EvoluTion And SoCiETy 583
virus (HIV), ill fit our concept of an intelligent, kindly designer, but they are easily
explained by natural selection (see Chapter 13).
No one has yet demonstrated a characteristic of any species that serves only to
benefit a different species, or only to enhance the so-called balance of nature—
for, as Darwin saw, “such could not have been produced through natural selec-
tion.” Because natural selection consists only of differential reproductive success,
it results in “selfish” genes and genotypes, some of which have results that are
inexplicable by intelligent design (see Chapter 12). We have seen that genomes are
brimming with sequences such as transposable elements that increase their own
numbers without benefiting the organism. We have seen maternally transmitted
cytoplasmic genes that cause male sterility in many plants, and nuclear genes that
have evolved to override them and restore male fertility (see Chapter 12). Such
conflicts among genes in a genome are widespread. Are they predicted by intel-
ligent design theory? Likewise, no theory of design can predict or explain features
that we ascribe to sexual selection, such as males that remove the sperm of other
males from the female’s reproductive tract (see Chapter 10). Nor can we rational-
ize why a beneficent designer would shape the many other selfish behaviors that
natural selection explains, such as cannibalism, siblicide, and infanticide.Evolution, and its mechanisms, observed
Anyone can observe erosion, and geologists can measure the movement of con-
tinental plates, which travel up to 10 cm per year. No geologist doubts that these
mechanisms, even if they accomplish only slight changes on the scale of human
generations, have shaped the Grand Canyon and have separated South America
from Africa over the course of millions of years. Likewise, biologists do not expect
to see anything like the origin of mammals played out on a human time scale, but
they have documented the processes that will
yield such grand changes, given enough time.
We know that genetic variation in all kinds
of phenotypic characters originates by mutation
(see Chapters 4 and 6). This variation has been
used by humans for millennia to develop strains
of domesticated plants and animals that differ in
morphology more than whole families of natural
organisms do. In experimental studies of labo-
ratory populations of microorganisms, we have
seen new advantageous mutations arise and
enable rapid adaptation to temperature changes,
toxins, or other environmental stresses. Evolu-
tionary biologists have documented hundreds of
examples of natural selection acting on genetic
and phenotypic variation, and of rapid adapta-
tion to new environmental factors (see Chapters
3, 5, and 6)—including resistance to pesticides
and antibiotics (FIGURE 22.4). Within the past
century, some populations (e.g., the apple mag-
got; see Chapter 9) have almost become differ-
ent species.
In summary, the major causes of evolution
have been extensively documented. The twoFIGURE 22.4 Cartoonist Garry Trudeau affirms the
importance of evolutionary science.22_EVOL4E_CH22.indd 583 3/22/17 1:49 PM