Frequently Asked Questions*
What is evolution?
Evolution in the broadest sense explains that what we see today is different from what existed in the past. Galaxies, stars, the solar system, and Earth have changed through time, and so has life on Earth.
Biological evolution concerns changes in living things during the history of life on Earth. It explains that living things share common ancestors. Over time, biological processes such as natural selection give rise to new species. Darwin called this process "descent with modification," which remains a good definition of biological evolution today.
Isn't evolution just an inference?
No one saw the evolution of one-toed horses from three-toed horses, but that does not mean that we cannot be confident that horses evolved. Science is practiced in many ways besides direct observation and experimentation. Much scientific discovery is done through indirect experimentation and observation in which inferences are made, and hypotheses generated from those inferences are tested.
For instance, particle physicists cannot directly observe subatomic particles because the particles are too small. They make inferences about the weight, speed, and other properties of the particles based on other observations. A logical hypothesis might be something like this: If the weight of this particle is Y, when I bombard it, X will happen. If X does not happen, then the hypothesis is disproved. Thus, we can learn about the natural world even if we cannot directly observe a phenomenon—and that is true about the past, too.
In historical sciences like astronomy, geology, evolutionary biology, and archaeology, logical inferences are made and then tested against data. Sometimes the test cannot be made until new data are available, but a great deal has been done to help us understand the past. For example, scorpionflies (Mecoptera) and true flies (Diptera) have enough similarities that entomologists consider them to be closely related. Scorpionflies have four wings of about the same size, and true flies have a large front pair of wings but the back pair is replaced by small club-shaped structures. If two-winged flies evolved from scorpionfly-like ancestors, as comparative
anatomy suggests, then an intermediate true fly with four wings should have existed—and in 1976 fossils of such a fly were discovered. Furthermore, geneticists have found that the number of wings in flies can be changed through mutations in a single gene.
Something that happened in the past is thus not "off limits" for scientific study. Hypotheses can be made about such phenomena, and these hypotheses can be tested and can lead to solid conclusions. Furthermore, many key mechanisms of evolution occur over relatively short periods and can be observed directly—such as the evolution of bacteria resistant to antibiotics.
Evolution is a well-supported theory drawn from a variety of sources of data, including observations about the fossil record, genetic information, the distribution of plants and animals, and the similarities across species of anatomy and development. Scientists have inferred that descent with modification offers the best scientific explanation for these observations.
Is evolution a fact or a theory?
The theory of evolution explains how life on Earth has changed. In scientific terms, "theory" does not mean "guess" or "hunch" as it does in everyday usage. Scientific theories are explanations of natural phenomena built up logically from testable observations and hypotheses. Biological evolution is the best scientific explanation we have for the enormous range of observations about the living world.
Scientists most often use the word "fact" to describe an observation. But scientists can also use fact to mean something that has been tested or observed so many times that there is no longer a compelling reason to keep testing or looking for examples. The occurrence of evolution in this sense is a fact. Scientists no longer question whether descent with modification occurred because the evidence supporting the idea is so strong.
Don't many famous scientists reject evolution?
No. The scientific consensus around evolution is overwhelming. Those opposed to the teaching of evolution sometimes use quotations from prominent scientists out of context to claim that scientists do not support evolution. However, examination of the quotations reveals that the scientists are actually disputing some aspect of how evolution occurs, not whether evolution occurred. For example, the biologist Stephen Jay Gould once wrote that "the extreme rarity of transitional forms in the fossil record persists as the trade secret of paleontology." But Gould, an accomplished paleontologist and eloquent educator about evolution, was arguing about how evolution takes place. He was discussing whether the rate of change of species is slow and gradual or whether it takes place in bursts after long periods when little change occurs—an idea known as punctuated equilibrium. As Gould writes in response, "This quotation, although accurate as a partial citation, is dishonest in leaving out the following explanatory material showing my true purpose—to discuss rates of evolutionary change, not to deny the fact of evolution itself." Gould defines punctuated equilibrium as follows:
Punctuated equilibrium is neither a creationist idea nor even a non-Darwinian evolutionary theory about sudden change that produces a new species all at once in a single generation. Punctuated equilibrium accepts the conventional idea that new species form over hundreds or thousands of generations and through an extensive series of intermediate stages. But geological time is so long that even a few thousand years may appear as a mere "moment" relative to the several million years of existence for most species. Thus, rates of evolution vary enormously and new species may appear to arise "suddenly" in geological time, even though the time involved would seem long, and the change very slow, when compared to a human lifetime.
If humans evolved from apes, why are there still apes?
Humans did not evolve from modem apes, but humans and modem apes shared a common ancestor, a species that no longer exists. Because we share a recent common ancestor with chimpanzees and gorillas, we have many anatomical, genetic, biochemical, and even behavioral similarities with these African great apes. We are less similar to the Asian apes—orangutans and gibbons—and even less similar to monkeys, because we share common ancestors with these groups in the more distant past.
Evolution is a branching or splitting process in which populations split off from one another and gradually become different. As the two groups become isolated from each other, they stop sharing genes, and eventually genetic differences increase until members of the groups can no longer interbreed. At this point, they have become separate species. Through time, these two species might give rise to new species, and so on through millennia.
Why can't we teach creation science in my school?
The courts have ruled that "creation science" is actually a religious view. Because public schools must be religiously neutral under the U.S. Constitution, the courts have held that it is unconstitutional to present creation science as legitimate scholarship.
In particular, in a trial in which supporters of creation science testified in support of their view, a district court declared that creation science does not meet the tenets of science as scientists use the term (McLean v. Arkansas Board of Education). The Supreme Court has held that it is illegal to require that creation science be taught when evolution is taught (Edwards v. Aguillard). In addition, district courts have decided that individual teachers cannot advocate creation science on their own (Peloza v. San Juan Capistrano School District and Webster v. New Lennox School District).(See Teaching About Evolution and the Nature of Science, Appendix A. National Academy of Sciences, Washington, D.C. 1998.)
Teachers' organizations such as the National Science Teachers Association, the National Association of Biology Teachers, the National Science Education Leadership Association, and many others also have rejected the science and pedagogy of creation science and have strongly discouraged its presentation in the public schools. In addition, a coalition of religious and other organizations has noted in "A Joint Statement of Current Law" that "in science class, [schools] may present
only genuinely scientific critiques of, or evidence for, any explanation of life on Earth, but not religious critiques (beliefs unverifiable by scientific methodology)." (See Teaching About Evolution and the Nature of Science, Appendices B and C, National Academy of Sciences, Washington, D.C., 1998.)
Some argue that "fairness" demands the teaching of creationism along with evolution. But a science curriculum should cover science, not the religious views of particular groups or individuals.
If evolution is taught in schools, shouldn't creationism be given equal time?
Some religious groups deny that microorganisms cause disease, but the science curriculum should not therefore be altered to reflect this belief. Most people agree that students should be exposed to the best possible scholarship in each field. That scholarship is evaluated by professionals and educators in those fields. Scientists as well as educators have concluded that evolution—and only evolution—should be taught in science classes because it is the only scientific explanation for why the universe is the way it is today.
Many people say that they want their children to be exposed to creationism in school, but there are thousands of different ideas about creation among the world's people. Comparative religions might comprise a worthwhile field of study, but not one appropriate for a science class. Furthermore, the U.S. Constitution states that schools must be religiously neutral, so legally a teacher cannot present any particular creationist view as being more "true" than others.