GENETIC ADAPTATION TO THE ENVIRONMENT 



An evaluation of man's adaptation 

 to the environment depends, obvi- 

 ously, on the use of the two key 

 words, adaptation and environment. 

 To begin with the latter, its use in 

 connection with adaptation usually 

 brings to mind the physical environ- 

 ment — climate, etc. — but the bio- 

 logical environment of a species, in 

 the form of disease or predators, is 

 also well known. Furthermore, many 

 of the important problems of man's 

 adaptation are now concerned with 

 the psychological or social environ- 

 ment. People are as much a part of 

 the environment as sunlight and rain- 

 fall, and the problems of man's in- 

 traspecific aggression and population 

 control must take into account adap- 

 tations to this environment. 



If the definition of environment is 

 extremely general, even nebulous, the 

 definition and uses of the concept of 

 adaptation are even more so. In fact, 

 there is considerable confusion as to 

 the nature of man's "adaptations" 

 because of the very loose use of the 

 term. General systems theory, for 

 which adaptation is a central concept, 

 can be applied to everything from 

 physical systems or phenomena to 

 cultural change. Even within the bi- 

 ological sciences there are many uses 

 of the term adaptation. However, the 

 most general use is to define genetic 

 adaptations, which are changes in 

 the gene frequency of a population 

 in response to or as a result of differ- 

 ences in the fitness of the genotypes. 

 Adaptation will here be used only in 

 this restricted, genetic sense. 



Darwinism Revived 



As an explanation of human ge- 

 netic differences, the concept of adap- 

 tation — or, what is synonymous, 

 natural selection — has only assumed 

 its rightful place in the past twenty 

 years — even though it was Darwin's 

 major contribution to biological sci- 

 ence. For almost 100 years after 



Darwin, biologists and anthropolo- 

 gists concentrated on constructing 

 taxonomies and phylogenies, which 

 were based on the similarities and 

 differences among populations of or- 

 ganisms and were based, implicitly 

 and explicitly, on the assumption 

 that these similarities and differences 

 were "non-adaptive." 



In anthropology, the switch to 

 adaptive explanations began about 

 1950, with Coon and others, and was 

 concerned with the visible, measur- 

 able features of individuals that are 

 commonly called racial traits. At 

 about the same time, there was new 

 work and rediscovery of old work 

 on the association of the ABO blood 

 groups and various diseases. The 

 rediscovery of the work done in the 

 1920's was comparable in a way to 

 the rediscovery of Mendel, in that its 

 significance was now recognized. 

 This recognition was due to the re- 

 discovery of natural selection as a 

 major factor in the evolution of 

 human differences. 



Again in the early 1950's, research 

 showed that sickle cell anemia and 

 thalassemia varied in frequency in 

 different "races"; they occurred with 

 extremely high frequencies in some 

 populations. Since these diseases 

 were known to be due to homo- 

 zygosity for a single gene (the situa- 

 tion is somewhat more complicated 

 now) and were extremely severe if 

 not lethal, their prevalence raised 

 some knotty problems for population 

 geneticists. With such selection 

 against these genes, there had to be 

 some other force balancing this ad- 

 verse selection and thereby causing 

 the high frequencies. Although there 

 is still some disagreement — mostly 

 as to details — it is generally accepted 

 that heterozygotes for the sickle-cell 

 gene have a resistance to falciparum 

 malaria; thus, adaptation, or natural 

 selection, is the major explanation 

 for differences among human popu- 

 lations in the frequency of the sickle- 



cell gene. This exampli 



that it is used in just about 



textbook. 



Science, like the rest of human 

 endeavor, evolves by a pendulum 

 process. Thus, when these three 

 trends re-introduced adaptation into 

 the study of human genetic variation, 

 adaptive explanations began to be 

 proposed for most genetic differences. 

 The result was an exaggeration of 

 the concept that was almost as faulty 

 as its total absence had been in 

 previous work. In the sickle-cell 

 example, the racial and polymorphic 

 traits that were explained by adapta- 

 tion required this concept; they were 

 obviously genetic differences, and 

 other explanations seemed inadequate 

 due to problems such as the extreme 

 selection against the sickle-cell gene. 

 After the pendulum swung, explana- 

 tions by adaptation were extended to 

 all genetic differences and to many 

 behavioral differences between popu- 

 lations. These extensions raise two 

 questions: (a) how many genetic dif- 

 ferences are explained primarily by 

 selection, and (b) how many of the 

 functional or behavioral differences 

 between populations are primarily 

 genetic and due to different adapta- 

 tions? 



Selection as an Explanation for 

 Genetic Difference 



The first question is now being 

 hotly debated by geneticists. The 

 debate began with Muller's discus- 

 sion of "our land of mutations" and 

 was continued with his paper in as- 

 sociation with Morton and Crow. 

 One could almost label this "the 

 American position," which considers 

 most genetic loci, or the allelic vari- 

 ability at most loci, as due to a 

 balance between mutation from the 

 normal allele and selection against 

 the abnormal variants. The state- 

 ment that most loci are generally 

 described in this way is reasonable; 



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