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Evolution as Fact AND Theory
Larry Flammer

EVO: Ten Questions Everyone Should Ask about Evolution: Directly downloadable from <> This is a series of ten short (10') videos by the Vimeo film makers.
In the first video, "What is evolution?", several biologists offer their definitions. In the process, students may find themselves confused. Evolution can apparently be both a fact and a theory, and can even have several different definitions, depending on the circumstances. When students watch this video, they may well ask about the meaning of evolution. You might want to ask for students to volunteer what they think the word means, and you will certainly get a variety of answers, many of which reflect some of the common misconceptions about evolution, or about science. The following comments are intended to help students see what evolution IS, what it's NOT, and why it can be considered both a fact and a theory.

First, some basic definitions:

A scientific fact is generally recognized as a critical observation of some phenomenon (object or event) by many independent critical observers, and their observations are virtually the same.

In contrast, the word "fact" in its non-scientific usage can refer to anything considered to be real.

A scientific theory is a deeply established explanation for a broad feature of the natural world that is well supported by an abundance of critical investigation and resulting evidence. It can include facts, laws, models, inferences and tested hypotheses (tentative explanations).

Biological evolution: some acceptable definitions:
1. Biological evolution is the process of change in populations of organisms over time (over many generations), as defined by one of the scientists in the first video.
2. It can also be defined as a change in gene frequencies in a population over generations.
3. Others will define evolution as the process in which a new species forms from a pre-existing species by gradual change over many generations.

Examples of evolution have actually been directly observed by many critical observers (both in the lab and in the field), making those examples facts. (Search for "examples of evolution" to confirm that.) Furthermore, there's an overwhelming collection of observations of fossils over time clearly showing major changes in major groups of organisms, e.g., new groups arising, earlier groups going extinct. Fossil studies have also clearly revealed many sequences over time that can most logically be interpreted as evidence that species have indeed arisen from previous species by accumulating gradual changes, usually over many generations. One of the best examples is the growing collection of the fossils of human ancestry. When students compare features of hominin skull replicas arranged over time, they can actually see the gradual accumulation of a mosaic of features, becoming less ape-like and more like modern human features. More recently, a variety of molecular studies confirm and continue to strengthen and clarify all the previous evidence. These multiple lines of evidence point to the same conclusion: life has evolved and is still evolving.

All of this critical evidence - those facts - all strongly point to evolution being a reality that makes sense of those facts. The evidence is now so compelling, with no evidence against it, that scientists treat evolution as a reality. When speaking to the general public, and even to other scientists, scientists may at times call evolution a fact (rather than a theory), invoking the broader, non-scientific use of "fact" that recognizes the well supported process of evolution, and because the layman's perception of "theory" as just an idea or hunch does not match that high level of scientific confidence based on the overwhelming evidence of evolution.

All critical efforts by biologists to test (try to disprove) evolution have failed, thereby consistently supporting the process we call "biological evolution" as the only viable explanation that fits all the evidence. And, there is no material evidence against evolution. That satisfies the requirements for a scientific theory.

Technically, evolution, in its totality, should not be called a fact; it does not quite match the generally accepted definition of a scientific fact. However, because it has been tested many times, and we see so many examples of the process, along with an abundance of evidence pointing to it, and no evidence against it, scientists do treat evolution as a reality. For all those reasons, the reality of the theory of evolution is very unlikely to ever change.

Therefore, biological evolution is both a reality and a theory.

That said, sometimes even scientists will refer to the "fact of evolution" using "fact" in its broader non-scientific sense to include anything considered to be real.
Natural selection is a fact, because it has been observed directly and indirectly by many critical observers independently. Furthermore, it has been so observed as the main mechanism that best explains how biological evolution takes place, so it's also a scientific theory.

Darwin's theory was actually the "Theory of evolution by natural selection" - namely that evolution (now seen as a fairly obvious process) happened (and is still happening) primarily by the mechanism of natural selection.

So, are evolution and natural selection the same thing? No they are not. Evolution is the general process where new species arise gradually from previous species. Natural selection is the primary mechanism by which evolution takes place.

CAUTION: Never say that "evolution is only a theory." That phrasing suggests that the word "theory" is being used in its vernacular sense, as a hunch or tentative idea, which biological evolution is definitely NOT.

Transitional Fossils (a sampling)
Classroom Cladogram of Vertebrate Evolution
Patterns in Time: Origins of Vertebrates

Evidence of Evolution: Examples where evolution has been directly and indirectly observed.

Video: Theory vs Hypothesis vs. Law... Explained![including the theory of the fact of evolution]
Meanings of these terms, and how they're used in science, and why science is so helpful.
Excellent, brief (7') presentation. Best used in intro to evolution, rather than NOS. Note the misuse of "fact" in regards to evolution.