"It is important to understand that the current questions about how life evolves in no way imply any disagreement over the fact of evolution." - Neil A. Campbell

Those who still dispute evolution often use the argument that it is “just a theory.” Stating that, as theory, scientists cannot even agree if it occurs.

This is an argument of those with little understanding of what evolution actually is. Evolution occurs. It is observable not only in the geologic record, but also in the world around us. Evolution means change, and populations of organisms certainly change over time. So where does the confusion lie regarding evolutionary theory?

Is Evolution ‘Just’ a Theory?

Non-biologists discussing the “theory of evolution” typically confuse two different aspects to evolutionary theory:

1. Does evolution occur?

2. How does evolution occur (i.e. what is the mechanism)?

Does Evolution Occur?

The first is a question of whether modern organisms evolved from ancestral organisms and are continuing to change over time. Biologists consider the existence of biological evolution to be a fact. It can be demonstrated today and the historical evidence for its occurrence in the past is overwhelming.

How Does Evolution Occur?

Beyond the fact that organisms do change over time, there is the question of how, or the mechanism change. Regarding this question, biologists are still discussing and debating the details of 'how,' and there are several theories regarding the mechanism of evolution.

Evolution as Fact & Theory

The late Stephen J. Gould, one of the great thinkers regarding evolution, explained the confusion as such: “…Facts and theories are different things…Facts are the world's data. Theories are structures of ideas that explain and interpret facts. Darwin continually emphasized the difference between his two great and separate accomplishments: establishing the fact of evolution, and proposing a theory -- natural selection -- to explain the mechanism of evolution.”

How Does Evolution Work?

Evolution can be defined as inheritable change in a population that ultimately results from the interaction of individuals with their environment. And its action over very large stretches of time explains the origin of new species, occasionally the elimination of existing species and ultimately the vast diversity of the biological world.

Today’s species are related to each other through common decent (ancestors that they share) and are products of evolution over billions of years.

Key concepts of evolution:

Any change must be inheritable (able to be passed on to the next generation).

These changes are regulated by natural selection.

Natural Selection

Natural selection is the theorized mechanism of evolution, the process in nature by which only the organisms that are best adapted to their environment tend to survive and transmit their genetic characteristics to the next generation. Individuals less well adapted to their environment tend to be eliminated.

More Information on Evolution

See the Science education website Science Prof Online for more information on evolution and natural selection or L. Moran's article Evolution is a Fact and a Theory or the Suite101 articles Darwin, Evolution & Natural Selection, Natural & Artificial Selection or Pre-Darwinian Evolutionary Theory.

Sources

Moran, L. (1993) Evolution is a Fact and a Theory.

Gould, S. J. (1981) Evolution as Fact and Theory. Discover, May issue.

Campbell, N.A. (1990) Biology 2nd ed., Benjamin Cummings, p. 434.