Mendelian Genetics with Selection
At
the time when Darwin wrote, nothing whatever was known about the
laws of heredity, and all that he had to go upon was the vague
notion that offspring tended to strike an average between the
characters of their parents, a supposition that went by the name of
'blending inheritance', and which occasioned for Darwin the
greatest difficulty with which he had to contend in formulating his
theory. In the first place, if blending inheritance were true, it
would mean that any new variation which appeared, even if
heritable, would be rapidly diluted by 'swamping', and in about ten
generations would have been obliterated. To compensate for this it
would be necessary to suppose that new variations were extremely
frequent. Since whole brothers, sons of the same father and mother,
share an identical heredity, any difference between them would have
to be due to new variation that had arisen during their own early
lives, and variation would have to affect practically all members
of a species. This problem of the supply of variation was a
difficulty which Darwin felt so acutely that it even led him to
look for a source of this supply in the supposed hereditary effects
of use and disuse.
This reliance on use and disuse as a source of
variation, without any effect on his main argument, is the only
part of Darwin's demonstration which has had to be abandoned, and
he would have welcomed the reasons for it. If only Darwin had
realised it, the solution to all these difficulties was being
provided by Gregor Mendel, but his results remained unknown until
1900, eighteen years after Darwin's death.
The Mendelian theory of the gene, as worked out
by T. H. Morgan and his colleagues with an unprecedented wealth of
experimental evidence from the breeding pen and from cytological
studies on the structure of the cell and its chromosomes, has
established as firmly as Newton's laws of motion or the atomic
theory that hereditary resemblances are determined by discrete
particles, the genes, molecules situated in the chromosomes of the
cells, which are transmitted to offspring in accordance with the
mechanism of germ-cell formation and fertilization, and conform to
distributional patterns known as Mendelian inheritance). The
researches of C. D. Darlington and others on the structure and
behaviour of the chromosomes have reached such a degree of
refinement and precision that each step in the mechanism of
Mendelian inheritance can actually be seen under the
microscope.
The genes preserve their separate identity; they
collaborate in the production of the characters of the individual
that possesses them, but they never contaminate each other; they
remain constant for long periods but, from time to time, they
undergo a change known as mutation and involving a change in the
characters which they control, after which they remain constant in
their new condition until they mutate again. It has been
conclusively proved that the theory of the gene applies to all
plants and all animals investigated, and that the mutation of genes
is the only known way in which heritable variation arises. The
modifications resulting from good or bad food supply, or from the
climatic conditions in which plants and animals live, are not
inherited and therefore without significance in evolution.
The history of the reception of Mendelian
genetics after its discovery has been peculiar. The earliest
mutations discovered, often called 'sports', were usually
deleterious and showed marked and discontinuous steps instead of
the gradual and continuous variation which Darwinian selectionists
looked for as the raw material of evolution. Selectionists
therefore rejected Mendelian genetics as the source of variation.
On the other hand the Mendelian geneticists, knowing that their
mutations were the only source of heritable variation, thought that
as they showed wide discontinuous steps and arose suddenly,
ready-made and apparently without long-continued selection,
selection was inoperative in evolution and they rejected it.
With the progress of knowledge it gradually
became obvious that each of the two schools of research objected to
the other for reasons which were baseless. As more and more genes
were identified and their effects studied, it became clear that the
wide and discontinuous mutations first observed were the more
easily detected extremes of a range in which the majority exert
only slight effects. For the same reason, these mutations were
deleterious because organisms are delicately adjusted systems, more
likely to be upset by large and discontinuous changes than by small
and gradual steps.
The Mendelian geneticists also had to learn two
lessons. On the one hand they discovered that although individual
genes are associated with particular characters, their control of
those characters is also affected by all the other genes, which
constitute an organized gene-complex. As a result of previous
mutations, gene-complexes of plants and animals in nature contain
many genes and these are sorted out and recombined at fertilization
in astronomically numerous possibilities of permutations. These
recombinations have been shown to bring about gradual and
continuous changes in the characters under the major control of
individual genes. Sir Ronald Fisher demonstrated the significance
of this by showing that a mutant gene that now exhibits the quality
known as dominance has gradually become dominant from a previous
intermediate condition. This is what has happened to those
mutations that confer a benefit on their possessors, and in their
case there has been a selection of gene-complexes in favour of
those which accentuate the effects of a favourable mutant, so that
these effects are manifested even if the mutant gene is inherited
from only one parent, which is the definition of dominance.
Conversely, with genes that place a handicap on their possessors,
there has been a selection of gene- complexes in favour of those
which suppress the effects of such genes so that they are
manifested only when the mutant gene is inherited from both parents
and the gene has gradually become recessive; or suppress them even
further when the effects of such a gene are obliterated and the
gene becomes what is known as 'modifier' without major control over
characters. It has even been demonstrated by E. B. Ford under
rigorous experimental conditions that one and the same mutant gene
can be made to become dominant in one strain and recessive in
another simply by selecting as parents those individuals whose
gene-complexes accentuate or diminish the effects of the
gene.
The Mendelian geneticists' second lesson was the
realisation that although the effects of the mutations which they
first observed appeared to be clear-cut, they were already the
results of selection in past gene- complexes. For these mutations
have occurred before, and the gene- complexes have become adjusted
to them. The fact that a single gene may now act as a switch
controlling the production of one or other character-difference
does not mean that this character- difference originally arose at
one stroke by one mutation of such a switch-gene, because it has
probably been built up gradually as a result of past selection in
the gene-complex.
It is therefore clear that mutations and
recombinations of genes provide the supply of variation on which
selection acts to produce evolution exactly in the way Darwin's
theory requires. Its requirements are exacting, for as T. H. Huxley
pointed out, some organisms have evolved slowly and others have
evolved fast, and he saw that natural selection was the only
mechanism that could satisfy both those requirements. It is able to
do so because Mendelian inheritance is capable of producing both
diversity and stability. As E. B. Ford has said, an immense range
of types must be available for natural selection to act upon, and
this is provided by mutation and recombination of genes; yet when a
favourable gene-complex has been achieved it must not be dissipated
and broken down, and this is provided against by the facts that the
genes do not blend or contaminate one another, and that they mutate
only rarely.
Ecological genetics describes the experimental
study of evolution and natural selection carried out by means of
combined field-work and laboratory breeding. The field work needed
in these investigation is of several kinds. It involves detailed
observation, usually conducted on successive generations, having
strict regard to the ecology of the habitats. It often requires
long-continued estimates of the frequency of genes, or the
characters controlled by genetic mechanisms, together with
population size. The research also involves founding of new
colonies in nature, not the more familiar technique of establishing
them in the closed laboratory.
A major experimental finding is that it is the
independent appearance of new mutations in each population, and the
independent course of genetic change within populations, that
together make up the driving force within the organism which tends
to make each isolated population gradually become different from
every other. The force outside the organism that aids the
process is simpler. Natural selection acts to adapt the population
to its surroundings. But no two patches of woodland, no two
freshwater ponds, will be absolutely identical, even if they lie in
the same area of country. They may differ in the precise nature of
their soil or water, in their range of temperature, or their
average temperature, or in the particular species of animal or
plant that may become unusually rare or unusually common in that
locality. Since each population has to adapt to slightly different
conditions, the two populations will gradually come to differ from
one another.
The history of a patch of sunflowers living in a
ditch in the Sacramento Valley of California provides a good
example of the way in which all these forces can gradually make two
populations become quite different from one another. The population
consisted of natural hybrids between the two annual sunflowers of
California, Helianthus annum and H. bolanderi. To
begin with, the original population gradually became split into two
by a drying-out of part of the ditch, the dry section being
colonized by grasses among which the sunflowers could not survive.
Over the space of five years, the dry grassy patch widened until it
had pushed the two separated sub-populations of sunflowers over 100
metres apart. One of these was now in a deeper part of the ditch,
which remained wet until late spring, while the other grew in a
shallower, drier position. The two sub-populations became different
in a number of characteristics, such as the shape of the flower
head as a whole, the number of sterile floret rays surrounding the
head, the shape of the base of the leaf, and the length of the
hairs on the stem and leaves. Even though bees could easily fly
from one population to the other, so that some cross-pollination
between them must have taken place, observations over the next
seven years showed that the differences between the two populations
did not disappear. Their environments differed sufficiently to
ensure that natural selection preserved the distinctiveness of the
two populations.
Exactly the same process takes place in animal
populations though, because they can move, the separate populations
may each cover a larger area. For example, two species of Carpenter
Bee, Xylocopa diversipes and X. nobilis, live in the
large East Indian island of Celebes. Three different colour
patterns of each species are known on Celebes, each making up one
or more separate populations. Another three colour patterns of
X. nobilis are known to exist on neighbouring islets.
Once populations have started to diverge in their
genetic adaptations in this way, the foundations for the appearance
of a new species have been laid. If two divergent populations
should meet again when the process has not gone very far, they may
completely hybridize and merge into one another. The further, vital
step towards the appearance of a new species is when hybrids
between the two independent populations do appear, but only along a
narrow zone where the two populations meet. Such a situation
suggests that, though continued interbreeding within this zone can
produce a population of hybrids, these hybrids cannot compete
elsewhere with either of the pure parent populations. This seems to
be the situation with the woodpecker-like flickers of North
America. The eastern yellow-shafted flicker, Colaptes
auratus, does mate with the western red-shafted flicker, C.
cafer, along a narrow 2OOO-mile-long stretch where the two
meet, but this zone of hybridization does not seem to be
spreading.
The principles revealed by this kind of research
that includes the analysis of variation and selection in the most
diverse forms within a population, show that they apply in higher
plants, butterflies, snails, or men. So much so indeed is this true
that by applying these fundamental concepts it has been possible to
make genetic predictions from one group to another and subsequently
to find them verified.
One of the most far-reaching results of recent
work on ecological genetics is the discovery that unexpectedly
great selective forces are normally operating to maintain or to
adjust the adaptations of organisms in natural conditions.
Selective advantages of 20 or 30 per cent and more are common and
indeed usual: a general conclusion derived from the study of a wide
range of forms whatever the control of the variation investigated
may be. That consideration forces us completely to readjust our
thoughts on evolution and to recognize that a population may adapt
itself very rapidly to changing conditions.
The direction of selection may sometimes alter
with time. In these circumstances, a particular character may have
a total advantage of small magnitude when considered over a number
of generations, yet swift adaptations to changing conditions are
possible since the opposed selective forces are generally
great.
If ever it could have been thought that mutation
is important in the control of evolution, it is impossible to think
so now; for not only do we observe it to be so rare that it cannot
compete with the forces of selection but we know that this must
inevitably be so. Mutation is originally responsible for the
diversity of the genetic units, but living organisms are the
product of evolution controlled not by mutation but by
powerful selection.
Among the aims of ecological genetics, the study
of evolution in wild populations is outstanding. Yet that project
is possible only when the process takes place rapidly. This it has
been found to do
(i) when marked numerical fluctuations affect
isolated communities;
(ii) when polygenic characters can be studied
either (a) in populations inhabiting ecologically distinct and
isolated areas or (b)even in the absence of isolation if subject to
very powerful selection-pressures;
(iii) when a species successfully invades a new
country or territory;
(iv) in all types of genetic polymorphism.
It proved essential to define the latter
condition precisely. Once that had been done, its analysis could be
carried out in species belonging to very distinct groups; a
procedure which has disclosed two of its fundamental
properties.
First, since the co-operation of several
major-genes is often necessary to promote advantageous adjustments,
the evolution of close linkage between them is favoured, so holding
together the appropriate characters for which they are responsible.
Consequently polymorphism is often controlled by means of a
super-gene.
Secondly, when a major-gene spreads in a
population, owing to changes in the environment or in the
gene-complex, we have seen that it will tend to develop
heterozygous advantage. It and its allele will therefore be
stabilised at the best attainable frequencies: that is to say,
polymorphisms are then generated, so that they must be very
widespread and important phenomena.
That is one of the reasons why genetics is
playing an increasing part in medicine. For, owing to the powerful
selection-pressures now known to operate, polymorphisms must be
associated with considerable, though balanced, advantages and
disadvantages, the existence of which they indeed advertise. Thus,
however trivial the characters may be by which we detect their
alternative phases, these must have reached their observed
frequencies because the genetic unit controlling them is of
importance to the organism. The fact that a quarter of the
population in western Europe is unable to taste phenyl-thio-urea
for long seemed a negligible matter to the medical geneticist. Its
association with thyroid disease has altered that point of view
now.
The advances brought about by a combination of
field-studies and laboratory genetics can be appreciated by looking
to the past. Charles Darwin expressed his belief that by choosing
the right material it might be possible actually to detect
evolutionary changes taking place in natural conditions at the
present time. For this purpose he said that long- continued
investigations and careful records would be needed, extending over
a period which he estimated at perhaps fifty years in species
reproducing annually. As usual, Darwin was right, but on this
occasion he was too pessimistic. The techniques of ecological
genetics have made it possible not only to observe evolutionary
changes in three or four generations in such forms but to evaluate
the force of selection in wild populations and to analyze its
immediate effects.