Human Natures. Genes, Cultures, and the Human Prospect
Frost, P. 2003. Review of Human Natures. Genes, Cultures, and the Human Prospect by P.R. Ehrlich, Reports of the National Center for Science Education 23(2): 33-34.
Genetics or culture —which has done more to shape our human nature? This age-old debate is central to Paul Ehrlich's tome on the evolution of human behavior, society, and civilization. Although he treats both sides of the debate fairly, often confessing his own uncertainty, Ehrlich clearly comes down on the side of culture. Hence the title, Human Natures, which stresses the diverse ways we've developed from a common blueprint.
In our species, cultural evolution has certainly taken over from genetic evolution. Culture can pass on and create adaptations without being constrained by generation time and the long wait for useful mutations. Yet genes still have some advantages. If a situation arises often enough, we are better off with a genetically preprogrammed response. "Hardwiring" avoids the delays of learning how to respond to a situation that may have serious consequences the first time. The "ouch!" response doesn't have to be learned. Although Ehrlich does give hardwiring some role, he generally discounts it for two reasons.
His first reason is "gene shortage." Our genomes have less than 100,000 genes and our brains 100-1,000 trillion synapses (p. 124). That's at least 1 gene for every 1 billion synapses. Ehrlich concludes from this, on the assumption that one gene has one effect, that relatively few synapses are hardwired. Actually, a single gene can have many effects. It may produce different amounts of protein for different purposes, or even different proteins, depending on the way it is regulated by other genes and the way these other genes are regulated. So the number of hardwired synapses could be much higher than 100,000 — in theory, as many as there are ways of combining 100,000 genes with each other. Furthermore, if gene shortage had indeed prevented hardwiring, even when advantageous, wouldn't natural selection have expanded the genome and reduced the proportion of junk DNA? Such selection has occurred in other species. The pufferfish packs the same genetic information into one-eighth the DNA that we do (Lewis 2002). Given our genome's unused capacity, new genes should have been easy to create when needed.
Ehrlich's second reason is "imprecision of selection" (p. 127). If a desirable effect is selected, the result is selection for a lot of other effects, most of them undesirable. So a behavior won't be hardwired unless it is really critical to survival. Ehrlich has certainly put his finger on a problem in evolution (and it is heartening to see him acknowledge that a single gene can have many effects). But the problem isn't insuperable. Typically, natural selection will bring about duplication of the gene, with one copy specializing in one set of effects and the other in another. Or it will adjust regulator genes that are associated with any undesirable effects. Such fine-tuning has made possible the evolution of anatomy from one-celled ameba to many-celled humans. Why not the evolution of behavior?
In the genes vs. culture debate, Ehrlich confesses no uncertainty on one point: Human races don't exist. He presents two undisputed facts. First, there is much more genetic variability within populations than between populations. "If all human beings except native Africans were wiped out, humanity would still retain somewhat more than 90 percent of its genetic variability" (p. 52). Second, genetic variability is largely discordant. "Whether we plot skin color, height, indices of nose or face shape, frequencies of genes controlling blood groups, or any other characteristic, the resulting maps are in most cases utterly different from one trait to the next" (p. 291). Human races are thus arbitrary: "Pick a different set of characteristics and you get a different set of 'races' ." (p. 292).
These two facts deserve closer scrutiny. First, it is true that genes vary more within human populations than between them. But we see the same thing when comparing many species. Genes vary more within single dog breeds than between dogs and wolves (Coppinger and Schneider 1995:33; Vila et al. 1997). Genes vary more within species of Lake Victoria cichlids than between them, despite clear interspecific differences in morphology and behavior (Klein et al. 1998). Genes vary so much within Lycaeides butterfly species, and so little between them, that we cannot tell these species apart by examining their mitochondrial DNA or allozyme alleles, again despite clear differences in morphology (Nice and Shapiro 1999).
Second, it is true that genetic variability is discordant across human populations. But the same discordance exists across many species. All six species of Darwin's ground finches form a genetically homogeneous genus with very little concordance between mitochondrial DNA, nuclear DNA, and morphology (Freeland and Boag 1999). Let's go back to those traits that concord so poorly within our species. How well do they concord across the species boundary between us and our primate cousins? ABO blood groups exist not only in humans but also in other primates (Klein et al. 1998). According to this trait, I probably have more in common with certain apes than with Dr. Ehrlich. Height and skin color? Again, many of us are closer to some apes than to other humans. How about dental traits? Our species would still be indistinguishable: a large suite of dental traits exists in sub-Saharan Africans and non-human primates but not in humans outside Africa (Irish 1998).
If a single-trait approach is so poor at distinguishing species, it could hardly be better at distinguishing races. The only sensible approach is a multi-trait one. In other words, we should aggregate information from many different characteristics by superimposing maps of their variability on top of each other. If we do this for humans, the resulting composite map unmistakably reveals four regional groups: 1) sub-Saharan Africans; 2) Europeans and West Asians; 3) East Asians and Amerindians; and 4) Australian Aborigines (Cavalli-Sforza et al. 1994; Mountain and Cavalli-Sforza 1997; Nei and Roychoudhury 1993).
But why must we aggregate so much to filter out the fuzziness in the data? The answer is that only a fraction of the genome changes when one population differentiates from another in response to differences in natural selection. The rest remains unchanged, either because the genes have little selective value or because they handle adaptive problems that are common to both populations. Over most of the genome, then, variability consists not of adaptive differences created by different selection pressures but rather of non-adaptive variations that similar selection pressures (or none at all) have left in place.
As the two populations become reproductively isolated, they no longer accumulate the same non-adaptive variations and so their genomes drift steadily apart. But this takes time. For instance, the two species of redpoll finches diverged some 50,000 years ago and clearly have distinct phenotypes, yet their mitochondrial DNA reveals a single undifferentiated gene pool (Seutin et al. 1995). It is no surprise, then, that so much genetic overlap exists among human populations. The earliest split among them — the Out of Africa event — is on the order of 40,000 years ago (Pritchard et al. 1999).
What about the fact that 90% of all human genetic variability lies in Africa? All that means is that the African gene pool is older and has accumulated more "junk" variability, a mere fraction of which was carried out of Africa by the small founder groups that peopled the rest of the world. The absurdity of drawing further conclusions is illustrated by another fact: a single chimpanzee subspecies has more genetic variability than all humans taken together (Gagneux et al. 1999). So what? All that means is that chimps have stayed put with the same gene pool for a longer time.
Has Ehrlich failed, then, in his quest to merge culture and genetics into a common understanding of the human prospect? Yes, but in this he is no worse than others. Such a quest, by virtue of the many areas it must cover, will enter some that lie beyond the limits of normal debate. In another age, these areas were sex and religion. Are we more enlightened today?
References
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