Heredity and Evolution in Human Populations: Revised Edition [Rev. ed. Reprint 2014] 9780674492660, 9780674492653

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Heredity and Evolution in Human Populations

HARVARD BOOKS IN BIOLOGY Number 1

Heredity and Evolution in Human Populations L. C. DUNN Professor of Zoology Emeritus and Former Director of the Institute for the Study of Human Variation, Columbia University, New York

REVISED EDITION

HARVARD UNIVERSITY PRESS CAMBRIDGE, MASSACHUSETTS

Copyright © 1959, 1965 by the President and Fellows of Harvard College All rights reserved Revised Edition, 1967 Second Printing, 1968 Distributed in Great Britain by Oxford University Press, London Library of Congress card catalog number 65-11617 Printed in the United States of America

Preface to the Revised Edition The first edition of this book, written in the summer of 1957, was addressed to people who wanted answers to questions about possible causes of changes in the biological constitution of human populations. It turned out that it was read by many who were looking not for easy answers or for final ones but for promising ones, that is, those that presented new views or opened new ways of study. This is the kind of interest that students have, and it was pleasant to find that the book proved useful to them. When a new printing was called for it was the students' needs that I kept chiefly in mind, and the few changes deal with some new things which have been learned recently. I have not tried to be exhaustive in this respect, since anyone who grasps the rationale of this kind of study can go on and read for himself what has happened. Consequently I have added a selected bibliography of recent books which themselves contain lists of references to the scientific publications from which the illustrative examples in this book were drawn. L. C. DUNN

Preface Human heredity and human evolution present fascinating problems which touch every one of us. I think I am entitled to point this out after writing a little book about them because I was not originally of this opinion. Genetics, as I was introduced to it as a student, was an exact science because its statements could be verified experimentally; since hurrian genetics could not be experimental, it must perforce consist only of applications of ideas derived from experiments with animals and plants. Ideas about evolution, on the other hand, were purely speculative and could not be tested at all by recourse to experiment. All this has changed, and I have changed my opinions too. Although I have spent most of my life in experimental genetics, I know that theories can be tested by other means than deliberately controlled experiment. Studies of human populations present many examples of this. One can imagine what arrangements of hereditary factors one ought to find in human populations and families if certain theories are true, and these arrangements can be observed. Theories of evolution can be tested both in this way, by observations of changes in populations in nature, and by deliberately designed experiments with rapidly reproducing animals, plants, bacteria, and viruses. The ideas derived from genetics and those from observations of species, races, and varieties of natural populations have now come to support and supplement each other. Inner consistency and relations between facts obtained from diverse sources, always a sign of maturation in a scientific field, begin to appear in the new field of population genetics. Its ideas are couched in general terms,

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PREFACE

applying to all populations with certain modes of mating and reproduction. I have tried to select f r o m these some ideas which seem to m e basic and relevant, and to illustrate them by observations, most of them recent, m a d e directly on h u m a n populations. I should not call the result a synthesis; that would be pretentious at this stage. It is rather an assessment of the promise inherent in these ways of looking at problems of human evolution. It is only problems of evolution on a small scale, modes of change in races and smaller communities to which I have addressed myself; microevolution is the term sometimes applied to problems on this level. Anyone wanting to read good discussions of the larger problems of evolution, should turn to G . G. Simpson's Meaning of Evolution and the more recent Mankind Evolving by T. Dobzhansky. I have more debts to colleagues and friends than can be acknowledged individually. Individual chapters have been read by persons with knowledge and critical judgment of the matters in them, but not wanting to involve them in errors of commission or omission for which I am responsible, I must thank them anonymously. I should like, however, to record my gratitude to my colleagues in the Institute f o r the Study of H u m a n Variation, especially Professor Theodosius Dobzhansky and Professor Howard Levene, for bearing with me in discussions of most of the problems treated in this book; and to my old teachers and friends under whose guidance I first became interested in heredity and evolution: Professor John H . Gerould at D a r t m o u t h College and Professor William E. Castle at Harvard University. T o my wife, Louise Porter Dunn, I am grateful for help and forbearance far beyond the call of duty. N E W YORK, N E W YORK

September

20,

1958.

L. C. D.

Contents I. VARIETY

3

II. THE PRINCIPLES OF HEREDITY APPLIED TO POPULATIONS

15

III. METHODS OF EVOLUTIONARY CHANGE IN HUMAN POPULATIONS

37

IV. GENES AND EVOLUTION V. RACE FORMATION

66

91

VI. ISOLATED POPULATIONS AND SMALL COMMUNITIES

106

VII. A LOOK AHEAD

132

A R E F E R E N C E LIST

149

Figures 1 . HUMAN CHROMOSOMES

8

2 . CHROMOSOMES IN HUMAN REPRODUCTION 3 . FORMATION OF GAMETES

4 . HYPOTHETICAL ARRANGEMENTS OF GENES 5 . RANDOM COMBINATION OF GENES

30

6 . DISTRIBUTION OF SICKLE-CELL GENE 7 . RH DIAGRAM

22

23

52

74

8 . TRANSMISSION OF HEMOPHILIA

85

9 . FREQUENCY OF BLOOD-GROUP GENE Β 1 0 . BLOOD-GROUP GENE Β IN EUROPE 1 1 . J U V E N I L E AMAUROTIC IDIOCY

126

99

97

27

Heredity and Evolution in Human Populations

I

To a biologist, one of the most remarkable features of the human population of this planet is its enormous variety. Here among the 2500 million members of the one species, Homo sapiens, we find no duplicates, except those rare cases of identical twins which since they arose from one egg count as one individual. Literally each person is biologically unique and declares this fact not only in his obvious physical features, but in the individual properties of his blood and other body fluids, in the operation of his sense organs, and in numerous details of chemical constitution and behavior. What is the biological meaning of this vast variety? The biological study of man and of other animal and plant species has established that it arises out of the interplay of two influences to which every living being is subject. One is the heredity transmitted to an individual by his parents through egg and sperm; the other is the varied conditions of life, the environments in which different individuals develop. The causes of hereditary variety are now known in outline. The transmission of heredity takes place by the passage of living units, genes, through the sex cells, egg and sperm, from parents to children. Reproduction consists essentially in the production by each gene of a replicate or copy of itself which passes into each new cell and into the new individual. The continuity of heredity and of life itself thus depends on the self-reproduction of the genes. Occasionally a copy is produced which is not quite identical, in its effects, with the parental form of the

4

VARIETY

gene. This kind of accident, of which the exact cause is not known, is called a mutation. The mutated gene then reproduces in its altered form so that two (or more) forms of each such gene occur in the same population. The thousands of such units which each parent transmits to the children are then shuffled and recombined in all possible combinations. The essential features of the biological mechanism by which this occurs are now well understood. The variety thus engendered is then acted upon by the sifting effects of the varied environments in which the species lives. Some combinations of traits prove to be biologically more successful in certain environments than in others. The end result is that local populations show adaptations to the conditions under which they have lived. The two processes just outlined are those of heredity and of evolution. In the last analysis they are both due to one basic peculiarity of living substance, the ability to reproduce itself by converting materials from the environment (food) into its own specific configuration, which is then perpetuated by the replication of hereditary units at each act of reproduction. It is interesting to reflect that the chief clues to our present understanding of each of these processes, of heredity and of evolution, were obtained about 110 years ago, and in relative independence of each other. On the evening of July 1, 1858, the Linnean Society of London met to hear two communications on the same subject by two biologists, neither of whom was present. The two papers bore the same title: "On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection." The authors were Charles Darwin and Alfred Russell Wallace. Each had hit upon the idea of natural selection as the basic clue to the mechanism of evolution while studying plants and animals in different parts of the

VARIETY

5

world, Wallace in Malaysia, Darwin in South America. Darwin's book of 1859 elaborated the idea. Its full title was On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. The chief facts on which the theory was based were, first, the enormous overproduction of offspring by all forms of life, of which only a few survived; and, second, the fact that not all offspring were alike but differed f r o m each other by individual variations. The inference from these facts was that, since some of the variations were transmitted by heredity, they rendered their possessors and descendants better fitted to survive. These were the "favored races" which eventually give rise to new species. The historical changes through which living matter evolved and differentiated were seen as a result of forces inherent in nature itself; appeal to agencies outside of nature— supernatural forces—was thus made unnecessary. It was a clear inference from the theory that if all life was part of the same historical process, guided by natural laws, then man was a part of it too. It was these last two features of Darwin's work that brought about the great intellectual revolution of the nineteenth century. Even as Darwin's and Wallace's preliminary reports were being read, there was being initiated in a monastery garden in Moravia another train of events which was to lead to an understanding of the mechanism of heredity. There, in 1856, Gregor Mendel began his experimental search for the rules governing the inheritance of the characters by which varieties of the garden pea are differentiated. The report of his seven years of work was read to the Brünn Natural Science Society on the evenings of February 8 and March 8, 1865. No one before he wrote his report had conceived so clearly the essential problem of heredity in terms which could be tested by deliberately designed experi-

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VARIETY

ments. It was, in fact, Mendel's method of conceiving a theoretical model of a living process in precise terms, and then of testing it by simple direct experiments, which gave rise to a new era. Out of these beginnings came the science of genetics with its new insights into the nature of living matter, not least of which has been the elucidation of the mechanism by which evolution occurs. HEREDITY

The essential feature of Mendel's discovery can be stated quite simply. It is that heredity is particulate, occurring by the transmission of discrete units known as genes. This was proved by observing the distribution among the descendants of single discrete traits in which the parents differed. Thus, in peas, a cross of a tall parent by a dwarf one both from true-breeding varieties produces only tall (hybrid) offspring. When bred together the hybrids produce three kinds of offspring in constant proportions: a quarter of them are tall and breed true like the tall grandparent; a half are tall but produce when interbred some dwarf offspring, like the hybrid tall plants; while a quarter are dwarf and breed true like the dwarf grandparent. Mendel explained this by supposing that pure tall plants transmit through each reproductive cell (gamete) a unit, or gene, leading to the tall habit of growth; the dwarf plants transmit similarly a gene for dwarf habit in each gamete. Union of these two gametes produces a hybrid with one of each kind of gene, although the tall gene has the prevailing influence on growth (known as dominance, but not a constant or essential feature of heredity). The crux of Mendel's theory lay in his explanation of what happens when such a hybrid forms its reproductive cells. Then, said Mendel, the two genes in the hybrid act as sharp alternatives, one kind (tall) entering half the gametes, the other kind (dwarf) going to the other

VARIETY

7

half. Every gamete thus receives one and one only of each pair of alternative genes. When hybrids are crossed together the gametes of one parent (Vi tall, Vi dwarf) and the gametes of the other (Vi tall, Vi dwarf) meet in random fertilization. These random combinations would be V2 tall X Vi tall = VA tall-tall; Vi tall X Vi dwarf + Vi dwarf X Vi tall = Vi tall-dwarf (hybrids); and Vi dwarf X Vi dwarf = V4 dwarf-dwarf. This constant distribution can be obtained only by assuming discrete units which separate sharply (segregate) when the gametes are formed. When present together in the same individual they do not blend or contaminate each other. Each gene retains its own integral unitary character. This rule was derived from each of the seven pairs of alternative traits in peas studied by Mendel and is the general rule in all organisms studied, from viruses and bacteria to man. We know now that the hereditary material transmitted by animals and plants generally consists of thousands of discrete units, genes, each of which follows the behavior described above, although we can observe this fact only when a gene has assumed by mutation two alternative forms, such as those leading to tallness or dwarfness. The genes are structures distributed in a definite order in larger bodies of the nucleus of each cell, the chromosomes, which occur in a number that is characteristic of each species. The pea plant, for example, has regularly 7 pairs of chromosomes; in man there are 23 pairs (Fig. 1). The chromosomes are visible under the microscope; individual genes are too small to be seen by present methods. This is not surprising, considering that chromosomes, each of which may contain hundreds or thousands of genes, are so small that a billion or more of them may fill no greater bulk than that of a drop of water. Research in the first 20 years following the rediscovery of Mendel's laws in 1900 established the general

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