Natural selection is a theory because it had been tested and verified countless times by experimentation and observation. It made accurate predictions and can be modelled mathematically. It is thus a scientific theory.
If you're wondering by any chance what a theory is in science, it is exactly what I described what natural selection went through. A scientific theory is a tested and verified hypothesis supported by evidence that explains a phenomenon we observe. Some examples of scientific theories are the theory of gravity, the theory of relativity, the germ theory of disease, theory of quantum mechanics, so on.
Here is what a theory is NOT. It is NOT a wild guess, opinion or speculation, unsupported by evidence, as the everyday term of theory would me. A theory in science is a very high status. It means the theory had passed through rigorious tests and were peer reviewed by many other scientists and accepted.
The theory is called the Special Theory of Relativity.
Geocentric theory.
The theory is Tectonics.
The theory of cosmology that Copernicus proposed is called the heliocentric theory. This theory placed the Sun, rather than the Earth, at the center of the solar system, with the planets orbiting around it.
The theory is called "plutocratic theory", which characterizes a plutocracy.
The theory is called plate tectonics.
Newton's Theory.
theory hypothosis
A theory.
The theory of Pangaea is called the continental drift theory. It proposes that Earth's continents were once joined together in a single large landmass that later broke apart and drifted into their current positions.
The theory is called Plate Tectonics.
Such a system is called a theory. For example, Darwin's Theory of Evolution, or Newton's Theory of Universal Gravitation, or Einstein's Theory of Relativity.