The term theory in general usage is often synonymous with something which may not be true, something which is speculative, something a little shaky. ‘That’s all very well in theory’ is a phrase often heard. Its opposite is often thought to be ‘practical’ or ‘solid’. In science it has a very different meaning. A theory is something which explains phenomena, usually by linking these phenomena together. For example, the theory of gravity explains why objects fall, or more precisely, the conditions under which objects fall or move in particular ways.
Theories in science are therefore evaluated on how much they explain, and how well they fit the evidence. A good theory is a powerful one, explaining much. A theory is not the opposite of a fact, but it explains facts, or bodies of knowledge. It will also predict observations that can be made in the ‘real’ world.
In science there is often a distinction between a hypothesis, which is an explanation or theory in the process of being tested, and a theory, which is more firmly accepted and consistent with the available evidence.
As Karl Popper proposed, a good scientific theory should be testable, and susceptible to disproof.
Evolutionary theory is one of the overarching theories of science, and the framework for biology. Theodosius Dobzhansky said “Nothing makes sense in biology except in the light of evolution”. Evolutionary theory sets out to explain the observations and facts of biology, past and present. Darwin and Wallace’s theory of evolution suggested that natural selection was the mechanism to explain form and function, and so account for change. Evolution is an outcome of natural selection – this is the core concept in evolutionary theory.