invent wild scenarios, but even some serious scientists specu- late 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 Cap- tain 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
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 draw- ings 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
Brain structure. The brain influences the desire for stimula- tion. 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 plea- sure and rewards them when it is found. Genetically altered mice with enhanced activity of the enzyme that makes dopamine explore their environments more; genet- ically 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 dopa- mine to get the same effect. Longer sequences are also associated with people wanting more adventure. Population variation. Overall, the heritability (genetic com-
ponent 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 sig- nificant, 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 dopa- mine 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 fit- ness, 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 dif- ferences in mating behavior between species of rodents. Prairie voles (Microtus ochrogaster) are usually monogamous, while the closely related meadow vole (Microtus pensylvanicus) is promiscu- ous. Prairie voles have more vasopressin receptors, perhaps due
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 lon- ger noncoding DNA regions near the vasopressin receptor gene. In 1999 a researcher inserted the prairie vole vasopressin recep- tor 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 sero- tonin levels, but not in a direct and simple way. It takes several weeks to work, because it must influence the whole brain sys- tem. 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 com- plex, 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 insta-
bility generates diversity in brain and sociobehavioral traits.” Science 308 (2005): 1,630–1,634. Summarized by Pennisi, Eliza- beth. “In voles, a little extra DNA makes for faithful mates.” Sci- ence 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 Chap- ters. New York: HarperCollins, 1999.
———. The Agile Gene: How Nature Turns on Nurture. New York: HarperPerennial, 2004.
abundant photosynthetic organism in the oceans, was not dis- covered until 1988. There is no end in sight for discovering new insect species. Partly this is because there are so many of them (350,000 species of beetles and counting!) but also because they are so hard to see.
Only an expert can recognize many of the distinctions between closely related species. Flies are often classified using the arcane science of chaetotaxy, which distinguishes them on the basis of the arrangements of their bristles. Clearly, one limiting factor to the discovery of new species is the availabil- ity of experts who can recognize that they are new.
About a million and a half species have been named. Some biologists estimate that there may be as many as 30 million to 100 million species in the world. Ecologist Terry Erwin calculated this estimate. He began by using pesticides to kill all the insects in certain tropical trees and collecting the insects that fell. By estimating how many of these insects were unique to certain species of trees, and extrapolating to the number of tree species, he was able to estimate the number of tropical insect species. From there he could estimate the num- ber of insect species in the world, and then of all other spe- cies, using existing proportions of each taxonomic category. Although his estimate is probably high, it is certain that there are many more species than those already discovered. Robert May estimates just under seven million (see table at right).
Biodiversity (as indicated by the number of families of fossilized marine and terrestrial organisms) has increased through geological time. After each of the five massextinctions in earth history (five arrows), biodiversity decreased (especially after the Permianextinction, largest arrow) but within a few million years began to recover and continue its increase. (Redrawn from Benton)