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.
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
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
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
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
Humans did not evolve
from modern apes, but humans and modern 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
Why can't we teach creation science in my
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.)
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.
Teaching About Evolution and the Nature of Science by the
National Academy of Sciences (Washington, D.C.: National Academy Press,