Encyclopedia of Evolution.pdf - Online Reading Center

Brain structure. The brain influences the desire for stimulation.
The nucleus accumbens is the pleasure center of the
mammalian brain. When rats are allowed to self-stimulate
this pleasure center (through an electrode), they do so,
even to the extent of neglecting food.
Brain chemistry. Dopamine stimulates animals to seek pleasure
and rewards them when it is found. Genetically
altered mice with enhanced activity of the enzyme that
makes dopamine explore their environments more; genetically
altered mice lacking the enzyme sit and starve. In
humans, the D4 dopamine receptor on chromosome 11
has a noncoding region of 48 bases that can be repeated
two to 11 times. The longer sequences of this region result
in less binding of dopamine—that is, it takes more dopamine
to get the same effect. Longer sequences are also
associated with people wanting more adventure.
Population variation. Overall, the heritability (genetic component
of variability among humans; see population
genetics) is 40 percent for the human tendency to seek
stimulation. Although the correlation of the form of D4
receptor with stimulation seeking was statistically significant,
it explained only 4 percent of the variation. If this
gene explains only 4 percent out of the 40 percent total
genetic effect, there might be 10 genes involved in dopamine
regulation or other aspects of seeking stimulation.
Therefore, one cannot claim to have found “the gene” that
explains this part of human behavior. There must be many
genes, and taken together they influence less than half of
the variation in this behavior pattern among humans.
Evolutionary advantage. What evolutionary advantage might
one or the other form of the D4 gene have had? Both forms
of the gene can confer advantages. Individuals with the
long form of the gene would pursue many sexual partners;
individuals with the short form would tend to care more for
their offspring. Both forms of the gene may enhance fitness,
in different ways, depending on the circumstances.
Mating behavior in mammals is also influenced by hormone
levels, which have a genetic basis. Variation in the noncoding DNA
associated with vasopressin receptors appears to explain the differences
in mating behavior between species of rodents. Prairie
voles (Microtus ochrogaster) are usually monogamous, while the
closely related meadow vole (Microtus pensylvanicus) is promiscuous.
Prairie voles have more vasopressin receptors, perhaps due
invent wild scenarios, but even some serious scientists speculate
that super rats and tree-sized weeds may evolve. This will
occur on such a long timescale that during one’s lifetime a
human will see virtually nothing but extinctions.
Biodiversity seems like such an easy thing to measure:
Just go out and look for species, write them down, and count
them up. Sir Joseph Banks, when he was the botanist on Captain
Cook’s voyages, increased the number of plant species
known to Western science by a quarter by this method. This
process of discovery has nearly been completed for mammals
and birds, which are easy for human explorers to see, though
biodiversity
to the longer noncoding region associated with the vasopressin
receptor gene, than meadow voles. Some prairie voles are more
monogamous than others and more attentive to their offspring. The
more monogamous and attentive prairie vole parents also have longer
noncoding DNA regions near the vasopressin receptor gene.
In 1999 a researcher inserted the prairie vole vasopressin receptor
gene, together with its associated noncoding DNA, into mouse
chromosomes. The resulting mice were more monogamous and
more attentive to their offspring than normal mice.
The overall message is that genes do not determine, but can
strongly influence, human behavior, which therefore has a strong
evolutionary component. First, even where there is a genetic
basis, many genes can be involved. The effects of the genes can
be complex and indirect. For example, Prozac influences serotonin
levels, but not in a direct and simple way. It takes several
weeks to work, because it must influence the whole brain system.
If its effect was only a straightforward effect on serotonin
levels, it would start to work right away. Second, there is often
evolutionary selection for different alleles of the same gene—for
example, both for seeking stimulation and for not seeking it, for
storing weight and for not storing weight. Both the pathway of
causation and the effects of natural selection can be very complex,
but very real.
Further Reading
Hamer, Dean, and Peter Copeland. Living with Our Genes: Why They
Matter More Than You Think. New York: Bantam, 1998.
Hammock, Elizabeth A. D., and Larry J. Young. “Microsatellite instability
generates diversity in brain and sociobehavioral traits.”
Science 308 (2005): 1,630–1,634. Summarized by Pennisi, Elizabeth.
“In voles, a little extra DNA makes for faithful mates.” Science
308 (2005): 1,533.
Lewontin, Richard. It Ain’t Necessarily So: The Dream of the Human
Genome and Other Illusions. New York: New York Review of
Books, 2000.
Pennisi, Elizabeth. “A genomic view of animal behavior.” Science
307 (2005): 30–32.
Ridley, Matt. Genome: The Autobiography of a Species in 23 Chapters.
New York: HarperCollins, 1999.
———. The Agile Gene: How Nature Turns on Nurture. New York:
HarperPerennial, 2004.
a few new species are occasionally found. A new species of
deer was discovered recently in Vietnam. There are 30,000
okapis (relatives of giraffes) in the jungles of Zaire, but they
were unknown to Western science until the 20th century.
Riwoche horses in Tibet were known only from cave drawings
and were assumed to be extinct, until some explorers
who got lost found them in a remote valley in 1995.
Biologists are still discovering many hundreds of species
of microbes, plants, and terrestrial arthropods such as insects
(see bacteria, evolution of; invertebrates, evolution
of). The photosynthetic bacterium Prochlorococcus, the most