PART XI — HUMAN ADAPTATION TO ENVIRONMENTAL STRESS 



but Morton and others have gone 

 beyond this position to say that most 

 selection, and particularly that in- 

 creased by inbreeding, is associated 

 with this type of locus and acts to 

 reduce genetic variation. The other 

 position is that many loci are balanced 

 polymorphisms in which the selection 

 against the two homozygotes balance 

 each other and result in genetic vari- 

 ability; the sickle-cell locus (/? hemo- 

 globin chain) is the most obvious 

 example. 



Both sides agree that both kinds 

 of loci exist. The questions are how 

 many of each and how important 

 for human genetic disease are the 

 two kinds. Recent estimates of the 

 number of polymorphic loci — in 

 other words, loci with at least two 

 different alleles with appreciate fre- 

 quencies in a significant number of 

 human populations — have ranged 

 around 30 to 35 percent of all human 

 loci. Thus, while the majority of loci 

 may well be mutational, problems 

 and arguments must still be resolved 

 in order to explain the 35 percent that 

 are polymorphic. 



How much of this polymorphism 

 is due to adaptation or natural selec- 

 tion, and how much is simply "neu- 

 tral" variation? This is one of the 

 major concerns at present. Again, 

 the Americans (and the Japanese, fol- 

 lowing Kimura) are working on the 

 assumption that most of this varia- 

 tion is "non-adaptive," while the 

 British are more skeptical. The ar- 

 gument seems to go back to the 

 famous encounters between Fisher 

 and Wright on the significance of 

 random genetic drift. In addition, 

 the assumption of "non-adaptive" 

 differences among human populations 

 is basic to the reconstruction of 

 phylogenies or taxonomies; recent 

 work on the adaptive significance of 

 racial differences thus tends to cast 

 doubt on previous work on race. 



However, the opposite position — 

 that all polymorphic differences are 

 adaptive — also rests on thin ground. 

 For example, in European populations 



the frequencies of cystic fibrosis ap- 

 proach those which would be labeled 

 polymorphic; in Eastern European 

 Jewish populations the frequency of 

 Tay-Sachs disease does, too. For 

 these two loci there has been con- 

 siderable speculation on the adaptive 

 significance of the abnormal allele, 

 although one need not assume selec- 

 tion for the abnormal allele in these 

 cases. Much work has also been 

 done on blood-group systems other 

 than ABO in an attempt to demon- 

 strate the action of natural selection. 

 Selection against heterozygote Rh 

 babies due to maternal-fetal incom- 

 patibility obviously exists, but the 

 attempts to show selection in relation 

 to environmental factors have not 

 been very successful. Thus, the posi- 

 tion that almost all genetic differences 

 among human populations are due 

 to adaptive selection cannot be said 

 to have been proven; on the other 

 hand, to assume that any locus is 

 "non-adaptive" because we cannot 

 demonstrate the selection that may 

 be involved is also tenuous. 



How much selection would be nec- 

 essary to develop the human genetic 

 differences we observe? To a great 

 extent, the answer appears to be 

 "infinitesimal" — or certainly within 

 the range of error of the measure- 

 ments on modern human populations 

 by which we are trying to detect 

 selection. This is due to the uncer- 

 tainties of sampling and the limited 

 size of the most significant human 

 populations presently under study, 

 such as the Yanomama Indians of 

 Brazil and Venezuela (see Figure 

 XI-1), who are among the few re- 

 maining hunters and primitive horti- 

 culturalists. And yet we know that 

 human populations do change. De- 

 spite their seemingly small genetic 

 differences (see Figure XI-2), Yano- 

 mama and Japanese are clearly very 

 different human types. Given this di- 

 lemma, we are likely to decide be- 

 tween competing theoretical positions 

 on the basis of their ability to explain 

 the totality of human genetic varia- 

 tion as it exists today and not on the 

 basis of hoping that we will ever 



be able to measure the amount of 

 selection that existed 10,000 or 20,000 

 years ago. And our explanations will 

 contain both instances where genetic 

 drift, or the "founder effect," were 

 far more important factors of gene- 

 frequency change than selection and 

 vice versa. 



Adaptation to Explain Behavioral 

 Differences 



So much for the increased effect 

 of our knowledge of adaptation on 

 known genetic differences. We now 

 turn to the other extension of the 

 concept of adaptation. To recapitu- 

 late, adaptation was revived as an 

 explanation of human genetic differ- 

 ences because there were certain dif- 

 ferences among human populations 

 that could not be explained without 

 it. Adaptation was then extended to 

 most other widespread genetic dif- 

 ferences; it was also extended by 

 anthropologists, physiologists, psy- 

 chologists, sociologists, and even edu- 

 cators to other biological or beha- 

 vioral differences among groups. If 

 a group could be shown to be geneti- 

 cally different in one trait, it was 

 assumed to be different in many 

 other traits and, in addition, any 

 differences that were found among 

 such groups were implicitly assumed 

 to be genetic. 



The resurgence of Darwinian think- 

 ing has been pronounced in behavi- 

 oral studies. These include a great 

 number of studies of physiological 

 responses to environmental stresses 

 such as temperature as well as studies 

 of the intellectual functioning of the 

 organisms, which usually fall under 

 the heading of behavioral genetics. 

 There is an important difference be- 

 tween these studies and earlier ones 

 concerning human differences that 

 required adaptive explanations. The 

 latter involved known structural dif- 

 ferences that could be related to gene 

 action. For example, the differences 

 in skin color among human popula- 

 tions are very pronounced, are known 

 to be genetic, and, although some 

 investigators do not think it is 



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