Encyclopedia of Evolution.pdf - Online Reading Center

iodiversity
How Much Do Genes Control Human
Behavior? (continued)
suggests that the birth of older siblings influences the hormonal
environment that a fetus experiences. Identical twins, separated at
birth, shared the same prenatal environment.
As an example of the interaction of genes and environment,
Hamer cites a Swedish study of adoptions and petty crimes. The
researchers assumed “bad genes” when both biological parents
were criminals, and “bad home” when both adoptive parents were
criminals. Here were the effects of genes and homes on the incidence
of petty crimes in the youth from these homes:
Good genes, good home: 3 percent petty crime rate
Good genes, bad home: 7 percent petty crime rate
Bad genes, good home: 12 percent petty crime rate
Bad genes, bad home: 40 percent petty crime rate
Good genes, good home (3 percent crime rate) was similar to
population at large. The influence of genes seems to exceed that of
environment, but the difference between 7 percent and 12 percent
may not be biologically significant. Hamer says that home environment
has most of its influence on the behavior of kids, but genes
have more of an effect on behavior in adulthood. Hamer therefore
suggests that bad environments put kids in juvenile hall but bad
genes put adults in jail.
Consider these other examples of genetic influence (not
genetic determination) on human behavior:
Appetite. Another example that Hamer cites is obesity. Body
weight is mostly an inherited trait. It takes hard work to change one’s
body weight. For some people it is easy to be thin—their bodies burn
the calories—while for others it is difficult to be thin, because their
bodies store calories. Is there a genetic basis for appetite?
Brain structure. In studies with mice, it has been found that
the hunger center of the brain is in the hypothalamus. If
the hunger center of the hypothalamus is removed, the
mice starve even in the presence of food; and if the hunger
center is stimulated, the mice overeat.
Brain chemistry. A mutation that causes obesity in mice was
found to influence the production of the protein leptin
(“thin” in Greek). Other mutations had the same effect—it
did not matter which mutation it was, so long as it influ-
tropical rain forests. Two acres (a hectare) of tropical rain
forest can contain as many tree or bird species as the entire
United States. The rain forests, though covering less than 5
percent of the Earth’s surface, may contain half of its species.
The equable climatic conditions of the tropics allow them to
be a museum of old species. It has so many species interactions
that it is also a prime nursery of new species.
Biodiversity has been repeatedly reduced by global catastrophes,
most notably the five mass extinctions in Earth
history. Biodiversity has increased after each mass extinction
event. Sometimes it takes a long time for biodiversity to
enced leptin production. Humans also have the leptin gene,
and when the human version of the gene is inserted into
mice, it works the same way as the mouse gene. The mutation
appears to increase appetite and lower metabolism.
Although humans have a leptin gene, the mutant leptin gene
was found in only a few grossly obese humans. Therefore
the genetic basis of human obesity must be mostly in other
genes. Mutations in many different metabolic genes—in
any genes that normally cause fat to burn faster—could
influence appetite and cause obesity. A mutation affecting
the production of a certain brain neuropeptide makes rats
eat even materials that taste bad or while receiving a shock.
The human counterpart might be that the brain chemical
serotonin is known to be associated with anxiety, depression,
and a craving for carbohydrates. Hormones influence
appetites in a complex fashion: Separate hormonal mechanisms
are involved in hunger for carbohydrates, for fats,
and for proteins, in a manner not understood.
Population variation. In twin studies, body weight had a 70
percent correlation. There is also a genetic effect on
type and location of body fat. A correlation was found
between people with potbellies and a gene that causes
blood vessels to constrict. This study was conducted in a
homogeneous population of Hutterites, in order to restrict
other sources of variability. Of course, environment is also
important. Many people of the Pima tribe in Arizona are
obese, but this is not the case with the Pima in Mexico.
The two populations separated just 1,000 years ago.
Evolutionary advantage. How does natural selection fit in to
these considerations? In the prehistoric past, individuals
who burned calories rather than saving them were less
likely to survive famines, unless they compensated for it
by violence. The ability to store calories, which evolutionary
geneticists call the “thrifty genotype,” was beneficial
in the human evolutionary past. Today, it can lead to obesity
(see evolutionary medicine).
Stimulation-seeking behavior. Another example of a genetic
influence on behavior is the desire for stimulation. This is an important
behavioral factor. People with an inclination to seek stimulation
are more likely to divorce; and one of the best predictors for
divorce is when one partner is inclined to seek stimulation and the
other is not.
recover. After the Permian extinction, more than a hundred
million years was required to restore the pre-disaster
diversity levels (see figure on page 48).
Many scientists estimate that a sixth mass extinction,
caused by human activity, is now under way. A combination
of human activities is causing these extinctions. The most
important is habitat destruction: Humans convert natural
habitats into farmlands and cities that are unsuitable for wild
species. Recovery will occur from this extinction event also.
New species will eventually evolve suitable to the new, disturbed
conditions created by humans. Science fiction writers