by John Pojeta, Jr. and Dale A. Springer 
Geologic time chart
Fossil Record
Change Through Time
Darwin's Theory
Mechanism for Change
Nature of Species
Nature of Theory
Paleontology, Geology & Evolution
Dating the Fossil Record
Examples of Evolution


References Cited

Suggested Readings
About the Authors

Paleontological Society


Evolution and the Fossil Record Mechanisms for Change (Previous Page || Next Page)

Biological evolution is not debated in the scientific community — organisms become new species through modification over time. “No biologist today would think of submitting a paper entitled ‘New evidence for evolution;’ it simply has not been an issue for a century” (Futuyma, 1986). Precisely how and at what rates descent with modification occurs are areas of intense research. For example, much work is under way testing the significance of natural selection as the main driving force of evolution. Non-Darwinian explanations such as genetic drift have been explored as additional mechanisms that explain some evolutionary changes. Darwin proposed that change occurs slowly over long periods of geologic time. In contrast, a more recent hypothesis called punctuated equilibrium proposes that much change occurs rapidly in small isolated populations over relatively short periods of geologic time. 

In Darwin’s time, the nature of inheritance and the cause of variation were very poorly understood. The scientific understanding of heredity began with the work of Gregor Mendel in the 1860s in Brno, Czech Republic. This understanding accelerated throughout the 20th century and now includes knowledge of chromosomes, genes, and DNA with its double helix.

Evolution could not occur without genetic variation. The ultimate source of variation can now be understood as changes or mutations in the sequence of the building blocks of the genetic material carried on the chromosomes in eggs and sperm. Many of these changes occur spontaneously during the process of creating copies of the genetic code for each egg or sperm. For example, the wrong molecule may become attached to the newly formed strand of DNA, or the strand may break and a portion can be turned around. Certain forms of radiation and chemical toxins can also cause mutations in the DNA.

Because the sequence of building blocks in DNA is the genetic foundation for the development of an individual’s features or characteristics, changes in the sequence can lead to a change in the appearance or functioning of an individual with that mutation. Although some changes may prove to be harmful or fatal, other changes produce variations that convey a survival advantage to the organism. It is these variations, when passed on, that give advantages to the next generation. 

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Copyright ©2001 All rights reserved. American Geological Institute produced in cooperation with The Paleontological Society.