Transitional Fossils: Archaeopteryx, co-opted structures, and exaptations
What Use is Half a Wing?
Big Idea: Evolution of complex structures
Agenda: 5/10 + 5/12
If you had to guess, what do you think the word "Exaptation" might mean? Hint: try breaking the word up into its parts. What does "Ex" make you think of? What does "aptation" sound a little like?
In In 1982, Stephen Jay Gould and Elisabeth Vrba proposed the idea of
Exaptations: Co-opted Evolutionary Structures
What do you think might be an example of an "exaptation" in Penguins? (Identify at least one structure. Describe how it is currently used, and how that is different from its adaptive use by penguin ancestors.)
Penguins have many special features that help them survive in the harsh conditions of Antarctica, like their waterproof feathers and thick layer of fat. However, these are common to many other species of seabird and most likely do not represent a change in function of those structures so much as an amplification from their ancestral form. In other features, however, the ancestors of penguins must have developed new structures over time that helped them do things they couldn't do before, like using their wings to swim underwater instead of fly. New adaptive structures, like the swimming flippers of penguins, built from old adaptive structures, like the flying wings of their seabird ancestors, are called "exaptations" and they show how amazing natural selection can be!
Exaptations in Penguins: You can't be good at everything
Darwin's theory of evolution of complex structures through small, incremental changes was challenged shortly after publication by Sir George Jackson Mivart
The Evolution of Complex Structures
What are some ways that evolutionary biologists might investigate that question? (What use is half a wing?)
One place that evolutionary biologists often start from is observations about adaptative traits in extant (living - not extinct) species. How might the highlighted structures in the modern pigeon be adaptive for flight?
Which features does Ichthyornis have in common with pigeons?
Which features does Archaeopteryx have in common with pigeons?
14. Personalised Feedback
Feathers are clearly present on Archaeopteryx. What does this suggest about the presence or absence of feathers on Ichthyornis, and how do you know?
Big Idea: Evolution of complex structures through incremental change
16. Personalised Feedback
Transitional fossil in the evolution of flight: Archaeopteryx What evidence suggests that Archaeopteryx is a relative of modern birds that evolved from theropod dinosaurs, rather than Pterosaur reptiles?
According to the information from the image below, at which point on the cladogram would it make the most sense to place the evolution of feathers?
Which features does Velociraptor have in common with pigeons?
Which features does T. rex have in common with pigeons?
At which point on the cladogram would it make the most sense to place the evolution of powered flight?
According to the comparison of fossils made here, we're able to see the stepwise (incremental) accumulation of traits leading to more complex structural and behavioral adaptations that Darwin predicted.
Transitional Fossils and Complex Traits
1 - 2 word answers: What are some ways that feathers might have been adaptive for Velociraptor and Archaeopteryx that don't involve powered flight? (Hint: what other ways do we see modern birds use their feathers?)
24. Personalised Feedback
How would you answer Mivart, if asked: "What use is half a wing?" What evidence do we have to support Darwin's claim that complex structures, like wings, could have evolved by natural selection through co-opted intermediary structures?