VARIATION AND EVOLUTION
Through the facts of heredity we have reached a new conception of the
individual. Hitherto we have been accustomed to distinguish between the
members of a family of rabbits like that illustrated on Plate I. by
assigning to each an individuality, and by making use of certain external
features, such as the coat colour or the markings, as convenient outward
signs to express our idea that the individuality of these different animals
is different. Apart from this, our notions as to what constituted the
individuality in each case were at best but vague. Mendelian analysis has
placed in our hands a more precise method of estimating and expressing the
variations that are to be found between one individual and another. Instead
of looking at the individual as a whole, which is in some vague way endowed
with an individuality marking it off from its fellows, we now regard it as
an organism built up of definite characters superimposed on a basis beyond
which for the moment our analysis will not take us. We have begun to
realise that each individual has a definite architecture, and that this
architecture depends {136} primarily upon the number and variety of the
factors that existed in the two gametes that went to its building. Now most
species exhibit considerable variation and exist in a number, often very
large, of more or less well-defined varieties. How far can this great
variety be explained in terms of a comparatively small number of factors if
the number of possible forms depends upon the number of the factors which
may be present or absent?
In the simple case where the homozygous and heterozygous conditions are
indistinguishable in appearance the number of possible forms is 2, raised
to the power of the number of factors concerned. Thus where one factor is
concerned there are only 2^1 = 2 possible forms, where ten factors are
concerned there are 2^{10} = 1024 possible forms differing from one another
in at most ten and at least one character. Where the factors interact upon
one another this number will, of course, be considerably increased. If the
heterozygous form is different in appearance from the homozygous form,
there are three possible forms connected with each factor; for ten such
factors the possible number of individuals would be 3^{10} = 59,049; for
twenty such factors the possible number of different individuals would be
3^{20} = 3,486,784,401. The presence or absence of a comparatively small
number of factors in a species carries with it the possibility of an
enormous range of individual variation. But every one of these individuals
has a perfectly definite constitution which can {137} be determined in each
case by the ordinary methods of Mendelian analysis. For in every instance
the variation depends upon the presence or absence of definite factors
carried in by the gametes from whose union the individual results. And as
these factors separate out cleanly in the gametes which the individual
forms, such variations as depend upon them are transmitted strictly
according to the Mendelian scheme. Provided that the constitution of the
gametes is unchanged, the heredity of such variation is independent of any
change in the conditions of nutrition or environment which may operate upon
the individual producing the gametes.
But, as everybody knows, an individual organism, whether plant or animal,
reacts, and often reacts markedly, to the environmental conditions under
which its life is passed. More especially is this to be seen where such
characters as size or weight are concerned. More sunlight or a richer soil
may mean stronger growth in a plant, better nutrition may result in a finer
animal, superior education may lead to a more intelligent man. But although
the changed conditions produce a direct effect upon the individual, we have
no indisputable evidence that such alterations are connected with
alterations in the nature of the gametes which the individual produces. And
without this such variations cannot be perpetuated through heredity, but
the conditions which produce the effect must always be renewed in each
{138} successive generation. We are led, therefore, to the conclusion that
two sorts of variations exist, those which are due to the presence of
specific factors in the organism and those which are due to the direct
effect of the environment during its lifetime. The former are known as
_mutations_, and are inherited according to the Mendelian scheme; the
latter have been termed _fluctuations_, and at present we have no valid
reason for supposing that they are ever inherited. For though instances may
be found in which effects produced during the lifetime of the individual
would appear to affect the offspring, this is not necessarily due to
heredity. Thus plants which are poorly nourished and grown under adverse
conditions may set seed from which come plants that are smaller than the
normal although grown under most favorable conditions. It is natural to
attribute the smaller size of the offspring to the conditions under which
the parents were grown, and there is no doubt that we should be quite right
in doing so. Nevertheless, it need have nothing to do with heredity. As we
have already pointed out, the seed is a larval plant which draws its
nourishment from the mother. The size of the offspring is affected because
the poorly nourished parent offered a bad environment to the young plant,
and not because the gametes of the parent were changed through the adverse
conditions under which it grew. The parent in this case is not only the
producer of gametes, but also a part of the environment of the young {139}
plant, and it is in this latter capacity that it affects its offspring.
Wherever, as in plants and mammals, the organism is parasitic upon the
mother during its earlier stages, the state of nutrition of the latter will
almost certainly react upon it, and in this way a semblance of transmitted
weakness or vigour is brought about. Such a connection between mother and
offspring is purely one of environment, and it cannot be too strongly
emphasised that it has nothing to do with the ordinary process of heredity.
The distinction between these two kinds of variation, so entirely different
in their causation, renders it possible to obtain a clearer view of the
process of evolution than that recently prevalent. As Darwin long ago
realised, any theory of evolution must be based upon the facts of heredity
and variation. Evolution only comes about through the survival of certain
variations and the elimination of others. But to be of any moment in
evolutionary change a variation must be inherited. And to be inherited it
must be represented in the gametes. This, as we have seen, is the case for
those variations which we have termed mutations. For the inheritance of
fluctuations, on the other hand, of the variations which result from the
direct action of the environment upon the individual, there is no
indisputable evidence. Consequently we have no reason for regarding them as
playing any part in the production of that succession of temporarily stable
forms which we term evolution. In {140} the light of our present knowledge
we must regard the mutation as the basis of evolution--as the material upon
which natural selection works. For it is the only form of variation of
whose heredity we have any certain knowledge.
It is evident that this view of the process of evolution is in some
respects at variance with that generally held during the past half century.
There we were given the conception of an abstract type representing the
species, and from it most of the individuals diverged in various
directions, though, generally speaking, only to a very small extent. It was
assumed that any variation, however small, might have a selection value,
that is to say, could be transmitted to the offspring. Some of these would
possess it in a less and some in a greater degree than the parent. If the
variation were a useful one, those possessing to a rather greater extent
would be favoured through the action of natural selection at the expense of
their less fortunate brethren, and would leave a greater number of
offspring, of whom some possessed it in an even more marked degree than
themselves. And so it would go on. The process was a cumulative one. The
slightest variation in a favourable direction gave natural selection a
starting-point to work on. Through the continued action of natural
selection on each successive generation the useful variation was gradually
worked up, until at last it reached the magnitude of a specific {141}
distinction. Were it possible in such a case to have all the forms before
us, they would present the appearance of a long series imperceptibly
grading from one extreme to the other.
Upon this view are made two assumptions not unnatural in the absence of any
exact knowledge of the nature of heredity and variation. It was assumed, in
the first place that variation was a continuous process, and, second, that
any variation could be transmitted to the offspring. Both of these
assumptions have since been shown to be unjustified. Even before Mendel's
work became known Bateson had begun to call attention to the prevalence of
discontinuity in variation, and a few years later this was emphasised by
the Dutch botanist Hugo de Vries in his great work on _The Mutation
Theory_. The ferment of new ideas was already working in the solution, and
under the stimulus of Mendel's work they have rapidly crystallised out.
With the advent of heredity as a definite science we have been led to
revise our views as to the nature of variation, and consequently in some
respects as to the trend of evolution. Heritable variation has a definite
basis in the gamete, and it is to the gamete, therefore, not to the
individual, that we must look for the initiation of this process. Somewhere
or other in the course of their production is added or removed the factor
upon whose removal or addition the new variation owes its existence. The
new variation springs into being by a {142} sudden step, not by a process
of gradual and almost imperceptible augmentation. It is not continuous but
discontinuous, because it is based upon the presence or absence of some
definite factor or factors--upon discontinuity in the gametes from which it
sprang. Once formed, its continued existence is subject to the arbitrament
of natural selection. If of value in the struggle for existence natural
selection will decide that those who possess it shall have a better chance
of survival and of leaving offspring than those who do not possess it. If
it is harmful to the individual natural selection will soon bring about its
elimination. But if the new variation is neither harmful nor useful there
seems no reason why it should not persist.
In this way we avoid a difficulty that beset the older view. For on that
view no new character could be developed except by the piling up of minute
variations through the action of natural selection. Consequently any
character found in animals and plants must be supposed to be of some
definite use to the individual. Otherwise it could not have developed
through the action of natural selection. But there are plenty of characters
to which it is exceedingly difficult to ascribe any utility, and the
ingenuity of the supporters of this view has often been severely taxed to
account for their existence. On the more modern view this difficulty is
avoided. The origin of a new variation is independent of natural {143}
selection, and provided that it is not directly harmful there is no reason
why it should not persist. In this way we are released from the burden of
discovering a utilitarian motive behind all the multitudinous characters of
living organisms. For we now recognise that the function of natural
selection is selection and not creation. It has nothing to do with the
formation of the new variation. It merely decides whether it is to survive
or to be eliminated.
One of the arguments made use of by supporters of the older view is that
drawn from the study of adaptation. Animals and plants are as a rule
remarkably well adapted to living the life which their surroundings impose
upon them, and in some cases this adaptation is exceedingly striking.
Especially is this so in the many instances of what is called protective
coloration, where the animal comes to resemble its surroundings so closely
that it may reasonably be supposed to cheat even the keenest sighted enemy.
Surely, we are told, such perfect adaptation could hardly have arisen
through the mere survival of chance sports. Surely there must be some
guiding hand moulding the species into the required shape. The argument is
an old one. For John Ray that guiding hand was the superior wisdom of the
Creator: for the modern Darwinian it is Natural Selection controlling the
direction of variation. Mendelism certainly offers no suggestion of any
such controlling force. It interprets the {144} variations of living forms
in terms of definite physiological factors, and the diversity of animal and
plant life is due to the gain or loss of these factors, to the origination
of new ones, or to fresh combinations among those already in existence. Nor
is there any valid reason against the supposition that even the most
remarkable cases of resemblance, such as that of the leaf insect, may have
arisen through a process of mutation. Experience with domestic plants and
animals shows that the most bizarre forms may arise as sports and
perpetuate themselves. Were such forms, arising under natural conditions,
to be favoured by natural selection owing to a resemblance to something in
their environment we should obtain a striking case of protective
adaptation. And here it must not be forgotten that those striking cases to
which our attention is generally called are but a very small minority of
the existing forms of life.
For that special group of adaptation phenomena classed under the head of
Mimicry, Mendelism seems to offer an interpretation simpler than that at
present in vogue. This perhaps may be more clearly expressed by taking a
specific case. There is in Africa a genus of Danaine butterflies known as
_Amauris_, and there are reasons for considering that the group to which it
belongs possesses properties which render it unpalatable to vertebrate
enemies such as birds or monkeys. In the same region is also found the
genus _Euralia_ belonging to the entirely {145} different family of the
Nymphalidae, to which there is no evidence for assigning the disagreeable
properties of the Danaines. Now the different species of _Euralia_ show
remarkably close resemblances to the species of _Amauris_, which are found
flying in the same region, and it is supposed that by "mimicking" the
unpalatable forms they impose upon their enemies and thereby acquire
immunity from attack. The point at issue is the way in which this seemingly
purposeful resemblance has been brought about.
One of the species of _Euralia_ occurs in two very distinct forms (Pl.
VI.), which were previously regarded as separate species under the names
_E. wahlbergi_ and _E. mima_. These two forms respectively resemble
_Amauris dominicanus_ and _A. echeria_. For purposes of argument we will
assume _A. echeria_ to be the more recent form of the two. On the modern
Darwinian view certain individuals of _A. dominicanus_ gradually diverged
from the _dominicanus_ type and eventually reached the _echeria_ type,
though why this should have happened does not appear to be clear. At the
same time those specimens which tended to vary in the direction of _A.
echeria_ in places where this species was more abundant than _A.
dominicanus_ were encouraged by natural selection, and under its guiding
hand the form _mima_ eventually arose from _wahlbergi_.
According to Mendelian views, on the other hand, {146} _A. echeria_ arose
suddenly from _A. dominicanus_ (or _vice versa_), and similarly _mima_
arose suddenly from _wahlbergi_. If _mima_ occurred where _A. echeria_ was
common and _A. dominicanus_ was rare, its resemblance to the more plentiful
distasteful form would give it the advantage over _wahlbergi_ and allow it
to establish itself in place of the latter. On the modern Darwinian view
natural selection gradually shapes _wahlbergi_ into the _mima_ form owing
to the presence of _A. echeria_; on the Mendelian view natural selection
merely conserves the _mima_ form when once it has arisen. Now this case of
mimicry is one of especial interest, because we have experimental evidence
that the relation between _mima_ and _wahlbergi_ is a simple Mendelian one,
though at present it is uncertain which is the dominant and which the
recessive form. The two have been proved to occur in families bred from the
same female without the occurrence of any intermediates, and the fact that
the two segregate cleanly is strong evidence in favour of the Mendelian
view. On this view the genera _Amauris_ and _Euralia_ contain a similar set
of pattern factors, and the conditions, whatever they may be, which bring
about mutation in the former lead to the production of a similar mutation
in the latter. Of the different forms of _Euralia_ produced in any region
that one has the best chance of survival, through the operation of natural
selection, which resembles the most plentiful _Amauris_ form. Mimetic
resemblance is a true phenomenon, but natural selection plays the part of a
conservative, not of a formative agent.
[Illustration: PLATE VI.]
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It is interesting to recall that in earlier years Darwin was inclined to
ascribe more importance to "sports" as opposed to continuous minute
variation, and to consider that they might play a not inconsiderable part
in the formation of new varieties in nature. This view, however, he gave up
later, because he thought that the relatively rare sport or mutation would
rapidly disappear through the swamping effects of crossing with the more
abundant normal form, and so, even though favoured by natural selection,
would never succeed in establishing itself. Mendel's discovery has
eliminated this difficulty. For suppose that the sport differed from the
normal in the loss of a factor and were recessive. When mated with the
normal this character would seem to disappear, though, of course, half of
the gametes of its progeny would bear it. By continual crossing with
normals a small proportion of heterozygotes would eventually be scattered
among the population, and as soon as any two of these mated together the
recessive sport would appear in one quarter of their offspring.
A suggestive contribution to this subject was recently made by G. H. Hardy.
Considering the distribution of a single factor in a mixed population
consisting of the heterozygous and the two homozygous forms he showed that
such a population breeding at random rapidly fell into a {148} stable
condition with regard to the proportion of these three forms, whatever may
have been the proportion of the three forms to start with. Let us suppose
for instance, that the population consists of p homozygotes of one kind, r
homozygotes of the other kind, and 2 q heterozygotes. Hardy pointed out
that, other things being equal, such a population would be in equilibrium
for this particular factor so long as the condition q^2 = pr was fulfilled.
If the condition is fulfilled to start with, the population remains in
equilibrium. If the condition is not fulfilled to start with, Hardy showed
that a position of equilibrium becomes established after a single
generation, and that this position is thereafter maintained. The
proportions of the three classes which satisfy the equation q^2 = pr are
exceedingly numerous, and populations in which they existed in the
proportions shown in the appended table would remain in stable equilibrium
generation after generation:--
p. 2q. r.
1 2 1
1 4 4
1 6 9
1 8 16
1 20,000 100,000,000
1 2n n^2
This, of course, assumes that all three classes are equally fertile, and
that no form of selection is taking place to the {149} benefit of one class
more than of another. Moreover, it makes no difference whether p represents
the homozygous dominants or whether it stands for the recessives. A
population containing a very small proportion of dominants and one
containing a similar proportion of recessives are equally stable. The term
dominant is in some respects apt to be misleading, for a dominant character
cannot in virtue of its dominance establish itself at the expense of a
recessive one. Brown eyes in man are dominant to blue, but there is no
reason to suppose that as years go on the population of these islands will
become increasingly brown eyed. Given equality of conditions both are on an
equal footing. If, however, either dominant or recessive be favoured by
selection the conditions are altered, and it can be shown that even a small
advantage possessed by the one will rapidly lead to the elimination of the
other. Even with but a 5 per cent selection advantage in its favour it can
be shown that a rare sport will oust the normal form in a few hundred
generations. In this way we are freed from a difficulty inherent in the
older view that varieties arose through a long-continued process involving
the accumulation of very slight variations. On that view the establishing
of a new type was of necessity a very long and tedious business, involving
many thousands of generations. For this reason the biologist has been
accustomed to demand a very large supply of time, often a great deal more
than the physicist is {150} disposed to grant, and this has sometimes led
him to expostulate with the latter for cutting off the supply. On the newer
views, however, this difficulty need not arise, for we realise that the
origin and establishing of a new form may be a very much more rapid process
than has hitherto been deemed possible.
One last question with regard to evolution. How far does Mendelism help us
in connection with the problem of the origin of species? Among the plants
and animals with which we have dealt we have been able to show that
distinct differences, often considerable, in colour, size, and structure,
may be interpreted in terms of Mendelian factors. It is not unlikely that
most of the various characters which the systematist uses to mark off one
species from another, the so-called specific characters, are of this
nature. They serve as convenient labels, but are not essential to the
conception of species. A systematist who defined the wild sweet pea could
hardly fail to include in his definition such characters as the procumbent
habit, the tendrils, the form of the pollen, the shape of the flower, and
its purple colour. Yet all these and other characters have been proved to
depend upon the presence of definite factors which can be removed by
appropriate crossing. By this means we can produce a small plant a few
inches in height with an erect habit of growth, without tendrils, with
round instead of oblong pollen, and with colourless deformed flowers quite
different {151} in appearance from those of the wild form. Such a plant
would breed perfectly true, and a botanist to whom it was presented, if
ignorant of its origin, might easily relegate it to a different genus.
Nevertheless, though so widely divergent in structure, such a plant must
yet be regarded as belonging to the species _Lathyrus odoratus_. For it
still remains fertile with the many different varieties of sweet pea. It is
not visible attributes that constitute the essential difference between one
species and another. The essential difference, whatever it may be, is that
underlying the phenomenon of sterility. The visible attributes are those
made use of by the systematist in cataloguing the different forms of animal
and plant life, for he has no other choice. But it must not be forgotten
that they are often misleading. Until they were bred together _Euralia
wahlbergi_ and _E. mima_ were regarded as perfectly valid species, and
there is little doubt that numbers of recognised species will eventually
fall to the ground in the same way as soon as we are in a position to apply
the test of breeding. Mendelism has helped us to realise that specific
characters may be but incidental to a species--that the true criterion of
what constitutes a species is sterility, and that particular form of
sterility which prevents two healthy gametes on uniting from producing a
zygote with normal powers of growth and reproduction. For there are forms
of sterility which are purely mechanical. The pollen of _Mirabilis jalapa_
cannot fertilise _M._ {152} _longiflora_, because the pollen tubes of the
former are not long enough to penetrate down to the ovules of the latter.
Hybrids can nevertheless be obtained from the reciprocal cross. Nor should
we expect offspring from a St. Bernard and a toy terrier without recourse
to artificial fertilisation. Or sterility may be due to pathological causes
which prevent the gametes from meeting one another in a healthy state. But
in most cases it is probable that the sterility is due to some other cause.
It is not inconceivable that definite differences in chemical composition
render the protoplasm of one species toxic to the gametes of the other, and
if this is so it is not impossible that we may some day be able to express
these differences in terms of Mendelian factors. The very nature of the
case makes it one of extreme difficulty for experimental investigation. At
any rate, we realise more clearly than before that the problem of species
is not one that can be resolved by the study of morphology or of
systematics. It is a problem in physiology.
* * * * *
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