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For instance, Mendel’s laws of assortment and independent segregation, which
initiated modern genetics, were modified when phenomena such as linkage and
meiotic drive were discovered, but Mendel’s underlying principle of inheritance
based on “particles” (genes) holds true today.
This process reflects one of the most important and valuable features of sci-
ence: even if individual scientists may be committed to a hypothesis, scientists as a
group are not irrevocably committed to it; they must, and do, change their minds if
the evidence so warrants. Indeed, much of science consists of seeking chinks in the
armor of established ideas. Thus science, as a social process, is tentative; it ques-
tions belief and authority; it continually tests its views against evidence. Scientific
claims, in fact, are the outcome of a process of natural selection, for ideas (and
scientists) compete with one another, so that the body of ideas in a scientific field
grows in explanatory content and power [60]. Science differs in this way from cre-
ationism, which does not use evidence to test its claims, does not allow evidence to
shake its a priori commitment to certain beliefs, and does not grow in its capacity
to explain the natural world.
The ideal of democracy doesn’t extend to ideas—some are simply wrong, and
as a purely practical matter, it is imperative that we recognize them as such [91]. In
everyday life, we assume and depend on natural, not supernatural, explanations.
Unlike the Puritans of Salem, Massachusetts, who in 1692 condemned people for
witchcraft, we no longer seriously entertain the notion that someone can be vic-
timized by a witch’s spell or possessed by devils, and we would be outraged if a
criminal successfully avoided conviction because he claimed “the Devil made me
do it.” We depend on scientific explanations, and we know that science has proven
its ability—because it works.
Is evolution a fact or a theory? Both. Recall from Chapter 1 that a theory, as the
word is used in science, doesn’t mean an unsupported speculation or hypothesis
(the popular use of the word). A theory is instead a big, well-supported idea that
encompasses other ideas and hypotheses and weaves them into a coherent fabric.
The word “fact” applies to hypotheses that have become so well supported by evi-
dence that we feel safe in acting as if they were true. To use a courtroom analogy,
they have been “proven” beyond reasonable doubt. Not beyond any conceivable
doubt, but reasonable doubt. By this criterion, evolution is a scientific fact. That is,
the descent of all species, with modification, from common ancestors is a hypoth-
esis that in the last 150 years or so has been supported by so much evidence, and
has so successfully resisted all challenges, that it has become a fact. This history
of evolutionary change—and the diversity of life—is explained by evolutionary
theory, the body of statements (about mutation, selection, genetic drift, develop-
mental constraints, and so forth) that together account for the various changes that
organisms have undergone.
It is chiefly the fact of evolution that creationists deny. Everyone who has stud-
ied biology should be able to counter creationist arguments and present evidence
for the fact of evolution.The Evidence for Evolution
The evidence for evolution has been presented throughout the preceding chapters
of this book (see especially Box 2B, p. 44–45). In this section we simply review the
sources of evidence and refer back to earlier chapters for detailed examples.The fossil record
Even though the fossil record is known to be very incomplete, paleontologists have
found many examples of transitional stages in the origin of higher taxa, such as22_EVOL4E_CH22.indd 579 3/22/17 1:49 PM