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

Archaeopteryx—the first bird in the fossil record

Figure 24.2 lower

Gradualism

Niles Eldredge and Stephen J. Gould

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

Lake Victoria cichlids

Desert pupfish

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

Is there evidence for rapid “quantum” speciation?

The Salton Sea, an inland saline sea, was formed in southern California in 1905-1907

In 1934, a new copepod crustacean, Cyclops dimorphus , was described from the Salton Sea