There is no alternative to the Theory of Evolution. There are questions that remain unanswered, gaps in the fossil record and plenty of discussion and debate about the details of the theory that Darwin put forth in 1859, but there are no competing theories as to how species evolve over time. As such, it is the foundation of biological science and widely accepted as true and accurate.
The statement "evolution is both a theory and a fact" is often seen in biological literature. Evolution is a "theory" in the scientific sense of the term "theory"; it is an established scientific model of a portion of the universe that generates propositions with observational consequences. Such a model both helps generate new research and helps us understand observed phenomena.
When scientists say "evolution is a fact", they are using one of two meanings of the word "fact". One meaning is empirical: evolution can be observed through changes in allele frequencies or traits of a population over successive generations.
Another way "fact" is used is to refer to a certain kind of theory, one that has been so powerful and productive for such a long time that it is universally accepted by scientists. When scientists say evolution is a fact in this sense, they mean it is a fact that all living organisms have descended from a common ancestor (or ancestral gene pool) even though this cannot be directly observed. This implies more tangibly that it is a fact that humans share a common ancestor with all living organisms. [Cavalier-Smith T (2006), "Cell evolution and Earth history: stasis and revolution"]
In the American vernacular, "theory" often means "imperfect fact" - part of a hierarchy of confidence running downhill from fact to theory to hypothesis to guess. Thus the power of the creationist argument: evolution is "only" a theory and intense debate now rages about many aspects of the theory. If evolution is worse than a fact, and scientists can't even make up their minds about the theory, then what confidence can we have in it? Indeed, President Reagan echoed this argument before an evangelical group in Dallas when he said (in what I devoutly hope was campaign rhetoric): "Well, it is a theory. It is a scientific theory only, and it has in recent years been challenged in the world of science - that is, not believed in the scientific community to be as infallible as it once was."
Well evolution is a theory. It is also a fact. And facts and theories are different things, not rungs in a hierarchy of increasing certainty. Facts are the world's data. Theories are structures of ideas that explain and interpret facts. Facts don't go away when scientists debate rival theories to explain them. Einstein's theory of gravitation replaced Newton's in this century, but apples didn't suspend themselves in midair, pending the outcome. And humans evolved from ape-like ancestors whether they did so by Darwin's proposed mechanism or by some other yet to be discovered.
Moreover, "fact" doesn't mean "absolute certainty"; there is no such animal in an exciting and complex world. The final proofs of logic and mathematics flow deductively from stated premises and achieve certainty only because they are not about the empirical world. Evolutionists make no claim for perpetual truth, though creationists often do (and then attack us falsely for a style of argument that they themselves favor). In science "fact" can only mean "confirmed to such a degree that it would be perverse to withhold provisional consent." I suppose that apples might start to rise tomorrow, but the possibility does not merit equal time in physics classrooms.
Evolutionists have been very clear about this distinction of fact and theory from the very beginning, if only because we have always acknowledged how far we are from completely understanding the mechanisms (theory) by which evolution (fact) occurred. Darwin continually emphasized the difference between his two great and separate accomplishments: establishing the fact of evolution, and proposing a theory - natural selection - to explain the mechanism of evolution. [Stephen J. Gould, " Evolution as Fact and Theory"; Discover, May 1981]
"Today, nearly all biologists acknowledge that evolution is a fact. The term theory is no longer appropriate except when referring to the various models that attempt to explain how life evolves... it is important to understand that the current questions about how life evolves in no way implies any disagreement over the fact of evolution." - Neil A. Campbell, Biology 2nd ed., 1990, Benjamin/Cummings, p. 434
We now know beyond any reasonable doubt that Darwinian evolution is a fact. There is overwhelming evidence that all living things are descended from a common ancestor by accidental mutations and unguided natural selection. Intelligent design is wrong!
"Let me try to make crystal clear what is established beyond reasonable doubt, and what needs further study, about evolution. Evolution as a process that has always gone on in the history of the earth can be doubted only by those who are ignorant of the evidence or are resistant to evidence, owing to emotional blocks or to plain bigotry. By contrast, the mechanisms that bring evolution about certainly need study and clarification. There are no alternatives to evolution as history that can withstand critical examination. Yet we are constantly learning new and important facts about evolutionary mechanisms." - Theodosius Dobzhansky, "Nothing in Biology Makes Sense Except in the Light of Evolution", American Biology Teacher vol. 35 (March 1973) reprinted in Evolution versus Creationism, J. Peter Zetterberg ed., ORYX Press, Phoenix AZ 1983
Examples of evolution that are readily apparent include the fact that modern populations are evolving and the fact that two closely related species share a common ancestor. The evidence that Homo sapiens and chimpanzees share a recent common ancestor falls into this category. There is so much evidence in support of this aspect of primate evolution that it qualifies as a fact by any common definition of the word "fact."
In other cases the available evidence is less strong. For example, the relationships of some of the major phyla are still being worked out. Also, the statement that all organisms have descended from a single common ancestor is strongly supported by the available evidence, and there is no opposing evidence. However, it is not yet appropriate to call this a "fact" since there are reasonable alternatives.
Scientific breakthroughs in paleontology, embryology, experimental selection, evolutionary developmental biology (evo-devo), and molecular phylogeny have proven Darwinian evolution beyond a shadow of doubt.
"One thing all real scientists agree upon is the fact of evolution itself. It is a fact that we are cousins of gorillas, kangaroos, starfish, and bacteria. Evolution is as much a fact as the heat of the sun. It is not a theory, and for pity's sake, let's stop confusing the philosophically naive by calling it so. Evolution is a fact." - Richard Dawkins, "The Illusion of Design"
Paleontology: Berkeley dino-bird expert Kevin Padian has just purchased the last missing link from a Chinese fossil dealer - thereby providing us with an unbroken record of ancestors and descendants from the Big Bang to the present.
Embryology: University of Chicago fruit fly geneticist Jerry Coyne and his colleague, historian Robert Richards, have declared a Scientific Consensus that vertebrate embryos really are most similar in their earliest stages--thereby proving once and for all that human embryos are just fish.
Experimental selection: Distinguished bacteriologist Richard Lenski of Michigan State University has observed after more than fifty thousand rounds of artificial selection that E. coli are still E. coli--thereby convincing himself and Even-More-Distinguished philosopher Robert Pennock that natural selection has the unlimited power to create new species, organs and body plans.
Evo-devo: Howard Hughes Medical Institute (HHMI) researcher Sean B. Carroll has announced that mutations affecting insect wing patterns account for all human diseases--thereby persuading HHMI that his research actually has something to do with medicine.
Molecular phylogeny: Retired Oxford bird behaviorist (and living legend) Richard Dawkins has asserted yet again that all molecules yield the same evolutionary tree--thereby earning himself a place in Guinness World Records for denying reality more times than Baghdad Bob.
"Since Darwin's time, massive additional evidence has accumulated supporting the fact of evolution - that all living organisms present on earth today have arisen from earlier forms in the course of earth's long history. Indeed, all of modern biology is an affirmation of this relatedness of the many species of living things and of their gradual divergence from one another over the course of time. Since the publication of The Origin of Species, the important question, scientifically speaking, about evolution has not been whether it has taken place. That is no longer an issue among the vast majority of modern biologists. Today, the central and still fascinating questions for biologists concern the mechanisms by which evolution occurs." - Helena Curtis and N. Sue Barnes, Biology 5th ed. 1989, Worth Publishers, p. 972
The final triumph of Darwinism will be officially celebrated at the U.S. National Academy of Sciences in Washington, DC, on April 27, 2011 - the 140th anniversary of Charles Darwin's historic Nature article on "Pangenesis." In that work, Mr. Darwin defended his theory that "gemmules" scattered throughout the body explain the inheritance of acquired characteristics. Although long derided by religiously motivated followers of Roman Catholic priest Gregor Mendel, pangenesis is currently enjoying a renaissance among Darwinists, who are confident that everything written by The Greatest Scientist Who Ever Lived will ultimately be proven true.
Meanwhile, there are rumors that the National Center for Science Education and the American Civil Liberties Union will share the Nobel Peace Prize later this year for promoting Darwin-only education and thereby saving civilization from the forces of darkness.
Understanding how we, as a society, have acquired knowledge is especially important when the ideas, perspectives, and discoveries were - and may still be - controversial. With respect to evolutionary biology, many key ideas, perspectives, and discoveries were intensely resisted when they were first put forward, and many continue to be resisted today, owing to real or perceived conflicts with certain religious beliefs.
It is common for people to claim, or ponder, that human beings evolved from monkeys, which is untrue. Rather, monkeys and humans have a common ancestor that they both evolved from over many millions of years. Humans share a common ancestor with modern African apes, like gorillas and chimpanzees. Scientists believe this common ancestor existed5 to 8 million years ago. Shortly thereafter, the species diverged into two separate lineages. One of these lineages ultimately evolved into gorillas and chimps, and the other evolved into early human ancestors called hominids. [PBS: Human Evolution]
Since the earliest hominid species diverged from the ancestor we share with modern African apes, 5 to 8 million years ago, there have been at least a dozen different species of these humanlike creatures. Many of these hominid species are close relatives, but not human ancestors. Most went extinct without giving rise to other species. Some of the extinct hominids known today, however, are almost certainly direct ancestors of Homo sapiens. [PBS: Origins of Human Kind]
Finally, Intelligent Design is not science. It is Creationism re-packaged to circumvent court rulings that have held that teaching Creationism in public schools violates the establishment clause of the U.S. Constitution.
Evolution describes the change over time of all living things from a single common ancestor. The "tree of life" illustrates this concept. Every branch represents a species, each connected to other such branches and the rest of tree as a whole. The forks separating one species from another represent the common ancestors shared by these species. In the case of the relatedness of humans and single-celled organisms, a journey along two different paths -- one starting at the tip of the human branch, the other starting at the tip of a single-celled organism's branch -- would ultimately lead to a fork near the base of the tree: the common ancestor shared by these two very different types of organisms. This journey would cross countless other forks and branches along the way and span perhaps more than a billion years of evolution, but it demonstrates that even the most disparate creatures are related to one another - that all life is interconnected. [PBS: Deep Time]
A "fact" means something that is so highly probable that it would be silly not to accept it. This point has also been made by others who contest the nit-picking epistemologists.
"So enormous, ramifying, and consistent has the evidence for evolution become that if anyone could now disprove it, I should have my conception of the orderliness of the universe so shaken as to lead me to doubt even my own existence. If you like, then, I will grant you that in an absolute sense evolution is not a fact, or rather, that it is no more a fact than that you are hearing or reading these words." - H. J. Muller, "One Hundred Years Without Darwin Are Enough" School Science and Mathematics 59, 304-305. (1959)
In any meaningful sense evolution is a fact, but there are various theories concerning the mechanism of evolution.
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Resources:
Stephen J. Gould, "Evolution as Fact and Theory", Discover, May 1981
Richard Dawkins, "The Illusion of Design", Darwin & Evolution, Nov 2005
"Cell Evolution and Earth History: Stasis and Revolution" - Thomas Cavalier-Smith
Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK
Muller, H. J. (1959). "One hundred years without Darwin are enough".
School Science and Mathematics 59: 304–305.
Theodosius Dobzhansky, "Nothing in Biology Makes Sense Except in the Light of Evolution"
American Biology Teacher vol. 35 (March 1973) reprinted in Evolution versus Creationism, J. Peter Zetterberg ed., ORYX Press, Phoenix AZ 1983
PBS: Evolution FAQ
PBS: Human Evolution
PBS: Origins of Human Beings
PBS: Evolution of Love
"Evolution Is Not 'Just a Theory'." Discredits the assertion that evolution is "just a theory", with an explanation of the meaning of the word 'theory' in a scientific context.
Talk Origins. Response to the claim that no examples of speciation have been observed.
Glenn Branch; Louise S. Mead (2008-06-06). ""Theory" in Theory and Practice" (PDF). Evo Edu Outreach, 2008, 1:287–289. Springer Science + Business Media. Retrieved 2008-07-21.
Wikipedia: "Evolution as theory and fact"
