Passage VII

Rock layers containing fossils (remains of past life) can give clues to the past. Those fossils in older (lower) layers can be compared to those in younger (higher) layers. This comparison often shows that life forms have changed over a period of time. Scientists who study fossils hypothesize that those changes occurred as a result of evolution. Evolution theory suggests that more recent forms have their origin in earlier forms and that the differences between them are due to changes in successive generations, resulting been in new species. Two mechanisms have proposed to explain how these changes occur over time.

Gradualism

This theory says the changes occur slowly, steadily, and gradually, in response to environmental changes. The entire species is transformed over millions of years into a new species by accumulating small changes over the generations. This happens over the entire geographic range of the species. The fossil record should show long sequences of intermediate forms leading from earlier to later forms. If no intermediate forms are found, gradualists claim it is due to the incompleteness of the fossil record. Either no rocks were deposited during that period, those that were have been eroded away, or conditions were such that no fossils were formed.

Episodic Evolution

Some scientists suggest that the changes in life forms are episodic (changing at irregular intervals). New species appear when small subpopulations of the earlier form become isolated in a small area, usually on the margin of the ancestor's geographic range (the region throughout which an organism naturally occurs). Different environmental conditions there cause rapid changes in this isolated population, and, in a few thousand years, a new species may form. In the small area, where the isolated population existed, the record should show sudden of replacement ancestor by descendant, without intermediate forms. Once formed, a new species should not show much change. Most evolution is concentrated in episodes that are brief compared to the time period in which the species existed.