J Med Genet: first published as 10.1136/jmg.8.4.444 on 1 December 1971. Downloaded from Annotation Journal of Medical Genetics (1971). 8, 444. Polymorphism and Protein Evolution The Neutral Mutation-Random Drift Hypothesis HARRY HARRIS From the Galton Laboratory, University College London, Wolfson House, London NW1 Enzyme and Protein Diversity 1969; Selander and Yang, 1969), and the horseshoe In recent years many examples of what are con- or king crab Limulus polyphemus (Selander et al, veniently referred to as enzyme and protein 1970). The results indicate that two or more polymorphisms have been discovered in the course relatively common alleles giving rise to electro- of surveys of human populations and of naturally phoretic differences may exist at some 25-40% of occurring populations of other animal species. loci coding for enzyme and protein structure. The term polymorphism is used in this context for The frequencies of the common alleles which any situation where members of a population can be make up the different polymorphisms vary consider- sharply classified into several distinct phenotypes in ably from case to case. However a useful index of terms of particular characteristics of the enzyme or the extent of the phenomenon in a particular popu- protein, and where at least two of the phenotypes lation is obtained by estimating the average hetero- zygosity per locus. This is the sum of the have an appreciable incidence (greater than 2%). pro-copyright. Most of these polymorphisms can be attributed to portion of individuals found to be heterozygous at the occurrence of two or more alleles each coding each locus divided by the total number of loci for a structurally distinct form of a polypeptide studied, including of course both 'polymorphic' and chain in the particular enzyme or protein. It is 'non-polymorphic' loci. Studies on some 26 thought that the structural difference between the arbitrarily chosen loci in European and negro polypeptides usually amounts to no more than a populations gave values in each population of just single amino-acid substitution, and that the allelic over 0-05 for the average heterozygosity per locus difference originated in a single mutational event which could be detected by enzyme electrophoresis http://jmg.bmj.com/ involving the change of only one base for another (Harris, 1970). However it was of interest that the in the sequence of several hundred or thousand contributions of different loci to this average bases in the DNA of the particular gene. But sometimes varied quite widely between the two direct evidence for this has so far only been ob- ethnic groups. Since it seems probable that at tained in a limited number of polymorphisms, most only about one third of all possible structural and there are certainly some exceptions where the changes in an enzyme produced by mutations polypeptide products of the two common alleles would be detected electrophoretically, the results on September 27, 2021 by guest. Protected differ by more than one amino acid, eg, the sheep suggest that the true value for average heterozy- haemoglobins A and B (Boyer et al, 1967). gosity per locus may be around 0-16. Put another Electrophoretic surveys of arbitrarily chosen way, one might expect that any single individual enzymes and proteins have been carried out in in these populations would be heterozygous at per- various naturally occurring animal populations to haps 16% of gene loci coding for enzyme structure. see how often such polymorphisms occur. A Less systematic data on some 30 other enzyme loci number of very different species have been studied in man appear to lead to essentially the same esti- in this way. They include man (Harris, 1966 and mate. Similar or higher estimates of average 1969), Drosophila pseudoobscura (Hubby and heterozygosity per locus have been obtained from Lewontin, 1966; Lewontin, and Hubby, 1966; electrophoretic studies of different enzyme and Prakash, Lewontin, and Hubby, 1969), mouse other proteins in naturally occurring populations of (Ruddle et al, 1969; Selander, Hunt, and Yang, Drosophila pseudoobscura, D. persimilis, Mus mus- culus, Peromyscus polionotus, and Limulus polyphe- Received 28 May 1971. mus (Selander et al, 1970). 444 J Med Genet: first published as 10.1136/jmg.8.4.444 on 1 December 1971. Downloaded from Polymorphism and Protein Evolution 445 It should be pointed out that these investiga- clinical abnormality is seen in the heterozygous tions have been largely confined to soluble enzymes state. and other proteins which can be examined electro- It is usual to regard any allele with a frequency in phoretically. It is possible, for example, that the population of less than 10-2 as 'rare'. In fact structural proteins or membrane-bound enzymes the majority of rare alleles which have so far been may differ in this respect, though there is as yet found appear to have frequencies much less than little evidence one way or another on this point. this. In surveys involving 2000 to 10,000 indi- Also of course only a very small number of different viduals particular 'rare' alleles are often found in enzymes and proteins have been investigated com- only one or two individuals. So their individual pared with the many thousands that actually occur frequencies appear to be generally less than 10-s in an organism. So the estimates of the fraction of and usually perhaps less than 10- 4. However, polymorphic loci or of average heterozygosity per because quite a number of different 'rare' alleles can locus must be regarded as very rough and tentative. evidently occur at any one locus, they may contri- Nevertheless it is clear that in each of several very bute appreciably to individual heterozygosity different species enzyme and protein polymorphism (Hopkinson and Harris, 1971). Rough estimates is a not infrequent phenomenon. It is certainly derived from electrophoretic surveys of enzymes very much more extensive than was generally in human populations suggest that any single indi- thought possible only a few years ago. vidual may be heterozygous for one or another rare Besides the alleles giving rise to these common allele at perhaps 0-30' of loci coding for enzyme polymorphic differences many more alleles deter- structure. mining distinctive variants have also been found to occur when particular enzymes or proteins have been examined by suitable procedures in sufficiently Mutation, Selection, and Drift large numbers of individuals. Human adult Thus it now seems likely that at most gene loci haemoglobin a and fi loci (Lehmann and Carrell, coding for enzyme or protein structure many 1969), and the enzyme glucose-6-phosphate dehy- different alleles occur among the members of natural drogenase (Kirkman, 1971) have for various reasons populations. At any particular locus the majority copyright. been studied from this point of view in more detail, are probably very rare, but there must of course and in very many more individuals than any other always be one allele which is very common, and proteins, and at each of the loci concerned some 50 often as we have seen there may be two. or more rare variants determined by different rare The alleles which are present in populations to- alleles have already been discovered. And the rate day may be presumed to have arisen by separate at which new examples are currently being reported mutations in individuals in earlier generations, suggests that many more remain to be identified. some perhaps relatively recently, others in the more http://jmg.bmj.com/ Other enzymes and proteins have been surveyed less remote past. And in terms of classical theory one extensively, but nevertheless in a variety of cases would expect that in general their incidence and (eg, serum albumin, transferrin, phosphoglucomu- distribution today will reflect the operation of tase, phosphohexose isomerase, placental alkaline natural selection over the course of many previous phosphatase, NADH diaphorase, etc) a series of generations on the flux of mutant alleles being different variants each evidently due to a different continuously generated by fresh mutations at a low rare allele at the structural locus concerned have but steady rate. been found. In general it is now beginning to Natural selection will tend to eliminate from the on September 27, 2021 by guest. Protected appear that as more individuals are examined for a population mutant alleles which have relatively particular enzyme or protein the number of different deleterious effects on individuals who carry them, rare variants that are discovered progressively in- and tend to cause the spread of those which are creases. Some of these rare variants give rise in advantageous. Since one would expect that a either homozygous or heterozygous states to overt single random alteration in a complex structure like clinical abnormality, but many appear to be an enzyme protein is more often likely to impair relatively harmless. Furthermore the great major- than to improve its function, very many more of the ity of rare variants found in population surveys do mutants that occur at any particular locus are likely not appear when family investigations are carried to be detrimental than advantageous. Thus the out, to be the product of fresh mutations occurring conclusion that the great majority of variants due to in the immediately preceding generation. The alleles at a single locus are rare, is in general keep- occasional exceptions have usually been variants ing with this expectation. (such as unstable haemoglobins) where overt But the discovery that at an appreciable J Med Genet: first published as 10.1136/jmg.8.4.444 on 1 December 1971. Downloaded from 446 Harry Harris proportion of loci, two common alleles occur rather* likely phylogenetic tree which would have given than just one was rather surprising, and its inter- rise with the minimum number of mutational pretation has engendered a great deal of discussion changes to the amino-acid sequences in the proteins and controversy.