21.3 Homology and Analogy
Key Takeaways
Homology is similarity inherited from a common ancestor, so the forelimb bones of a human, a bat, and a whale are homologous despite different uses.
Analogy is functional similarity that is not inherited from a recent common ancestor, and an insect wing and a bird wing are analogous.
Analogous traits often result from convergent evolution, and a shared job by itself does not prove homology.
Using homologous limb bones for different jobs does not cancel that homology.
Vestigial structures are reduced remnants of ancestral structures, such as hind-limb bones in some whales.
21.3 Homology and Analogy
Homology is similarity inherited from a common ancestor. Structures match because the ancestor already had them and descendants modified that inheritance. Analogy is a shared function that was not inherited from a recent common ancestor. The parts do a similar job but were built separately. Analogous traits often come from convergent evolution. This section separates those similarities and names the reduced remnant, a vestigial structure.
Homologous forelimbs
The forelimb bones of a human, a bat, and a whale are homologous. Each has a humerus, a radius and an ulna, and then carpals, metacarpals, and phalanges. A human hand grasps. A bat hand is long and supports a flight membrane. A whale flipper is short, stiff, and used for swimming. Different uses do not cancel the homology. The common ancestor was a tetrapod that already had this skeleton. A whale flipper is a mammal arm inside a swimming outline, not a fish fin built from scratch. You can test the claim with the bone list: humerus, forearm pair, and digits in both animals means the tetrapod forelimb.
A horse foreleg and a bird wing skeleton belong in the same comparison. A bird wing is a feathered forelimb. Some bones are fused or shortened, but the limb is still the ancestral tetrapod arm. As skeletons, a human arm and a bird wing are homologous. As flight organs, a bird wing and an insect wing are analogous, because no winged ancestor gave both of them wings.
Changing a shared structure for different jobs is divergent evolution. Divergence starts from homology. Convergent evolution produces analogy when unrelated lineages solve a similar problem, which is what insect and bird wings show. Different jobs are not a reason to call homologous bones analogous.
Analogous wings and eyes
An insect wing and a bird wing are the standard analogous pair. Both produce lift. An insect wing is a sheet of cuticle from the exoskeleton, with no humerus, radius, or ulna. A bird wing is bone, muscle, and feathers. The common ancestor of insects and birds did not fly with either kind of wing. A shared job does not prove homology.
The camera eye of a squid and the camera eye of a vertebrate are a second analogous pair. Both form an image with a lens and a retina. They did not come from a recent common ancestor that already had this camera design. A vertebrate retina sends nerve fibers in front of the photoreceptors and has a blind spot where those fibers exit. A squid retina does not block the light path that way and has no such blind spot. That structural difference is evidence of separate history, which is stronger than the shared ability to see.
Analogy is about where the similarity came from, not about whether the function is real. Both wings support flight, and both eyes form images. A shared verb is not the test. Two structures can both fly, or both see, and still be analogous. Two structures can do different jobs and still be homologous. Ask whether the common ancestor already had the structure.
Vestigial structures
A vestigial structure is a reduced remnant of a part that was larger, and more fully used for its original job, in an ancestor. Some whales have small hind-limb bones, including pelvic remnants, buried in the body wall. Whales do not walk on them. The bones are remnants of terrestrial hind limbs, homologous to the hind limbs of land mammals and vestigial as walking legs. A remnant can still anchor a muscle. Vestigial means reduced relative to the ancestral job. Do not require the bone to be functionless, and do not treat it as a new invention.
Some snakes keep tiny pelvic remnants of a limbed ancestor. Those bones are reduced homologous limbs, not a new organ and not an analogy to a lizard's full leg.
| Pair | Kind of similarity | Why |
|---|---|---|
| Human arm and bat wing, compared as skeletons | Homologous | Same forelimb bones, inherited from a common tetrapod ancestor, used for grasping versus flight |
| Whale flipper and horse foreleg, compared as skeletons | Homologous | Same limb bones from a common ancestor, used for swimming versus running |
| Insect wing and bird wing, compared as flight surfaces | Analogous | Both make lift, but the structure differs and their common ancestor had no wings |
| Squid eye and vertebrate camera eye | Analogous | Both form images with a lens, but the eyes evolved separately and the retinas are arranged differently |
Warning
Similar function does not by itself prove homology. An insect wing and a bird wing share a job and are analogous. Divergent use does not cancel homology of the limb bones either. A human arm, a bat wing, and a whale flipper are used differently, and those bones are still homologous.
A short decision rule
- If the common ancestor already had the structure, the similarity is homology, even when one descendant flies and another swims.
- If the common ancestor lacked the structure, a matching job is analogy.
- Hind-limb bones in some whales are vestigial remnants of homologous hind limbs.
- A bird wing is homologous to a human arm as a forelimb skeleton, and analogous to an insect wing as a flight surface.
Homology records ancestry. Analogy records a repeated solution. Vestigial structures record ancestry after the old job has been reduced.
The bones of a human arm, a bat wing, and a whale flipper include a humerus, a radius, an ulna, and digits. The limbs are used for grasping, flying, and swimming. Which reading is correct?
The bones are analogous, because a shared function is what the word homology means.
The bones are homologous. They were inherited from a common tetrapod ancestor, and different uses do not cancel that homology.
The bones are analogous, because limbs that do different jobs cannot have been inherited from a common ancestor.
The bones are vestigial in the human, the bat, and the whale, because each limb is only a remnant of a fish fin.
Which pair is analogous rather than homologous?
A whale flipper and a horse foreleg, compared as the same series of limb bones.
Hind-limb bones buried in some whales and the walking legs of a land mammal, compared as remnants of the same limb.
An insect wing and a bird wing, compared as flight surfaces.
A human arm and a bat wing, compared as forelimb skeletons.
Which statement correctly separates these terms?
A vestigial structure is a brand-new organ that has no counterpart in any ancestor.
If two structures are used differently, they must be analogous.
If two structures do the same job, they must be homologous.
Similar function by itself does not prove homology, and using a limb for a different job does not cancel homology of the underlying bones.
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