I thought the following might be helpful in discussing the "nature vs. nurture" debate. (Also, a great read is Tim Noakes talk before the W **** The nature/ nurture issue in human populations * Some physical variation in humans (other than variation in sex) is the result of genetic factors. Some examples include body build, skin color, height, and high altitude adaptations. However, all of these differences are subject to environmental and cultural forces. * Some populations of humans are more, or less, susceptible to certain infectious diseases. Examples include: measles, tuberculosis, and malaria. There are biological and cultural components to these differences. * With regard to body build, Bergman's rule states that body weight tends to be lower in warm climates and Allen's rule states that protruding body parts are shorter in cooler areas. * These rules about body shape and limb size refer to thermal regulation mechanisms. Global variation in humans tends to support these rules, but note that this is, once again, a case of correlation without necessary causation. Much of the variation in body build and shape in humans is the result of environmental and developmental, not genetic, forces. * For example, where mean temperature is 70-82 degrees F, mean weight for humans is 110-120 pounds; where mean temperature is 40 degrees F, mean weight is 140 pounds. * There are many body type and physiological differences between some populations that are a combination of genetic and social/cultural factors. Bone density (highest among Africans), muscle fiber type distribution, fat distribution (lowest among equatorial populations, mostly notably Africans and some Aboriginal communities) are genetic proscribed, with some plasticity. * Height is very sensitive to nutrition and to stress in childhood. * Genetics has something to do with susceptibility to illness, although culture plays a significant role. Women in the U.S., for example, have lost much of the advantage they once had over men in coronary heart disease rates. This change is the result of rapid cultural/behavioral change. * There are genetic differences across populations in susceptibility to disease. Things like susceptibility to Tay-Sachs disease among European Jews and their Diaspora descendants, susceptibility to sicklemia among Central Africans and their Diaspora descendants, susceptibility to skin cancer, cystic fibrosis, and multiple sclerosis among Northern Europeans and their descendants. Population frequencies of many polymorphic genes vary with "race". For example, a condition called primaquine sensitivity is responsible for the intensity of the reaction to certain drugs among African, Mediterranean, and Asian men. Another mutated gene accounts for the sensitivity of the Japanese to alcohol. Other genetic polymorphisms (found in specific population groups) are associated with sensitivity to specific foods, type one diabetes, QT syndrome (a heart disease), asthma, thrombophilia (bleeding disorder), and an inability to metabolize common drugs like codeine, beta-blockers and antidepressants. * Across populations, however, most of the differences in susceptibility to disease is the result of cultural and environmental factors. Three examples of the relationship between environment, biology, and culture: * 1. Sickle cell anemia: This results from a mutation in the gene complex that regulates the production of hemoglobin. * Before the advent of modern therapies, those with the homozygous condition for sicklemia did not usually live to reproductive age. Those who were homozygous for normal hemoglobin were susceptible to malaria. The heterozygous condition apparently protected against malaria. * Malaria was wiped out in many areas, but the use of DDT and other agents has now resulted in resistant strains of mosquitoes. From the 1970s until today, malaria has become one of the leading causes of death in some African and Asian countries. * Malaria may not have come to Africa until about 2000 ya. Metal tools made slash and burn agriculture much more effective. The plasmodium parasite that causes malaria is carried by the Anopheles gambiae vector. Slash and burn agriculture apparently led to the creation of pools where the Anopheles mosquitoes breed. * This case illustrates the microevolutionary response of humans to environmental changes brought on by cultural interventions. * 2. Lactase deficiency: Lactose is the sugar in milk. It can not be absorbed without the aid of an enzyme, lactase. This enzyme is secreted by infants from the wall of the small intestine, but lactase production stops at about four years of age in many populations. Adults who are lactase deficient can not drink milk without suffering gas, bloating, diarrhea, pain. * From 2-20 percent of Europeans (and Euroamericans) are lactase deficient. About 70% of African American and over 90% of Thai, Chinese, and Japanese are lactase deficient. * Keeping cattle and consuming milk products appears to have resulted in natural selection for lactase in adulthood. Pastoral people who have historically depended on milk in their diet are adult producers of lactase. * Some lactase deficient peoples consume milk products, but in the form of yoghurt or cheese. * 3. High-altitude adaptation: In the Himalayas and Andes, people live at 12,000 ft and up. The maximum altitude for humans appears to be about 18,000 ft. (Mt. Everest is 29,000). * People who live all their lives at high altitudes have larger lungs, bigger chests, larger hearts and higher blood pressure in the lungs. They can extract more oxygen from the same volume of air than can people who live at sea level. * These traits appear to be a combination of acclimatization and genetics. However, human physiology is plastic enough for people to live at either sea level or at high altitudes. About skin color: * Variation in human thought and behavior is neither genetic nor immutable. Variation in human behavior and thought is tied to culture and culture is malleable -- subject to social, economic, and environmental forces. * Differences in human skin color are the consequence of different amounts of melanin in the skin. Melanin protects the dermis from damage by ultraviolet rays. Ultraviolet rays facilitate the production of production vitamin D, and thus the management of calcium. * In tropical latitudes, a higher level of melanin minimizes the danger of hypervitaminosis D. It also minimizes the danger of skin cancer. In northern latitudes, a lower level of melanin maximizes the production of vitamin D. Hope this clarifies some of the points of debate and makes it clear that behavior ( and athletic performance) is a bio-social phenomenon. -- Jon Entine RuffRun 6178 Grey Rock Rd. Agoura Hills, CA 91301 (818) 991-9803 [FAX] 991-9804 http://www.jonentine.com
