Evolution Now: A Century after Darwin

241 Pages·1982·23.998 MB·English

by  John Maynard Smith (eds.)

EVOLUTION NOW
EVOLUTION NOW
A CENTURY AFTER DARWIN
EDITED BY
John Maynard Smith
SCHOOL OF BIOLOGICAL SCIENCES,
UNIVERSITY OF SUSSEX
M
© Nature 1982
Softcover reprint of the hardcover 18t edition 1982
978-0-333-33595-6
All rights reserved. No part of this publication
may be reproduced or transmitted, in any form
or by any means, without permission.
First published 1982 by
NATURE in association with
THE MACMILLAN PRESS LTD
London and Basingstoke
Companies and representatives throughout the
world
Typeset by Oxprint Ud, Oxford
ISBN 978-0-333-33603-8 ISBN 978-1-349-86046-3 (eBook)
DOI 10.1007/978-1-349-86046-3
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CONTENTS
Introduction 1
The Drigin of Life 7
The Evolution of the Genome 39
Lamarckian Inheritance and the Puzzle of Immunity 91
The Pattern of Nature 107
Evolution-Sudden or Gradual? 125
The Evolution of Behaviour 183
INTRODUCTION
LUDWIG Boltzmann once wrote that the nineteenth century
would be remembered as the century of Darwin. One hundred
years after Darwin's death this judgement still seems perceptive.
No other writer had such a profound effect on the way we see
ourselves, and no other brought about so great an extension in
the range of subjects which we regard as explicable by scientific
theory. Here, however, I shall confine myself to his contribution
to evolutionary biology, and shall forget that he was the founder
of ecology and ethology, and made significant contributions to
geology and psychology. It was, after all, his formulation of the
theory of evolution by natural selection that was decisive.
In recent years there have been daims-in the daily press, on
television, and by retired cosmologists-that Darwin may have
got it wrong. Some excuse can be found in the fact that Darwin
has indeed been criticised by scientists working in a variety of
fields-for example palaeontology, taxonomy and embryology.
At least one group of scientists have daimed that a new evolu
tionary paradigm is on the way. The most controversial of these
issues are debated in this book. However, to see Darwinism as
being under serious threat would, I think, be a false perception.
The error arises because Darwin's theory is so central to modem
biology that any new idea may first be seen (as Mendelian genetics
was seen) as being in conflict with Darwinism.
This volume presents some current controversies and recent
advances in evolutionary biology, by reprinting papers published
in the last few years. But let me, as a background, first give abrief
history of evolutionary ideas since Darwin. In the Origin o[ Spedes,
Darwin aimed to establish two things. First, he argued that evolu
tion had in fact happened (that is, that all existing organisms are
descended from one or a few simple ancestral forms), and, second,
that the main cause of evolutionary change was the natural selec
tion of variations that were in their origin non-adaptive. The
main weaknesses of his position were that he had no adequate
theory of genetics, and that he could give no satisfactory account
2 INTRODUCTION
of the origin of the variations on which selection would later act.
In genetics, Darwin was a Lamarckist. That is, he thought that if
organisms acquired characteristics by use and disuse during their
lifetimes, this would influence the nature of their offspring. In
thinking this, he was sharing an opinion held by almost all his
contemporaries.
The first major advance after Darwin was made by August
Weismann, who argued for the independence of the 'germ line'
(the celllineage leading from the fertilized egg to the germ ceHs,
egg and sperm, which form the starting point of the next genera
tion) from the 'soma' (the celllineage from fertilized egg to adult
body). As a boy, I acquired, from reading the preface to Shaw's
Back to Methuselah, a picture of Weismann as a cruel and ignorant
German pedant who cut the tails off mice to see if their offspring
had tails. What a ridiculous experiment! Since the mice did not
actively suppress their tails as an adaptation to their environ
ment, no Lamarckist would expect the loss to be inherited. Much
later, I discovered that Weismann was not as I had imagined him.
His experiment on mice was performed only because, when he
first put forward his theory, he was met with the objection that,
(as was, it was claimed, weH known) if a dog's tail is docked, its
children are often tailless-an early use of what J.B.S. Haldane
once called Aunt Jobisca's theorem, 'It's a fact the whole world
knows'.
Much more interesting are Weismann's reasons for proposing
his theory, and its implications for Darwinism. At first sight, his
reasons were poor. It is often not the case (for example, in higher
plants) that the germ line is a lineage distinct from the soma. Even
when it is distinct, the material and energy it needs are supplied
by the soma. In Weismann's day, the experimental evidence for
the 'non-inheritance of acquired characters' was weak. Why,
then, did he believe it? I think that the clue lies in his remark that,
if one were to come across.a case of the inheritance of an acquired
character, it would l-e as if one were to send a telegram to China
and it arrived translated into Chinese. This is the first use known
to me of the information analogy in heredity. Weismann did not
accept the inheritance of acquired characters because he could
not conceive of a mechanism of 'reverse translation', whereby the
hypertrophied muscles of the blacksmith could be translated into
genes (he called them 'ids') which could, in the next generation,
cause the growth of large muscIes.
If Weismann was right, this greatly strengthened Darwin's
theory. Natural selection, instead. of being just one of the possible
INTRODUCTION 3
proeesses leading to evolutionary adaptation, beeomes the only
proeess (at least, until the evolution of organisms sufficiently
intelligent to learn from their parents).
The next important step was the rediseovery of Mendel's laws
at the start of this eentury, and the formulation of the chromo
some theory of heredity. The first impact of Mendelism on evolu
tionary biology was distinctly odd. The early Mendelians saw
themselves as anti-Darwinians; Darwin's banner was held aloft
by the biometrie school, who concentrated on measuring the
correlations between relatives, and who regarded genes as meta
physical entities. The Mendelians saw the 'mutations' they studied
as each being the potential starting point of new species, and
the continuous variation studied by the biometricians as evolu
tionarily irrelevant; the biometricians saw mutations as patho
logical deviants doomed to early elimination by selection, and
continuous variation as the stuff of evolution. The argument, led
by Bateson on the one side, and Pearson on the other, fore
shadowed the current debate between punctuationists and
gradualists, discussed below. It is part of the larger debate be
tween those who see the world as eontinuous and those who
think it proeeeds in jerks.
The debate, at least in the form in which it then presented itself,
was largely settled by the work of the population geneticists,
Fisher, Haldane and Wright. Two points were made clear. First,
the continuous variation studied by the biometricians could be
explained by alternative alleles at many loci, each by itself having
a small effect on the phenotype. Second, even rather small dif
ferences in fitness between genotypes are sufficient to determine
the direction of evolutionary change, despite mutation being
mainly in an opposite direction.
The work of the population geneticists prepared the way for
the 'modem synthesis' of evolutionary biology, developed in the
period 1930-1950 by a group including Dobzhansky, Ford, Julian
Huxley, Mayr, Muller, Rensch, Simpson and Stebbins. It is hard
in a few sentences to describe what these men did. In effect, they
showed that the 'neo-Darwinian' mechanism-natural selection
in Mendelian populations-was sufficient to explain the evolu
tionary process as it could be observed in nature. Dobzhansky,
Ford and others measured genetic variability and natural selec
tion in wild populations. Mayr and Rensch (for animals) and
Stebbins (for plants) studied geographical variation within and
between species, and discussed how new species might arise.
Simpson argued that the fossil record could best be understood in
4 INTRODUCTION
Darwinian terms. Most research in evolutionary biology since
that time has been carried out in the framework of the modern
synthesis. Particular efforts have been made in areas which, at
least at first sight, seem to be difficult to explain in terms of
natural selection, for example, the evolution of social behaviour
and of sex and breeding systems.
Since 1950, developments in molecular biology have had a
growing influence on the theory of evolution. The 'central dogma'
of molecular biology, according to which information can pass
from nucleic acid to protein, but not from protein to nucleic acid,
provides a molecular explanation for Weismann's principle, thus
leaving natural selection as the only agent of adaptation. Impor
tant as this is, however, two additional points should be made.
First, even if 'reverse translation' of amino acid sequences into
base sequences were possible, this would not provide a general
mechanism for Lamarckian inheritance, because most develop
mental adaptations do not involve the production of new protein
sequences. Second, there are good reasons why, even if living
organisms have arisen independently many times in the uni
verse, Lamarckian processes should playa minor role in their
evolution. Most 'acquired characters' are non-adaptive--they are
the results of age, injury and disease. Therefore, a hereditary
mechanism which transmitted such characters to offspring
would work against the evolution of adaptation. Hence the one
way flow of information from nucleic acid to protein may have
been a necessary feature of an hereditary mechanism able to
support evolution. In physics, the second law of thermo
dynamics asserts that entropy will increase in a closed physical
system. In biology, Weismann's principle, together with the
principle of natural selection, makes possible the maintenance,
and even the increase, of information in open biological systems.
Molecular biology has had an impact on evolutionary theory in
other ways. Protein electrophoresis has provided a way of
measuring the genetic variability of populations; its main value
may be in enabling us to discover more about the breeding
structure of populations. Sequence data on proteins and nucleic
acids can be used to work out the phylogenetic relationships of
existing organisms. In this context, sequences have the advantage
over morphological data in that they provide a means of estimating
the number of genetic changes separating two forms. The in
formation we are acquiring about how DNA is arranged in
chromosomes may at last give us some insight into the evolu
tionary significance of chromosome structure. Two questions in
INTRODUCTION 5
particular are being asked. First, does the large-scale arrange
ment of genes on chromosomes have any significance for de
velopment, or is it merely a way of ensuring accurate gene
segregation during cell division? Second, does all the DNA in the
genome perform some useful function in the survival or repro
duction of the organism, or is some part of it 'selfish' or 'para
sitic'? The second question raises a set of problems which are
logically similar to those which have been debated for some time
by students of the evolution of social behaviour, that is, questions
about the levels at which selection acts and the differences be
tween what Dawkins has called 'replicators' and 'vehicles'.
There is, however, one area of molecular biology which seems
to me to lag behind the rest. This is the study of the evolution of
prokaryotes (organisms such as bacteria lacking a proper cell
nucleus). The modem synthesis of the 1940s was concemed with
eukaryotes (organisms with a cell nucleus, usually sexual and
diploid). Its essential achievement was to bring together two
previously separate disciplines-the chromosome theory of
heredity and the study of natural populations. The same syn
thesis is now required for the prokaryotes. There is an abundant
knowledge of their genetics, but as yet no adequate synthesis of
that knowledge with a study of the natural history ofbacteria. For
example, we have little idea of the significance of conjugation for
bacterial populations; it is as if we had no idea of the significance
of sexual reproduction for populations of birds or insects. Popu
lation thinking has been weIl developed for fully half a century,
but has yet to be adopted by microbiology.
The papers printed below have been grouped under six topics,
each with abrief introduction aimed at doing three things. First, I
explain how the topic is related to Darwin's ideas. Second, I have
tried to help non-specialists to find their way through papers
which are sometimes rather technical. Some of the critical tech
nical terms are defined in the introductory passages, but it is
inevitable that some words will be unfamiliar to some readers.
Nevertheless, I would urge readers to press on regardless; it
should usually be possible to grasp the gist of the argument.
Finally, I have allowed myself the indulgence of expressing my
own opinion on some of the more controversial issues.
The selection of papers has inevitably been somewhat arbitrary.
We have not aimed at reprinting just the most important papers
published in recent years. Instead, we have concentrated on the
more controversial fields at the expense of others in which
equally valuable work is being done. Some topics have been