The Fossil Record
Fossils form best when there is slow decomposition andrapid burial of plant or animal parts.
The extent of fossilization depends on the habitat and the structure of the organisms.
Pollen Leaves
1. A tree lives in aswampy habitat.The tree dropsleaves, pollen, andseeds into the mud,where decompositionis slow.
2. The tree falls.The trunk andbranches breakup as they rot.
Figure 24.1a upper
3. Flooding brings in sand and mud, burying the remains of the tree.
4. Over many years, the mountains erode and the swamp is filled with sediment. The habitat dries.
Sand and gravel
Buried material from swamp
Bedrock
Figure 24.1a lower
IntactThe pollen was preserved intact because nodecomposition occurred.
CompressionSediments accumulated on top of the leaf and compressed it into a thin carbon-rich film.
CastThe branch decomposed after it was buried. Thisleft a hole that filled with dissolved minerals,faithfully creating a cast of the original.
PermineralizedThe wood decayed very slowly, allowing dissolved minerals to gradually infiltrate the cells and then harden into stone.
Figure 24.1b
Four types of Fossils
Punctuated equilibrium
Presumes that most evolutionary change occurs during speciation events
Most species stay pretty much the same once they come into being. This lack of change is called stasis.
Speciation events normally occur in isolated, peripheral populations
We would not expect to see these small, transient populations in the fossil record
This view of macroevolution nicely explains the lack of “missing links”
Is there evidence for rapid “quantum” speciation?
Lake Victoria, one of the African rift lakes, was formed between 500,000 and 700,000 mybp
170 species of cichlid fishes occur there today
All but three are endemic
Is there evidence for rapid “quantum” speciation?
Lake Nabugabo is only 4000 years old
Five species differ from all species of Lake Victoria species
Clear example of speciation in less than 4000 years
Is there evidence for rapid “quantum” speciation?
Hedylepta is a genus of moths found only on the Hawaiian Islands
Polynesians introduced the banana plant to Hawaii about 1,000 years ago
Five species of Hedylepta now form a clade of banana-eating moths
These species are most closely related to palm-eating Hedylepta species