Is Evolution a Fact or a Theory?

Darwinian Evolution: Just a Theory? 
Charles Darwin's Evolutionary Mechanism 
of Natural Selection 


Those who dispute the existence of 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 scientific theory and evolution actually are. 

Article Summary: "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. This article explores the misinterpretation of the word theory as it applies to evolution.

Darwinian Evolution: Just a Theory?
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.

In This Diagram Mutations With Natural Selection Result in a Population Darker In Color
Instructor's Corner
Page last updated: 11/2015
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 How Does Evolution Work?
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Charles Darwin ,1881,  72 years old.
Charles Darwin ,1881, 
72 years old.
The formula for Newton's Law of Universal Gravitation
The formula for Newton's Law of Universal Gravitation
How Does Evolution Occur?
Beyond the fact that organisms do change over time, there is the question of how, the mechanism of change. What drives evolutionary change? Regarding this question, biologists are still discussing the details of how, and throughout history there have been several theories proposed regarding the mechanism of evolution.

Evolution: Fact or 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.”

"While laws rarely change, theories change frequently as new evidence is discovered. Instead of being discarded due to new evidence, theories are often revised to include the new evidence in their explanation. The Theory of General Relativity explains why things fall, and has adapted as new technologies and new evidence have expanded our view of the universe." - Robert Kampf, The Happy Scientist

Evolution via natural selection is a scientific theory. It is an explanation of why and how evolution occurs, and, as an theory, is expected to develop and be modified as new tools are developed and new information is uncovered. For example, DNA sequencing is a modern tool that has contributed valuable information to our understanding of the evolutionary relationships between organisms.

"A scientific law is not "better" or "more accurate" than a scientific theory. A law explains what will happen under certain circumstances, while a theory explains how it happens." - from website Evolution FAQ

Why Isn't Evolution a Scientific Law?
If evolution is true, if it is really happening, why isn't it it considered a a scientific law? For example, gravity is both a law and a theory. A scientific law explains what 
will happen under certain conditions and can usually be expressed as a mathematical formula. 

scientific theory starts as a hypothesis or aset of hypotheses that must be thoroughly tested, and must accurately explain and predict what we see in the natural world. A theory is an explanation of why and how things happen.

Although the evolution is accepted by biologists as a fact, evolution cannot be expressed in a concise mathematical equation, so it is not a scientific law.