Encyclopedia of Evolution.pdf - Online Reading Center

• Fitness risks. By repressing the binding of insulin, the
hormone Klotho acts as an anti- aging hormone in mice.
Underexpression of the gene for this hormone accelerates
aging in, and overexpression extends the life span of, mice.
However, the longer-lived mice have lower fertility.
The expense of producing a long-lived body may so
greatly impair reproduction that natural selection will favor
shorter-lived organisms, if this allows them a greater total
reproductive output. For animals, the continual renewal of
components that are damaged, such as the beak of a woodpecker,
would be enormously expensive—and that expense
would come at the cost of reproductive fitness. Since very old
individuals would be rare even in a population of biologically
immortal animals (most would already have died from
accidents), natural selection has not favored the evolution of
adaptations that prolong the lives of very old individuals (see
essay, “Why Do Humans Die?”). The reptiles with some of
the longest life spans are turtles, and the mammals with some
of the longest life spans (aside from humans) are porcupines,
both of which are relatively sluggish and well protected from
injury. Therefore natural selection will not eliminate mutations
that bring harm only to older individuals. One reason
is that, in most populations, few individuals live to be old.
Another reason is that even those individuals that do live
to be old will have very little reproductive life left ahead of
them. Scientists such as biologist Peter Medawar have pointed
out that pleiotropic genes, which have more than one effect,
may be responsible for aging (senescence). Such genes may
enhance fitness in younger individuals even if they also reduce
the survival of older individuals.
Those few organisms that do live for a very long time
(all of them trees) either possess very expensive adaptations,
or else grow slowly. An example of the former is the giant
sequoia (Sequoiadendron giganteum), many of which are
more than 2,000 years old, and which produce very thick
fire-retardant bark. An example of the latter is the bristlecone
pine (Pinus longeava) in California. Many of these
pines are more than 4,500 years old but have grown less
than 30 feet (10 m) tall during that time, because of the
harshly cold and dry environment in which they live. Many
of the world’s oldest and largest trees, such as the ancient
lime (linden) trees of England and Germany, are not actually
the original trees but are the outgrowth of branches or
roots that the original tree produced, from which the original
trunks have been lost.
Virginia opossums (Didelphus virginiana) on the mainland
of the Southeastern United States have evolved in the
presence of predators, but on Sapelo Island there are no
predators. The island opossums have evolved delayed old-age
senescence (as measured by the breaking time of their collagen
fibers). They also reproduce relatively more in their second
year of life, and less in their first year, compared to mainland
opossums. This pattern suggests that under more dangerous
conditions, natural selection favors earlier reproduction and
favors (or at least does not disfavor) more rapid aging.
Humans are unusual not only in the enormous amount
of care they give to offspring but also in the enormously pro-
life history, evolution of
longed post-reproductive life span. In most animals, once
reproduction is finished, the individuals die. A comparison
among different primates shows that chimps and humans
have slightly longer infancy and juvenile periods, relative to
the life span up to the end of reproduction, than do other
primates. But the most striking difference is the human postreproductive
period, which adds another one-third to the life
span (see table on page 241). In humans, the post-reproductive
life span can be almost as long as the juvenile and reproductive
life spans combined. The human with the longest life
span, verified by a birth certificate, was Jeanne Calment, who
died in Arles, France, at the age of 122 in 1997.
The explanation usually given for this is that the older
individuals possess a wealth of knowledge that can prove
immensely valuable to individual families and to the tribe as
a whole. This body of knowledge can be so large that it takes
a lifetime to teach. A bird can learn its repertory of songs
and how to forage in a single season, but human knowledge
fills a lifetime longer than the reproductive life span. Since
the invention of writing, this knowledge can be passed from
one generation to another impersonally, but through most of
human evolutionary history old people, not books or databases,
were the repository of tribal knowledge, especially
about the uses of the many wild species. Science writer Natalie
Angier calls grandmothers the “Alexandrian libraries for
preliterate tribes.” This cultural knowledge would directly
benefit the wise old person’s descendants.
Evolutionary scientists have had a particularly difficult
time explaining the origin of menopause. Many evolutionary
scientists consider menopause to be a nonadaptive side effect
of the evolution of human life history. Menopause evolved
accidentally, and since very few old women survived in prehistoric
societies, there was no selection against menopause.
In contrast, some evolutionary biologists, such as Kristin
Hawkes, say that menopause is adaptive. Dubbed the “grandmother
hypothesis,” this theory says that a woman may have
greater inclusive fitness (see altruism) by devoting herself to
the nurture of her grandchildren than by continuing to have
her own children, once she is old. This may explain why
older women seldom reproduce, but how could natural selection
have favored the sudden enforced cessation of reproduction?
The reason may be that continued reproduction, even
continued reproductive ability, in older women may present
physiological risks:
• Breast cancer. There is a correlation between the number
of menstrual cycles and the risk of breast cancer, since the
hormones that the woman’s body produces after ovulation
(estrogen and progesterone) stimulate breast cell division.
Continuation of menstrual cycles into old age could
therefore represent a risk. Some evolutionary scientists
conclude from this that menopause reduced breast cancer
incidence in older women. Other evolutionary scientists
point out that women have far fewer menstrual cycles in
primitive societies than in civilized societies, because they
were usually pregnant or lactating. Breast cancer incidence
is much lower in societies with high reproductive rates. In