EvoluTion And SoCiETy 575
nucleic acids had to have had any particular sequence.
If there are many possible sequences with such prop-
erties, the probability of their formation rises steeply.
Moreover, we do not need to know anything about
the origin of life in order to understand and document
the evolution of different life forms from their common
ancestor.- Mutations are harmful and do not give rise to complex
new adaptive characteristics.
Most mutations are indeed harmful and are purged
from populations by natural selection. Some, how-
ever, are beneficial, as shown in many experiments (see
Chapters 5 and 6). Complex adaptations are usually
based not on single mutations, but on combinations
of mutations that jointly or successively increase in fre-
quency as a result of natural selection. - Natural selection merely eliminates unfit mutants, rather
than creating new characters.
“New” characters, in most cases, are modifications of
pre-existing characters, which are altered in size, shape,
developmental timing, or organization (see Chapters 2
and 20). This is true at the molecular level as well (see
Chapter 14). Natural selection “creates” such modifi-
cations by increasing the frequencies of alleles at sev-
eral or many loci so that combinations of alleles, initially
improbable because of their rarity, become probable
(see Chapter 6). Observations and experiments on both
laboratory and natural populations have demonstrated
the efficacy of natural selection. - Chance could not produce complex structures.
This is true, but natural selection is a deterministic, not a
random, process. The random processes of evolution—
mutation and genetic drift—do not result in the evolution
of complexity, as far as we know. When natural selec-
tion is relaxed, complex structures, such as the eyes of
cave-dwelling animals, slowly degenerate, due in part to
selection for antagonistic pleiotropic effects. - Complex adaptations such as wings, eyes, and biochemi-
cal pathways could not have evolved gradually because
the first stages would not have been adaptive. The full
complexity of such an adaptation is necessary, and it
could not arise in a single step by evolution.
This was one of the first objections that greeted On the
Origin of Species, and it has been christened “irreduc-
ible complexity” by advocates of intelligent design. Our
answer has two parts. First, many such complex fea-
tures, such as hemoglobins and eyes, do show various
stages of increasing complexity and functional advan-
tage among different organisms (see Chapters 2, 14, and
20). Second, many structures have been modified for a
new function after being elaborated to serve a different
function (see Chapters 2 and 20).- If an altered structure, such as the long neck of the
giraffe, is advantageous, why don’t all species have
that structure?
This naïve question ignores the fact that different spe-
cies and populations have different ecological niches
and environments, for which different features are
adaptive. This principle holds for all features, including
“intelligence.” - If gradual evolution had occurred, there would be no
phenotypic gaps among species, and classification
would be impossible.
Many disparate organisms are connected by inter-
mediate species, and in such cases, classification into
higher taxa is indeed rather arbitrary (see Chapter 2).
In other cases, gaps exist because of the extinction of
intermediate forms (see Chapters 17 and 20). More-
over, although much of evolution is gradual, some
advantageous mutations with large, discrete effects
on the phenotype have probably played a role (see
Chapter 20). Whether or not evolution has been
entirely gradual is an empirical question, not a theo-
retical necessity. - The fossil record does not contain any transitional forms
representing the origin of major new forms of life.
This very common claim is flatly false, for there are many
such intermediate forms (see Chapters 2, 17, and 20). - Vestigial structures are not vestigial, but functional.
According to creationist thought, an intelligent Cre-
ator must have had a purpose, or design, for each
element of His creation. Thus all features of organisms
must be functional. For this reason, creationists view
adaptations as support for their position. However,
nonfunctional, imperfect, and even maladaptive struc-
tures are expected if evolution is true, especially if a
change in an organism’s environment or way of life has
rendered them superfluous or harmful. As noted
earlier, organisms display many features, at both the
(continued)
BOX 22A
Refuting Antievolutionary Arguments (continued)
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