Some will say that evolution is “just a theory”. In everyday language, a theory is a hunch. However, scientists use the word theory in a way that is completely different from the use of that word in popular culture. In science, everything starts out by observing a phenomenon or a group of phenomena and recording empirical evidence. A scientist then forms a hypothesis to explain that observation. The scientist makes predictions about behaviors that can be tested. Tests are then conducted. Tests can be experiments, but this does NOT mean that they must be repeatable real-time in a lab environment. They can be observations of additional evidence, including forensic evidence (such as fossils), sometimes through a different branch of science, to test if they are consistent with the original hypothesis. If anything doesn’t fit, the hypothesis must be updated or scrapped. The hypothesis is then sent for review by other scientists so that they can also test it. If the hypothesis holds up to scrutiny, usually over the course of several years or decades, it becomes a theory.
An established scientific theory is like the Ronald Reagan adage, “Trust, but verify.” Once a scientific explanation about some observed phenomenon has been is elevated to the level of “theory,” we trust that it is true enough to the point that we feel comfortable teaching it. However, we constantly test and verify all scientific theories, and this is done continuously through the scientific method by scientists all over the world.
Like evolution, gravity is also a theory. Our current theory of gravitation is simply known as the “Theory of Gravitation” and it is based on Einsteins General Theory of Relativity. It augments (but does not replace) Newton’s Theory of Gravitation which was formulated over 200 years earlier.
Some will argue that gravity is a Law, thinking that this should elevate its stance above other theories. This is not how science is done. The Law of Gravitation which was formulated by Newton is ONLY a mathematical description of the behavior of gravity. It says nothing about what causes gravity, and indeed, the Law of Gravitation is LESS ACCURATE than the modern Theory of Gravitation which takes into account relativistic effects.
In science, nothing ever progresses past the stage of a theory. Theories don’t grow up to be laws. A law is a mathematical model of a theory, and in many cases, it is only a simplistic expression of the theory which excludes edge cases (in the case of gravity, the edge cases being apparent when the object(s) in question are moving close to the speed of light).
Similarly, our current theory of evolution is known as the Modern Evolutionary Synthesis and it augments (but does not replace) Darwin’s theory of evolution by natural selection.
Darwin theorized that evolution works by variation and natural selection, and we still know this to be true. However, Darwin had no idea what caused the variation. At the time, Mendelian genetics was still in its infancy, and nobody knew about DNA. We now have a wealth of evidence for what causes variation: genetic mutations.
There are many different ways that genetic mutations can arise and increase the complexity of the genetic code, such as point mutations, gene duplications, deletions, chromosomal inversions, frameshift mutations, repeat expansions, horizontal gene transfer, endogenous retroviruses, translocations, and others. New information is added to the genome by means of duplications and subsequent divergence in function. In plants, an entire genome duplication can create a new species in a single generation.
And for those who think that genetic mutations cannot occur at a rate that can explain current biodiversity, you are mistaken.
If a theory has been accepted for a lengthy period of time, some will call it a fact. This is what some people mean when they say that evolution, in general, is a fact. However, the precise definition of the word “fact” within science is a piece of empirical data that is based on direct observation. For example, “The frequency of alleles in the genes of this population of mice has changed over time.” Using that fact, we can also say it is a fact that these mice have evolved. A theory is something that is used to explain facts, but it can also be a fact in a larger sense.
Evolution is not a fringe theory, like cold fusion, where the data are new enough and the number of researchers is small enough that it is difficult to ascertain whether biases, ulterior motives, or simple errors have tainted the results. Evolution is one of the most tested and well-supported theories in all of science and it is upheld by the research performed by scientists of many different religious and cultural backgrounds (including evangelical Christians) and technical specialties over the past 150 years. Since the time of its inception by Charles Darwin and Alfred Wallace, it has survived numerous occasions where entirely new fields of inquiry could have falsified it, including Mendelian genetics, nuclear physics (for verifying ages of rocks and fossils), developmental biology, molecular biology, and now genomics. Instead, these fields provided some of the strongest supporting evidence yet discovered.
We actually have far more evidence for (macro)evolution, from multiple diverse fields of study, than we do for the modern theory of gravity! Gravity and evolution are both theories, but the evidence that supports both theories is so strong that they can both be safely taught as facts.