Home > Uncategorized > Appendix 1: Major theories supporting the evolution of population specific difference in cognition

Appendix 1: Major theories supporting the evolution of population specific difference in cognition

Appendix 1: Major theories supporting the evolution of population specific difference in cognition

Selection due to differences in Population Density

Cochran and Harpending, 2009. The 10,000-year explosion: How civilization accelerated human evolution (Here’s a decent review: Gorelik and Shackelford, 2010. A review of Gregory Cochran and Henry Harpending, The 10,000 Year Explosion: How Civilization Accelerated Human Evolution.


Wade, 2007. Selection Spurred Recent Evolution, Researchers Say

Hawks, et al., 2007. Recent acceleration of human adaptive evolution

Human populations have increased vastly in numbers during the past 50,000 years or more (1). In theory, more people means more new adaptive mutations (2). Hence, human population growth should have increased in the rate of adaptive substitutions: an acceleration of new positively selected alleles.

Can this idea really describe recent human evolution? There are several possible problems. Only a small fraction of all mutations are advantageous; most are neutral or deleterious. Moreover, as a population becomes more and more adapted to its current environment, new mutations should be less and less likely to increase fitness. Because species with large population sizes reach an adaptive peak, their rate of adaptive evolution over geologic time should not greatly exceed that of rare species (3).

But humans are in an exceptional demographic and ecological transient. Rapid population growth has been coupled with vast changes in cultures and ecology during the Late Pleistocene and Holocene, creating new opportunities for adaptation. The past 10,000 years have seen rapid skeletal and dental evolution in human populations and the appearance of many new genetic responses to diets and disease (4).

In such a transient, large population, size increases the rate and effectiveness of adaptive responses. For example, natural insect populations often produce effective monogenic resistance to pesticides, whereas small laboratory populations under similar selection develop less effective polygenic adaptations (5). Chemostat experiments on Escherichia coli show a continued response to selection (6), with continuous and repeatable responses in large populations but variable and episodic responses in small populations (7). These results are explained by a model in which smaller population size limits the rate of adaptive evolution (8). A population that suddenly increases in size has the potential for rapid adaptive change. The best analogy to recent human evolution may be the rapid evolution of domesticates such as maize (9, 10).

Selection due to paleoclimate

Ash and Gordon. Brain Size, Intelligence, and Paleoclimatic Variation. (See note 77).

Jensen, 1998. Population Differences In Intelligence: Causal Hypotheses. In: The g Factor: The Science of Mental Ability

Jensen, 2006. Comments on correlations of IQ with skin color and geographic–demographic variables. Intelligence

Kanazawa, 2008. Temperature and evolutionary novelty as forces behind the evolution of general intelligence

Lynn, 1991. The evolution of race differences in intelligence

Lynn and Vanhanen, 2006. IQ and global inequality

Rushton, 2000. Race, evolution, and behavior: A life history perspective

Templer and Arikawa, 2006. Temperature, skin color, per capita income, and IQ: An international perspective

Both Rushton (1995, 1997, 2000) and Lynn (1991) have pointed out that ethnic groups in colder climates score higher on intelligence tests than ethnic groups in warmer climates. They contend that greater intelligence is needed to adapt to a colder climate so that, over many generations, the more intelligent members of a population are more likely to survive and reproduce. Their temperature and IQ analyses have been descriptive rather than quantitative, however. In the present quantitative study, we predicted a negative correlation between IQ and temperature. We hypothesized that correlations would be higher for mean winter temperatures (January in the Northern Hemisphere and July in the Southern Hemisphere) than for mean summer temperatures. Skin color was conceptualized as a variable closely related to temperature. It is viewed by the present authors as a multigenerational reflection of the climates one’s ancestors have lived in for thousands of years. Another reason to predict correlations of IQ with temperature and skin color is the product–moment correlation reported by Beals, Smith, and Dodd (1984) of 0.62 between cranial capacity and distance from the equator. Beals et al. based on findings from every continent and representing 122 ethnically
distinguishable populations. Jensen (1998) reasoned that natural selection would favor a smaller head with a less spherical shape because of better heat dissipation in hot climates. Natural selection in colder climates would favor a more spherical head to accommodate a larger brain and to have better heat conservation.

Implausible? Refer here: Roth, et al., 2010. Learning capabilities enhanced in harsh environments: a common garden approach

“We found that birds from the harsh northern population, where selection for cognitive abilities is expected to be high, significantly out performed conspecifics from the mild southern population…our results support the idea that environmental severity may be an important factor in shaping certain aspects of cognition.”

Selection due to geographic novelty

Jensen, 1998. Population Differences In Intelligence: Causal Hypotheses. In: The g Factor: The Science of Mental Ability

Kanazawa, 2004a. The Savanna Principle. Managerial and Decision Economics

Kanazawa, 2004b. General intelligence as a domain-specific adaptation

In a recent book, IQ and the Wealth of Nations, Lynn and Vanhanen (2002) compile a comprehensive list of “national IQs” (the mean IQ of populations) of 185 nations in the world. One striking feature of their data is that the national IQs in sub-Saharan Africa are significantly lower than those in the rest of the world (68.8 vs. 89.1), t(183)  15.88, p  .001. This, of course, makes perfect sense from my perspective of general intelligence as a domain-specific adaptation for evolutionary novelty. Since our ancestors spent most of their evolutionary history in sub-Saharan Africa, it is evolutionarily more familiar to the human brain than the rest of the world, which is more evolutionarily novel. If general intelligence evolved as a means to deal with evolutionarily novel situations, then it follows that it should evolve more rapidly in the rest of the world than in the ancestral environment of the sub-Saharan Africa…. In contrast, I offer a different evolutionary psychological explanation for the evolution of general intelligence as a domain specific adaptation in the sphere of evolutionary novelty. If there were a sufficient number of novel, non recurrent problems throughout human evolutionary history, any genetic mutation that allows its carrier to think and reason logically would have been selected for. Given the extraordinary constancy and continuity of the EEA, general intelligence in its evolutionary origin was not general at all, and its importance was limited to occasional problems that other evolved psychological mechanisms could not solve. Its universal and enormous importance today reflects the fact that we now live in an evolutionarily novel world.

Selection due to continuous disease burden/lack of continuous disease burden

Eppig, et al., 2010. Parasite prevalence and the worldwide distribution of cognitive ability

We also propose a complementary hypothesis that may explain some of the effects of infectious disease on intelligence. As we mentioned, it is possible that a conditional developmental pathway exists that invests more energy into the immune system at the expense of brain development. In an environment where there has consistently been a high metabolic cost associated with parasitic infection, selection would not favour the maintenance of a phenotypically plastic trait. That is, the conditional strategy of allocating more energy into brain development during periods of health would be lost, evolutionarily, if periods of health were rare. Peoples living in areas of consistently high prevalence of infectious disease over evolutionary time thus may possess adaptations that favour high obligatory investment in immune function at the expense of other metabolically expensive traits such as intelligence. Data do not currently exist on temporal variation of the severity of infectious disease across the world over human history. For genetically distinct adaptations in intelligence to exist based on this principle, parasite levels must be quite consistent over evolutionary time.

Gene-cultural selection

Miller. Are Pleiotropic Mutations and Holocene Selective Sweeps the Only Evolutionary-genetic Processes Left for Explaining Heritable Variation in Human Psychological Traits? The Evolution of Personality and Individual Differences. Buss and Hawley, ed.

“Selection pressure cannot have remained the same after human dispersals out of Africa and the Upper Paleolithic revolution, especially since the rise of agriculture, domestication, money, and institutionalized monogamy, These changes may not have had enough time to produce complex, new, cross-culturally universal psychological adaptations …but they could have had dramatic effects on the patters of genetic variations underlying personality, psychopathology, and cognitive traits.

New mutations that happen to have beneficial effects on one trait are likely to have pleiotropic effects on other traits that reduce their net fitness benefits by a factor of two, on average (Otto, 2004). For example, if selection in favor of general intelligence suddenly became more intense in some popultations, this could have favored the spread of new IQ-boosting alleles even if those alleles have a range of harmful side-effects on physical or mental health that … p.385-387

Kanazawa, 2007. The g-culture coevolution

Cochran and Harpending, 2009. The 10,000-year explosion: How civilization accelerated human evolution

Laland, Odling-Smee, and Myles, 2010. How culture shaped the human genome: bringing genetics and the human sciences together

Abstract | Researchers from diverse backgrounds are converging on the view that human evolution has been shaped by gene–culture interactions. Theoretical biologists have used population genetic models to demonstrate that cultural processes can have a profound effect on human evolution, and anthropologists are investigating cultural practices that modify current selection. These findings are supported by recent analyses of human genetic variation, which reveal that hundreds of genes have been subject to recent positive selection, often in response to human activities. Here, we collate these data, highlighting the considerable potential for cross-disciplinary exchange to provide novel insights into how culture has shaped the human genome

Stearns, Byars, Govindaraju, and Ewbank, 2010. Measuring selection in contemporary human populations.

Abstract | Are humans currently evolving? This question can be answered using data
on lifetime reproductive success, multiple traits and genetic variation and covariation in those traits. Such data are available in existing long-term, multigeneration studies — both clinical and epidemiological — but they have not yet been widely used to address contemporary human evolution. Here we review methods to predict evolutionary change and attempts to measure selection and inheritance in humans. We also assemble examples of long-term studies in which additional measurements of evolution could be made. The evidence strongly suggests that we are evolving and that our nature is dynamic, not static.

Acquisition of useful genes via interbreeding

Cochran and Harpending, 2009. The 10,000-year explosion: How civilization
accelerated human evolution

The key point here is that it would take only a very limited amount of interbreeding for modern humans to have picked up almost every Neanderthal allele with any significant advantage. Limited interbreeding would mean that neutral genes in hu- mans today would look almost entirely African—which they do—while at the same time we might carry a number of func- tional alleles that originated in Neanderthals.Those alleles would be ones that mattered, ones that made a difference…

…And yet, European Neanderthals probably faced many of the same life problems that African humans did.To some de- gree, big brains may have been solving the same problems in both populations. Even if that is the case, though, we can be certain that those problems were not solved in exactly the same way.

Increased cognitive complexity due to technological innovations

Gottfredson, 2007. Innovation, fatal accidents, and the evolution of general intelligence

Although migrating to new climates (or climate change in situ) may have sparked much innovation, it was these innovations that made daily life more cognitively complex. Each technological advance in taming adversity (e.g. hearths for cooking and heating inside closed shelters) could increase the need to anticipate, recognize, prioritize, and quickly mitigate its potential side-effects. Migration into successively less hospitable climes spurred new technologies that, individually or collectively, could ratchet up the g-related risk gradients for accidental death. A migration ratchet effect comports with the pattern of genetic divergence among certain human populations who have ancestral origins in different regions of the world [.]

Ecological Harshness

Hart, 2007. Understanding Human History:

Section 1 – The effects of cold climates
The genetic differences that affect the average intelligence of the various races arose in the same fashion — natural selection — that physical differences between the races did.

In tropical regions, groups with an average IQ of 70 or less can survive. We know this because there are such groups living today in sub-Saharan Africa (see Table 15-2). However, surviving in a cold climate (such as Siberia) poses much greater difficulties. Human beings cannot survive in such climates without:
1) Warm clothing.
2) Sewing needles, to make that clothing with. (These were not invented until
about 30 kya.)
3) Warm, sturdy housing capable of keeping out rain, wind, and cold.
4) Methods for accumulating food and storing it for the winter months.

Section 2 – Selection for high intelligence

Each of the items on that list — and many others needed to survive in a harsh, cold environment — required considerable intelligence. Since individuals with low IQs had poor prospects of surviving in Siberia, those genes which helped to create high intelligence were more likely to reach succeeding generations, while those which tended to result in lower intelligence were eliminated from the gene pool.

Furthermore, this selective process would also operate on groups. If the average intelligence of the members of a band was low, it was likely that the entire band would die out, including those individuals who were of high enough intelligence to have survived and reproduced if they had been members of a more capable group. This selection process began as soon as HSS moved out of Africa into regions where there were cold winters, and it became increasingly powerful as human groups moved into colder and colder climates.1 The process continued for tens of thousands of years, gradually resulting (in regions with very harsh climates) in groups evolving with average IQs of 100 or higher.

Section 3 – Why wasn’t higher IQ an advantage in all regions?

Higher intelligence would be an advantage everywhere if there were no costs accompanying it. But what matters are not merely the positive benefits of a trait, but also the costs involved. The changes in human anatomy and physiology that lead to higher intelligence do not come cost free. For example, one way for a subgroup of human beings to develop higher intelligence is for that group to evolve larger brains. However, larger brains impose at least three important costs:

1) Larger brains require larger amounts of energy. Since its neurons must be repeatedly recharged, the brain requires an amount of energy that is far out of proportion to its size. (A typical human brain contains only about 2% of the body’s mass, but uses about 20% of a resting person’s energy.) Larger brains, of course, require more energy than smaller ones.

2) Larger brains require larger heads, which creates strains on the muscular and skeletal structure. Standing on two legs is a complex problem in balancing, and weight situated near the top of our structure is particularly hard to balance. Larger brains and heads make this problem even greater.

3) Larger brains (and heads) require wider female pelvises, and the wider pelvises result in less efficiency in walking and running. The large size of the human head creates serious difficulties in childbirth (which is why human females typically have far more difficulty in delivering their young than do females of most other species). As brains and heads became larger, wider female pelvises were required to accommodate them. Since it was difficult for such a change to be confined to one sex only, the male members of those groups also wound up with wider pelvises. But wider pelvises and hips result in lower running speeds (see section 14-7) and less efficiency in both walking and running.

The combination of costs that are involved in having larger, more powerful brains explains why many human groups did not evolve brains as large as those typically found in north Asians and north Europeans. However, in cold climates, the advantages of higher intelligence outweighed the costs just described.

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