Just what is the theory of evolution?

The theory of evolution purports to account for a large number of observations about (1) life as we now observe it and about (2) certain facts that seem to imply certain things about past life on this world - facts referred to collectively as "the fossil record." But "theory of evolution" is slightly a misnomer. To be strictly correct, it is the "theory of natural selection" or, alternatively, the "theory of descent with modification."

Evolution is change over time. It is observed that organisms do change over time. Evolution is a fact to that extent. The theory of natural selection uses that fact and several others to explain (a) why we observe the changes that we do observe, (b) how the world came to be populated with all the species that are now observed to inhabit the world and (c) why we find what we do find in the fossil record.

I will set forth what the theory asserts about those points. Then I will present some of the facts that scientists think can best be accounted for by supposing the theory to be true.

The theory of natural selection asserts:

• Life on Earth originally was simple, presumably unicellular, and lacking in diversity, comprising possibly only a single species as species are currently defined.
• Descendant organisms exhibit some variation from parent organisms, and those descendants that are better suited to their environment are more likely to survive and reproduce than those less suited. Favorable variations that are heritable will tend to be preserved and transmitted to succeeding generations.
• By this process, favorable heritable variations will accumulate over time, resulting in increased diversity within the population where they occur.
• Sufficient variation between two subgroups within a population will cause reproductive isolation, i.e., the two subgroups will be unable to interbreed with each other. When this occurs, two species now exist where there was one before.
• Given sufficient time, this process will lead to multiplication of species, which usually (though not invariably) will tend toward greater diversity.
• The fitness of an organism is relative to its environment. Different environments will offer selective pressure for different characteristics. The geographic distribution of species will thus depend, at any given time, on regional environmental variations, and the characteristics of species within a given locale will depend on environmental variations over time.
• Changes in species occur without teleological consideration. Given sufficient time, beginning with the simplest possible organisms, random variations acted on by selective environmental pressure will suffice to account for observations of the fossil record and the present diversity of species.

Here is a more succinct statement, from the Talk.Origins Web site:

The theory specifically postulates that all of the earth's known biota are genealogically related, much in the same way that siblings or cousins are related to one another. Thus, macroevolutionary processes necessarily entail the transformation of one species into another and, consequently, the origin of higher taxa.

http://www.talkorigins.org/faqs/comdesc/

So, why do scientists think this is what happened? The following essays present only a sample of what they think is the evidence. It is based on The Scientific Case for Common Descent by Douglas Theobald at Talk.Origins, from which the above quotation was taken.

For those interested, some of Theobald's own sources are:

• Dawkins, R. (1996) The Blind Watchmaker. New York, Norton.
• Feynman, R. P. (1985) QED: The Strange Theory of Light and Matter. Princeton, NJ: Princeton University Press.
• Freeman, S. and Herron, J. C. (2004) Evolutionary analysis. Third edition. Upper Saddle River, NJ: Pearson/Prentice Hall.
• Futuyma, D. (1998) Evolutionary Biology. Third edition. Sunderland, MA: Sinauer Associates.
• Geological Society of America (2001) "Evolution." http://www.geosociety.org/aboutus/position.htm
• Gould, S. J. (2002) The Structure of Evolutionary Theory. Cambridge, MA: Belknap Press of Harvard University Press.
• Mayr, E. (1991) One Long Argument. Cambridge, MA: Harvard University Press.
• Rhodes, F. H. T. (1983) "Gradualism, punctuated equilibria, and the origin of species." Nature 305: 269-272.
• Ridley, M. (1993) Evolution. Boston: Blackwell Scientific.

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(This page last updated on August 12, 2010.)