonflies. The main difference is size, about 70 ÷ 10 = 7 times the wingspan, which shows that the group has changed over time. Full credit uses at least two similarities, one difference, and says the fossil is a relative rather than the same kind of animal.
Question 17 of 20 · Short Answer
Invented counts of skull differences among four living species: K-L 2, K-M 7, K-N 8, L-M 7, L-N 8, M-N 5. Describe the branching diagram these data support and explain your reasoning.
K and L differ least (2), so they share the most recent common ancestor. M and N differ by 5, less than either differs from K or L (7 or 8), so they form a second pair. The diagram has two branches, one with K and L and one with M and N, joined at an older common ancestor. Full credit identifies both pairs and uses the smallest differences as evidence.
Question 18 of 20 · Short Answer
A flying squirrel of North America and a sugar glider of Australia both glide on a flap of skin stretched between their front and back legs. The sugar glider mother has a pouch, like a kangaroo; the flying squirrel has the chisel-like, ever-growing front teeth of a squirrel. Are the gliding flaps evidence that the two are close relatives? Explain.
No. The gliding flaps are analogous: they do the same job but evolved separately. The other structures point to different relatives: the pouch links the sugar glider to kangaroos and other pouched mammals, and the teeth link the flying squirrel to other squirrels and rodents. Full credit calls the flaps analogous and uses at least one other structure for each animal.
Question 19 of 20 · Short Answer
A fossil has no skin, muscles or DNA left. Explain how scientists can still compare it with modern organisms to infer relationships, and name one limit of this method.
They compare the hard parts that are preserved, such as the number, shape and arrangement of bones, teeth and shells, with the same structures in modern organisms. Shared structures built on the same plan suggest a common ancestor. A limit is that soft features (skin, organs, color, behavior) are usually missing, so some traits cannot be compared, and similar shapes can be analogous. Full credit names the structures used and one real limit.
Question 20 of 20 · Short Answer
A student says that manatees must be closely related to fish because they live in water and have flippers. Use anatomy to respond.
Manatees breathe air with lungs, have a few hairs on their bodies, and their flippers contain an upper arm bone, two forearm bones and finger bones, the same plan found in other mammals' front limbs. Their flat tail moves up and down, unlike a fish's tail fin. These homologous structures place manatees with mammals; the flipper shape is analogous to a fish fin and only shows a shared way of life. Full credit uses at least two structures and the idea of analogous similarity.
0 of 20 answered · 0 correct
06
Frequently Asked Questions
10 Questions
What does MS-LS4-2 mean?
MS-LS4-2 asks middle school students to explain anatomical similarities and differences among modern organisms, and between modern and fossil organisms, to infer evolutionary relationships. The code means Middle School, Life Science, core idea 4 (Biological Evolution: Unity and Diversity), performance expectation 2.
What grade is MS-LS4-2 taught in?
It is a middle school standard for grades 6-8, usually taught in a life science unit on evolution together with MS-LS4-1 (the fossil record) and MS-LS4-3 (embryos). The grade varies by school and state.
What is the difference between homologous and analogous structures?
Homologous structures are built on the same plan because they were inherited from a common ancestor, even if they do different jobs, such as a bird wing and a human arm. Analogous structures do a similar job but are built differently, such as a bird wing and a butterfly wing, so they are not evidence of a close relationship.
Does MS-LS4-2 have an Assessment Boundary?
No. MS-LS4-2 has a Clarification Statement but no Assessment Boundary. The Clarification Statement puts the emphasis on explaining relationships from similarities and differences in the gross appearance of anatomical structures.
What does "gross appearance" of anatomical structures mean?
It means the overall visible form of a structure: how many bones there are, their shape and how they are arranged, what a shell or tooth looks like. It does not mean cells, chemicals or DNA, which students study later.
If two animals share a structure, did one evolve from the other?
No. Two living species that share structures are more like cousins: both descend from an older common ancestor. A modern turtle did not come from a modern lizard, for example, but both groups share ancestors that lived long ago.
How can students compare fossils when only bones are left?
Students compare the hard parts that fossils preserve, such as bones, teeth and shells, with the same parts in living organisms. Rare fossils, such as frozen mammoths or insects in amber, also preserve skin, hair or whole bodies.
Do students need DNA evidence for MS-LS4-2?
No. MS-LS4-2 is about anatomy. DNA comparisons are beyond this standard and appear in high school in HS-LS4-1, where students see that DNA usually gives the same relationships as anatomy, as it does for mammoths and Asian elephants.
How should teachers handle student questions about beliefs when teaching MS-LS4-2?
Respectfully. Evolution is the well-supported scientific explanation for the patterns in anatomy, fossils and embryos, and the standard asks students to understand and explain that evidence. Teachers can welcome questions, keep the focus on what the evidence shows, and respect that families hold a range of views.
What comes after MS-LS4-2?
MS-LS4-3 adds evidence from embryos, which can show relationships that adult anatomy hides. In high school, HS-LS4-1 asks students to communicate how anatomy, fossils, embryos and DNA together support common ancestry and biological evolution.
07
Related Standards
5 standards
These standards connect to MS-LS4-2: prerequisites to review first, parallel standards at the same level, and next steps that build on it.
Before this lesson
3-LS4-1Prerequisite
Analyze fossil data for evidence of organisms and environments of long ago
Lesson coming soon
4-LS1-1Prerequisite
Argue that internal and external structures support survival, growth and reproduction
Lesson coming soon
Alongside
MS-LS4-1Parallel
Analyze fossil record patterns of existence, diversity, extinction and change
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