Homologies in Vertebrate Skeletons
Biologists who study evolution classify animals according to the form and function of different body structures.
The term homology is used when different animals each have a body structure with the same genetic basis,
inherited from a common ancestor. For example, a human arm and a bird’s wing are homologous structures,
since both were inherited from a common vertebrate ancestor. When animals have body structures that are
similar in function but not in ancestry, the term analogy is used. For example, the wings of insects and the
wings of birds are analogous structures, since they have the same function but not the same ancestry. Body
structures can be both homologous and analogous if they share a common heredity and a common function.
The wings of bats and the wings of birds are both homologous and analogous structures, since they were both
derived from the foreleg of an ancestral vertebrate and are both used for flying. In this investigation you will
use the concepts of homology and analogy to compare different vertebrate skeletons.
PRELAB
1. Read the instructions for the investigation and answer Prelab questions 1 through 3 on the Lab Report.
2. Refer to Figure 31-A and look at the pectoral, pelvic, and anal fins on the fish skeleton. The pectoral and
pelvic fins are paired, while the anal fin is a single fin on the ventral midline of the fish’s body.
3. Examine the tail vertebrae, or bones, of the fish. Answer Prelab question 4 on the Lab Report.
anterior dorsal fin
posterior dorsal fin
nasal
tail vertebrae
caudal fin
operculum
pectoral fin
Figure 31A-1 Fish Skeleton
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rib
pelvic fin
anal fin
4. Refer to Figure 31 A-2 and look at the bones of a frog’s forelimb and the bones of the frog’s hind limb.
Answer Prelab questions 5 and 6 on the Lab Report.
cervical vertebra
suprascapula
phalanges
metacarpals
carpals
radio-ulna
vertebra
humerus
sacral vertebra
ilium
urostyle
femur
pubis
tibiofibula
ischium
phalanges
metatarsals
tarsals
Figure 31A-2 Dorsal view of a frog skeleton
5. Refer to Figure 31A-3 and look at the bones of a bird’s forelimb and compare them to the bones of a bird’s
hind limb. Answer Prelab question 7 on the Lab Report.
6. Observe the caudal vertebrae and the pygostyle in Figure 31A-3. Answer Prelab question 8 on the Lab
Report.
7. Now refer to Figure 31A-4 and look at the bones of the human forelimb and compare them to the bones of
the human hind limb. Answer Prelab question 9 on the Lab Report.
8. Follow the spinal column down to where it ends in Figure 31A-4. Answer Prelab question 10 on the Lab
Report.
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carpometacarpus
phalanges
skull
mandible
radius
ulna
cervical vertebra
scapula
humerus
thoracic vertebra
lumbar vertebra
coracoid
clavicle
ilium
ribs
ischium
sternum
patella
femur
fibula
pubis
caudal vertebra
pygostyle
tibiotarsus
tarsometatarsus
phalanges
Figure 31A-3 Bird Skeleton
INVESTIGATION PROCEDURE
1. Examine the models of the fish, frog, bird, and human skeletons on exhibit in the lab. Notice the similarities
and differences among the limb and bones. Since all these animals are thought to derive from a common
ancestor all forelimbs, hind limbs, and tails (if present) are homologous.
2. Look closely at a fish skeleton. Use Figure 31A-1 as a reference. Observe the fish vertebral column and
ribs. Note the absence of bony limbs. Answer Investigation question 1 on the Lab Report.
3. Observe the frog skeleton while using Figure 31A-2 as a reference. Notice the position of the forelimbs on
the frog’s body. Answer Investigation question 2 on the Lab Report.
4. Observe the hind limbs of the frog. Answer Investigation questions 3 and 4 on the Lab Report.
5. Look at a bird skeleton and Figure 31A-3. Observe the forelimb (wing). Notice the reduction in the number
of phalanges, or finger bones, in the bird’s forelimb as compared to the frog’s forelimb. Answer
Investigation questions 5 and 6 on the Lab Report.
6. Observe the hind limb and tail bones of the bird skeleton. Answer Investigation question 7 on the Lab
Report.
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7. Look at a human skeleton and Figure 31A-4. Observe the position of the forelimb and note the attachment
of the forelimb to the shoulder. Note the structure and placement of the human thumb. Answer
Investigation questions 8 through 10 on the Lab Report.
8. Observe the human hind limb’s attachment at the hip. Compare the human hind limb to that of a frog and a
bird. Answer Investigation questions 11 through 13 on the Lab Report.
skull
mandible
cervical vertebra
clavicle
rib
scapula
sternum
thoracic vertebra
humerus
radius
lumbar vertebra
ilium
sacrum
coccyx
pubis
ulna
carpals
metacarpals
phalanges
ischium
femur
patella
tibia
fibula
tarsals
metatarsals
phalanges
Figure 31A-4 Human Skeleton
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Homologies in
Vertebrate Skeletons
Name
Class
Date
PRELAB QUESTIONS
1. What is homology?
2. What is analogy?
3. Are the wings of birds and the wings of bats homologous or analogous structures?
4. Of what larger structure do the tail bones seem to be a part?
5. Name the bones that make up a frog’s forelimb.
6. Does a frog have a tail?
7. Name the bones that make up the hind limb of a bird.
8. What do the caudal vertebrae and pygostyle combine to form in the bird?
9. The forelimb of the frog has a radio-ulna bone which is actually two bones fused together. How do the
bones in the human forelimb differ from those in the frog?
10. What is the name of the last bone in the human spinal column?
INVESTIGATION
OBSERVATIONS AND DATA
1. Are the paired fins of the fish skeleton homologous to the forelimbs and hind limbs of the frog, bird, and
human? Explain.
2. Are the forelimbs attached to the dorsal, ventral, or lateral surface(s) of the frog’s body?
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3. Are the frog’s hind limbs larger or smaller than its forelimbs?
4. How are the size and shape of the frog’s hind limbs and feet an advantage to the animal?
5. Are the forelimbs attached to the bird’s dorsal, ventral, or lateral surfaces. Compare this attachment to the
frog’s forelimb.
6. Why is it useful for a frog to have many phalanges or fingers? Why is it less important for a bird to have
many phalanges?
7. Compare the hind limbs of the frog skeleton and of the bird skeleton.
8. Using degrees, describe how far a human arm can rotate?
9. How is a human thumb different from the homologous phalanges in the frog and the bird?
10. What makes the human hand better adapted for picking up objects than the frog’s hand?
11. Compare the attachment of the frog and human hind limbs.
12. How are the bones of the human foot arranged differently than the bird’s foot bones?
13. Suggest a reason for the differences seen in the foot bones of birds and humans.
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