The Case of Sickle-Cell Anemia 47insect wings. These did not arise so that insects might fly,
but rather as structures that were used to “row,” and later
skim, across the surface of the water.^11 Later, the larger
ones by chance proved useful for purposes of flight. In
both the kiwi eggs and the insect wings, what we see is
natural selection operating as “a creative scavenger, taking
what is available and putting it to new use.”^12
The adaptability of organic structures and functions, no
matter how much a source of wonder and fascination, nev-
ertheless falls short of perfection. This is so because natural
selection can only work with what the existing store of ge-
netic variation provides; it cannot create something entirely
new. In the words of one evolutionary biologist, evolution
is a process of tinkering, rather than design. Often tinker-
ing involves balancing beneficial and harmful effects of a
specific allele, as the case of sickle-cell anemia illustrates.The Case of
Sickle-Cell Anemia
Among human beings, sickle-cell anemia is a particularly
well-studied case of adaptation (Figure 2.7). This pain-
ful disease, in which the oxygen-carrying red blood cellsextrapolation, biologists assume that the same adaptive
mechanisms also work on behavioral traits. These theo-
ries and how they influence the evolution of humans will
be discussed in Chapter 4.
Natural selection may also promote stability, rather
than change. Stabilizing selection occurs in populations
that are already well adapted or where change would be
disadvantageous. In cases where change is disadvanta-
geous, natural selection will favor the retention of allele
frequencies more or less as they are. However, the evolu-
tionary history of most life forms is not one of constant
change, proceeding as a steady, stately progression over
vast periods of time; rather, it is one of prolonged pe-
riods of relative stability or gradual change punctuated
by shorter periods of more rapid change (or extinction)
when altered conditions require new adaptations or
when a new mutation produces an opportunity to adapt
to some other available environment. According to the
fossil record, most species survive between 3 and 5 mil-
lion years.^9
Although it is true that all living organisms have many
adaptive characteristics, it is not true that all characteris-
tics are adaptive. All male mammals, for example, possess
nipples, even though they serve no useful purpose. To fe-
male mammals, however, nipples are essential to repro-
ductive success, which is why males have them. The two
sexes are not separate entities, shaped independently by
natural selection, but are variants upon a single body plan,
elaborated in later embryology. Precursors of mammary
glands are built in all mammalian fetuses, enlarging later
in the development of females, but remaining small and
without function in males.
Nor is it true that current utility is a reliable guide to
historical origin or future use. For one thing, traits that
seem nonadaptive may be co-opted for later use, and
traits that appear adaptive might have come about due to
unrelated changes in the pattern of growth and develop-
ment. For instance, the unusually large size of a kiwi’s egg
enhances the survivability of kiwi chicks, in that they are
particularly large and capable when hatched. Nevertheless,
kiwi eggs probably did not evolve to this large size because
the size is adaptive. Rather, kiwis evolved from an ances-
tor that was the size of an ostrich, and in birds, egg size
reduces at a slower rate than does body size. Therefore, the
outsized eggs of kiwi birds seem to be no more than a de-
velopmental byproduct of a reduction in body size.^10
Similarly, an existing adaptation may come under
strong selective pressure for some new purpose, as did
stabilizing selection Natural selection acting to promote
stability rather than change in a population’s gene pool.(^11) Kaiser, J. (1994). A new theory of insect wing origins takes off. Science
266 , 363.
(^12) Dorit, R. (1997). Molecular evolution and scientific inquiry, misperceived.
American Scientist 85, 475.
(^9) Thompson, K. S. (1997). Natural selection and evolution’s smoking gun.
American Scientist 85, 516.
(^10) Gould, S. J. (1991). Bully for brontosaurus (pp. 109–123). New York:
Norton.
Otorohanga Zoological Society
This x-ray showing the unusually large size of a kiwi egg illustrates
that evolution does not continue by preplanned design but rather by
a process of tinkering with preexisting body forms.