Homology
The humerus, radius, and ulna appear in a human arm, a whale flipper, and a bat wing. What do we call structures like these that share the same underlying bone arrangement?
Homologous structures
A dolphin and a shark both have streamlined bodies and dorsal fins. These structures evolved independently to solve the same problem. What do we call structures like these?
Analogous structures
Humans have a coccyx at the base of the spine that serves no structural function in locomotion. What kind of structure is this?
A vestigial structure
The eyes of an octopus and a human are structurally nearly identical despite these animals being very distantly related. What is the general term for similarity that arises independently rather than through shared ancestry?
Homoplasy
Name the three types of structural evidence for evolution examined at the stations in this unit.
Homologous structures, vestigial structures, and analogous structures
A fruit bat and a human both have a radius and ulna in their forelimbs. What does this shared bone structure suggest about their evolutionary past?
They likely share a common ancestor — the bones were inherited from the same ancestral lineage
At the analogous structures station, you tried to color-code a pterosaur wing, a bat wing, and a cicada wing using the same bone colors from the homology station. What happened — and what did that reveal?
The color-coding broke down for the cicada because its wing has no bones — only veins and membrane. This revealed that similar function does not mean similar internal structure or shared ancestry.
The Iberian mole has eyes permanently sealed beneath an unbroken layer of skin. What is unusual about the way this happened — and how is it different from simply closing your eyes?
While the eye is somewhat developed, the mole never developed eyelids. During embryonic development skin grew continuously over where the eyelid opening would form and never reopened. The eye is not closed — it simply never had an opening.
A harmless scarlet kingsnake has color bands nearly identical to the venomous coral snake. Is this an example of analogy, homoplasy, or both — and why?
This is homoplasy but not analogy. The similar color pattern did not evolve to perform the same function — it evolved through mimicry, a distinct mechanism where resemblance to another species provides a survival advantage.
The Archaeopteryx fossil has teeth, clawed wing digits, and a long bony tail — but also feathers and a wishbone. Why is it significant as evidence for evolution?
It is a transitional fossil showing a mixture of features from two groups — non-avian theropod dinosaurs and modern birds — providing physical evidence that birds share ancestry with dinosaurs
At the homology station you color-coded bones across four specimens. What was the point of using the same color for the same bone across different animals?
To show that the same bones are present across very different animals — revealing shared ancestry through structural organization.
A fellow student argues that because bat wings and bird wings both enable flight, bats and birds must be closely related. How would you respond using evidence?
Flight evolved independently in each animal group. The wings are analogous at the level of function but the internal structures differ — bird wings have fused digits and a carpometacarpus while bat wings have elongated fingers supporting a membrane. Similar function does not imply close relationship.
On the Function Meter, the mole eye is genuinely difficult to place. Why — and what does that ambiguity tell you about vestigiality?
The mole eye retains light-sensitivity for day/night rhythm regulation even though it cannot form images. Vestigiality is not always all-or-nothing — structures can be reduced and partially functional, which is what makes the concept interesting and the Function Meter a useful tool.
How is homoplasy different from analogy? Use a specific example for each to make the distinction clear.
Analogy involves similar structures performing the same function — dolphin and shark fins both help the animal to swim. Homoplasy is refers to traits that look the same and may or may not share a function — an example would be the webbing of a penguins flipper and the membrane of a bat's wing.
A fellow student says that because Archaeopteryx is older than Deinonychus, it proves Deinonychus evolved from Archaeopteryx. What is wrong with this argument?
Older age does not mean direct ancestry. Archaeopteryx and Deinonychus are separate branches sharing a common ancestor further back. This reversal actually demonstrates that evolution produces branching trees, not linear ladders.
A student says: "A whale flipper and a bird wing are homologous because they both help the animal move through a fluid." What is wrong with this reasoning?
Homology is determined by shared bone structure and common ancestry, not shared function. Function alone cannot establish homology.
Why is function alone an unreliable guide to evolutionary relationship? Give one example from the stations to support your answer.
Function reflects similar environmental pressures, not ancestry. The cicada and bat both have wings but share no recent common ancestor — they arrived at the same solution independently. Using function alone would lead you to group them together incorrectly.
If a vestigial structure is underused or useless, why hasn't natural selection eliminated it completely? Give two distinct reasons.
First, if maintaining the structure costs little energy it imposes no significant loss of survival — there is no pressure driving its removal. Second, losing a structure as a process is gradual and may not have had enough time to eliminate it entirely.
Why does homoplasy create problems for scientists trying to reconstruct evolutionary history using physical characteristics alone?
Homoplasy produces similarities that look like shared ancestry but are not. If a scientist groups animals based on a similar trait that actually evolved independently, the resulting tree will be wrong.
Why is the bat wing fossil record so incomplete — and what does that incompleteness tell us about the nature of scientific evidence?
Three independent factors converge: the evolution was rapid (fewer transitional generations), forest habitats are chemically hostile to preservation, and bat skeletons are small and fragile.
Highly derived bird forelimbs have lost several digits and fused multiple wrist bones into the carpometacarpus. Why does this make homology harder to identify — and what kind of evidence would you use to establish it anyway?
Fusion and loss complicate the one-to-one bone correspondence. Developmental and fossil evidence — tracing how the bones form embryologically and finding transitional forms — can reveal homology even when adult anatomy is highly modified.
Bird wings and bat wings share homologous bones but are considered analogous as flight structures. Explain how a single pair of structures can be both homologous and analogous simultaneously.
Homology and analogy operate at different levels. The underlying bones — humerus, radius, ulna, digits — are homologous because they derive from the same ancestral tetrapod forelimb. The wing as a flight structure is analogous because powered flight evolved independently in each group from a non-flying common ancestor.
Whales have small pelvis bones embedded in muscle with no connection to the rest of the skeleton. A student argues this proves whales evolved from land mammals. What would a scientist say about the strength of that argument?
The vestigial pelvis is consistent with whale evolution from land mammals and is good supporting evidence. However, a single line of evidence rarely proves an evolutionary claim on its own. The argument is strengthened by combining it with other evidence — fossils, homologous limb bones in early whale fossils, and molecular comparisons placing whales with hippos.
Limblessness in Snakes and Earthworms is a tricky comparison to make. Both are limbless, not due to evolution of a new trait, but rather through separate losses of limbs. How would you classify this — and what would you need to know to determine whether it is homoplasy, analogy, or both?
This is looks like homoplasy - the trait arose not because of shared ancestry or similar environmental pressures resulting in analogy. Instead it is the result of chance that they look similar in body shape.
Using homology, vestigiality, and homoplasy together, construct the strongest possible argument that living things share common ancestry. What does each line of evidence contribute that the others cannot?
Homology shows that distantly related animals share the same structural elements — the same bones arranged the same way — which is best explained by inheritance from a common ancestor. Vestigiality shows that structures persist even after losing function, implying animals carry a history of what their ancestors needed. Analogy strengthens the case: when similar structures arise independently in unrelated lineages, it demonstrates that living things may adapt to similar environments similarly to each other.