46 CHAPTER 2 | Genetics and Evolution
lethal, this allele’s frequency would still decrease only
gradually. Even with complete selection against those
homozygous for this allele, the allele would persist in the
offspring of heterozygotes. In the first several genera-
tions, the frequency of the allele would decrease at a rela-
tively rapid rate. However, with time, as the frequency
of the recessive allele drops, the probability of forming
a recessive homozygote also drops, so that it would take
many generations to realize even a small decrease in al-
lele frequency. This is compounded by the fact that a
human generation takes about twenty-five years (forty
generations would span over a thousand years). Nev-
ertheless, even such small and slow changes can have a
significant cumulative impact on both the genotypes and
phenotypes of any population.
As a consequence of the process of natural selection,
populations generally become well adapted to their en-
vironments. For example, consider the plants and ani-
mals that survive in the deserts of the western United
States. Members of the cactus family have extensive root
networks close to the surface of the soil, enabling them
to soak up the slightest bit of moisture; they are able to
store large quantities of water whenever it is available;
they are shaped so as to expose the smallest possible sur-
face to the dry air and are generally leafless as mature
plants, thereby preventing water loss through evapora-
tion; and a covering of spines discourages animals from
chewing into the juicy flesh of the plant. Desert animals
are also adapted to their environment. The kangaroo rat
can survive without drinking water; many reptiles live in
burrows where the temperature is lower; most animals
are nocturnal or active only in the cool of the night. Byeliminated from the population by disease, predation,
or starvation. Obviously, the survival of the fittest has
some bearing on natural selection. But there are many
cases in which “less fit” individuals survive, and even do
quite well, but do not reproduce. They may be incapable
of attracting mates, or they may be sterile, or they may
produce offspring that do not survive after birth. For ex-
ample, among the Uganda kob, a kind of antelope native
to East Africa, males that are unable to attract females
form bachelor herds in which they live out their lives. As
members of a herd, they are reasonably well protected
against predators, and so they may survive to relatively
old ages. They do not, however, pass on their genes to
succeeding generations.
Ultimately, all natural selection is measured in terms of
reproductive success—mating and production of viable
offspring who will in turn carry on one’s genes. Reproduc-
tive success is also a powerful social phenomenon in some
human societies where a woman’s social worth is assessed
in terms of her ability to bear children. In these contexts
infertility becomes a human rights issue, as described in
the Anthropology Applied feature.
The change in genetic variants in human popula-
tions can be very slow. For example, if an environment
changed such that a recessive allele that had been pres-
ent in humans at a modest frequency suddenly became
reproductive success The relative production of fertile
offspring by a genotype. In practical terms, the number of off-
spring produced by individual members of a population is tal-
lied and compared to that of others.© Camille Tokerud/Getty ImagesAcross the globe, newborn
babies weigh on average be-
tween 5 and 8 pounds. Sta-
bilizing selection seems to be
operating here to keep infant
size well matched to the size
of the human birth canal for
successful childbirth. Natural
selection can promote stabil-
ity as well as change.
©^
Camille Tokerud/Getty Images