Behavioural evolution

Although the selective effects of psychological pressures are difficult to measure, they must also play a role. Michael Marmot and his colleagues have shown that an individual's position in a work hierarchy has an impact on his or her health (Singh-Manoux et al., 2005). The members at the top level of the hierarchy live healthier lives than those at the bottom level - or even those who occupy positions slightly below the top level!

Each population's geographic origin number of individuals and frequency o) haplogtöup D chromosomes are gruen in parentheses as follows:

t. Southeastern and Southwestern Banlu (Soulh Africa. a, 0%)i 2. San ¡Namibia, 7, 0%); 3. Mbuti Pygmy (Democralic Republic of Congo 14 7 1%); 4 Masaal (Tanzania. 26, 11.5%); 5. Sandawe (Tanzania, 33, 22.7%): 6, Burunge (Tanzania. 28, 12.5%); 7, Turu (Tanzania, 27, 7.4%); 8, Northeastern Bantu (Kenya. 12, 4.2%); 9. 8,ate Pygmy (Central African Republic. 32. 6.3%). ID, Zime (Cameroon, 25. S%i 11, Sakola Pygmy (Cameroon, 27. 0%); 12, Bamoun (Cameroon, 29, 5.2%); 13, Yoruba (Nigeria, 24, 2.1%); 14. Mandenka (Senegal. 24, 4.2%); IS. Mozabile (Algeria (Mzab region). 29, 31%); 16 Druze [Israel (Carmel region), 45, 52.3%]; 17, Palestinian [Israel (Central), 43. 46.5%]; 18, Beduum [Israel (Negev "region). 46. 37%]; 19, Harare (Pakistan. 19,15.8%); 20. Balochi (Pakisian, 23.13%}; 21, Pathan (Pakistan. 21. 40.5%); 22. Burusho (Pakistan, 23.28.3%); 23, Makrani (Pakistan. 25, 32%); 24. Biahui (Pakistan, 24, 33.3%); 25. Kaiash (Pakistan. 25, 60%); 26, Sindlli (Pakistan. 25. 44%). 27. Hezhen (China, 9. 5.6%); 28, Mongola (China, 9. 11.1%); 29, Daur (China. 6,6 3%); 30. Orogen (China. 10, 5%); 31. Miaoiu (China. 10.10%): 32, Ytzu (China, 10, 25%); 33, Tuiia China 10 20%); 34. Han (China, 42. 17 3%); 35, Xlbo (China, 9. 0%). 36. Uygur (China. 10. 30%); 37. Dai (China. (0, 25%): 38, Lahu (China 10 10%)- 39 She (China. 7, 21,4%); 40. Naxi [China. 9, 11.1%); 41, Tu (China, 9, 16.7%); 42, Cambodian (Cambodia, I0, 0%), 43. .Japanese (Japan 28 10.7%) 44 Yakul (Russia (Siberia region), 24, 12.5%], 45, Paouan (New Guinea. 16 59.4%): 46 NAN Melanesian (Bougainville 18. 11 1%j 47 French Basgue (Franco. 15. 40%). 48. French (France. 29, 50%); 49, Sardinian (lialy, 27. 46.3%); 50, Morth Italian [Italy (Bergamo region), 12. 45.8% ¡; 51. Tuscan (lUiiy, 8, 37,5%); 52, Orcadian (Orkney Islands, 16, 40.61«.); 53. Russian (Russia. 25. 33%); 54, Adygei [Russia (Caucasus region) 15. 40%]- 55 Karitiana (Brazil, 34, 0%): 56, Surji (Brazil. 21,0%); 57, Colombian (Colombia 13, 3.8%); 56, Pima (Mexico, 25, 2%); SQ, Maya (Mexico, 24, 12.5%).

\ . 3.2 ■Worldwide frequencies of ASPM haptogroup D chromosomes {defined as having the derived G aJieJe at the A44S71G diagna

1i i..'.:, | 1', 'UlsCl'. <, II 1, Tof ', 1 HI; 1I; i IV , I \ I,,s

Reproduction also applies psychological pressures. The introduction of effective means of birth control has provided personal choice in reproduction for more people than at any time in the past. We can only surmise how selection for genes that influence reproductive decision-making will affect our future evolution. There is some evidence from twin studies, however, that heritability for reproductive traits, especially for age at first reproduction, is substantial (Kirk et al, 2001). Thus, rapid changes in population growth rates such as those being experienced in Europe and the former Soviet Union are likely to have evolutionary consequences.

Concerns over dysgenic pressures on the human population resulting from the buildup of harmful genes (Muller, 1950) have abated. It is now realized that even in the absence of selection, harmful mutant alleles will accumulate only slowly in our gene pool and that many human characteristics have a complex genetic component so that it is impossible to predict the effects of selection on them. Alleles that are clearly harmful - sickle cell, Tay-Sachs, muscular dystrophy and others - will soon be amenable to replacement by functional alleles through precise gene surgery performed on germ line cells. Such surgery, even though it will be of enormous benefit to individuals, is unlikely to have much of an effect on our enormous gene pool unless it becomes extremely inexpensive.

One intriguing direction for current and future human evolution that has received little or no attention is selection for intellectual diversity. The measurement of human intellectual capabilities, and their heritability, is at a primitive stage. The heritability of IQ has received a great deal of attention, but a recent meta-analysis estimates broad heritability of IQ to be 0.5 and narrow heritability (the component of heritability that measures selectable phenotypic variation) to be as low as 0.34 (Devlin et al., 1997). But IQ is only one aspect of human intelligence, and other aspects of intelligence need investigation. Daniel Goleman has proposed that social intelligence, the ability to interact with others, is at least as important as IQ (Goleman, 1995), and Howard Gardner has explored multiple intelligences ranging from artistic and musical through political to mechanical (Gardner, 1993). All of us have a different mix of such intelligences. To the extent that genes contribute to them, these genes are likely to be polymorphic - that is, to have a number of alleles, each at appreciable frequency, in the human population.

This hypothesis of polymorphic genes involved in behaviour leads to two predictions. First, loci influencing brain function in humans should have more alleles than the same loci in chimpanzees. Note, however, that most of the functional polymorphic differences are likely to be found, not in structural genes, but in the regulatory regions that influence how these genes are expressed. Because of the difficulty of determining which genetic differences in the polymorphisms are responsible for the phenotypic effects, it may be some time before this prediction can be tested.

The second prediction is that some type of balancing selection, probably with a frequency-dependent component, is likely to be maintaining these alleles in the human population.

When an allele has an advantage if it is rare but loses that advantage if it is common, it will tend to be maintained at the frequency at which there is neither an advantage nor a disadvantage. If we suppose that alleles influencing many behaviours or skills in the human population provide an advantage when they are rare, but lose that advantage when they are common, there will be a tendency for the population to accumulate these alleles at such intermediate frequencies. And, as noted earlier, if these genes are maintained by frequency dependence, the cost to the population of maintaining this diversity can be low.

Numerous examples of frequency-dependent balancing selection have been found in populations. One that influences behaviours has been found in Drosophila melanogaster. Natural populations of this fly are polymorphic for two alleles at a locus {for, standing for forager) that codes for a protein kinase. A recessive allele at this locus, sitter, causes larvae to sit in the same spot while feeding. The dominant allele, rover, causes its carriers to move about while feeding. Neither allele can take over (reach fixation) in the population. Rover has an advantage when food is scarce, because rover larvae can find more food and grow more quickly than sitter larvae. Sitter has an advantage when food is plentiful. If they are surrounded by abundance, sitter larvae that do not waste time and effort moving about can mature more quickly than rovers (Sokolowski etal., 1997).

It will be fascinating to see whether behaviour-influencing polymorphisms such as those at the for locus are common in human populations. If so, one type of evolutionary change may be the addition of new alleles at these loci as our culture and technology become more complex and opportunities for new types of behaviours arise. It is striking that the common MCPH1 allele has not reached fixation in any human population, even though it has been under positive selection since before modern humans spread to Europe. It may be that this allele is advantageous when it is rare but loses that advantage when it becomes common. There appear to be no behavioral effects associated with this allele (Mekel-Bobrov et al., 2007), but detailed studies may reveal small differences. Natural selection can work on small phenotypic differences as well as large, and much of our recent evolution may have resulted from selection for genes with small effects.

3.4.2 The future of genetic engineering

There has been much speculation about the effects of genetic engineering on the future of our species, including the possibility that a 'genetic elite may emerge that would benefit from such engineering to the exclusion of other human groups (e.g., Silver, 1998). Two strong counterarguments to this viewpoint can be made.

First, the number of genes that can potentially be modified in our species is immense. Assuming 50,000 genes per diploid human genome and 6 billion individuals, the number of genes in our gene pool is 3 x 1014. The task of changing even a tiny fraction of these genes would be enormous, especially since each such change could lead to dangerous and unexpected side effects. It is far more likely that our growing understanding of gene function will enable us to design specific drugs and other compounds that can produce desirable changes in our phenotypes and that these changes will be sufficiently easy and inexpensive that they will not be confined to a genetic elite (Wills, 1998). Even such milder phenotypic manipulations are fraught with danger, however, as we have seen from the negative effects that steroid and growth hormone treatments have had on athletes.

Second, the likelihood that a 'genetic elite' will become established seems remote. The modest narrow (selectable) heritability of IQ mentioned earlier shows the difficulty of establishing a genetic elite through selection. Heritabilities that are even lower are likely to be the rule for other physical or behavioural characteristics that we currently look upon as desirable.

Attempts to establish groups of clones of people with supposedly desirable characters would also have unexpected and unpredictable effects, in this case because of the environment. Clones of Bill Gates or Mother Teresa, growing up at a different time and in a different place, would turn into people who reflected the influences of their unique upbringing, just as the originals of such hypothetical clones did. And, luckily, environmental effects can work to defang evil dysgenic schemes as well as Utopian eugenic ones. 'The Boys from Brazil' notwithstanding, it seems likely that if clones of Adolf Hitler were to be adopted into well-adjusted families in healthy societies they would grow up to be nice, well-adjusted young men.

3.4.3 The evolution of other species, including those on which we depend

Discussions of human evolution have tended to ignore the fact that we have greatly influenced the evolution of other species of animals and plants. These species have in turn influenced our own evolution. The abundant cereals that made the agricultural revolution possible were produced by unsung generations of primitive agriculturalists who carried out a process of long-continued artificial selection. Some results of such extremely effective selection are seen in Indian corn, which is an almost unrecognizable descendent of the wild grass teosinte, and domesticated wheat, which is an allohexaploid with genetic contributions from three different wild grasses. The current immense human population depends absolutely on these plants and also on other plants and animals that are the products of thousands of generations of artificial selection.

One consequence of climate change such as global warming is that the agriculture of the future will have to undergo rapid adaptations (see also Chapter 13, this volume). Southern corn leaf blight, a fungus that severely damaged corn production in the Southeast United States during the 1970s, was controlled by the introduction of resistant strains, but only after severe losses. If the climate warms, similar outbreaks of blight and other diseases that are prevalent in tropical and subtropical regions will become a growing threat to the world's vast agricultural areas.

Our ability to construct new strains and varieties of animals and plants that are resistant to disease, drought, and other probable effects of climate change depends on the establishment and maintenance of stocks of wild ancestral species. Such stocks are difficult to maintain for long periods because governments and granting agencies tend to lose interest and because societal upheavals can sometimes destroy them. Some of the stocks of wild species related to domestic crops that were collected by Russian geneticist Nikolai Vavilov in the early part of the twentieth century have been lost, taking with them an unknown number of genes of great potential importance. It may be possible to avoid such losses in the future through the construction of multiple seed banks and gene banks to safeguard samples of the planet's genetic diversity. The Norwegian government recently opened a bunker on Svalbard, an Arctic island, designed to hold around 2 million seeds, representing all known varieties of the world's crops. Unfortunately, however, there are no plans to replicate this stock centre elsewhere.

Technology may aid us in adjusting to environmental change, provided that our technological capabilities remain intact during future periods of rapid environmental change. To cite one such example, a transgenic tomato strain capable of storing excess salt in its leaves while leaving its fruit relatively salt-free has been produced by overexpression of an Arabidopsis transporter gene. The salt-resistant plants can grow at levels of salt 50 times higher than those found in normal soil (Zhang and Blumwald, 2001). The ability to produce crop plants capable of growing under extreme environmental conditions may enable us to go on feeding our population even as we are confronted with shrinking areas of arable land.

Continue reading here: Future evolutionary directions

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