Isn't evolution just a theory?

Reliance on theory is critical to every branch of science. Many scientific theories are so well established that no new evidence is likely to alter them substantially. We all accept that…

• the Earth orbits the Sun (heliocentric theory),
• living things are made of cells (cell theory),
• matter is composed of atoms (atomic theory), and
• the Earth's surface is divided into solid plates that move over geological time (theory of plate tectonics).

Each theory has been barraged with scientific tests and still stands intact.

This is true of the Theory of Evolution, too, which is why scientists do not find it controversial. Any perceived controversy comes from people who do not understand the scientific process.

By contrast, all of the following scientific theories were developed more recently than the Theory of Evolution. None are considered controversial.

Atomic Theory. Atomic theory states that all matter is composed of units called atoms. Atoms are discrete and cannot be divided further without destroying the substance. For instance, an oxygen atom divided is no longer oxygen.

Germ Theory of Disease. This theory states that most diseases are caused by germs, which are microscopic organisms that infect other living things. Modern medicine would be lost without this crucial understanding of many diseases.

Plate Tectonics. This theory describes how the Earth's outermost surface, called the crust, changes over time. It states that the continents rest on plates that form the Earth’s crust. These plates shift and change over time. Much like a conveyor belt, new crust is constantly being formed—at a snail’s pace!—at deepsea ridges; and old crust is being drawn under the edges of other plates at equally-slow rates.

Theory of Relativity. Einstein's theories of general and special relativity revolutionized how physicists understand the world. These theories describe how spacetime and gravity interact and how objects behave at speeds near the speed of light.

Although there is only one Theory of Evolution, there are many proposed mechanisms for how it works – mutation, gene flow, genetic drift, and sexual selection. In fact, evolution occurs via all of these mechanisms. The questions evolutionary biologists deal with center around the relative contributions each mechanism makes under various circumstances.

Of these, the most well-known is Darwin’s Theory of Evolution by Natural Selection, also known as “descent with modification.”

The Theory of Evolution by Natural Selection is fairly straightforward.

• Offspring are very similar to their parents, but are not exact copies.
• Differences from one generation to the next can be either beneficial or harmful to the offsprings’ chances for survival, and therefore, their ability to produce offspring.
• Offspring with changes that are beneficial are more likely to survive and those with harmful changes are less likely to survive and reproduce.
• Thus over time beneficial changes accumulate and harmful ones are weeded out.