The Ghost in the Machine (21 page)

 

* This metaphor is almost literally applicable to mistakes made in the
protein manufacture in micro-organisms due to 'nonsense syllables'
appearing in the RNA code. [5]

 

That was an example of harmonising the consequences of a potentially
favourable
mutation. Let me now quote another example of evolutionary
self-repair after a potentially
harmful
mutation.

 

 

 

The Case of the Eyeless Fly

 

 

The fruit fly has a mutant gene which is recessive, i.e., when paired
with a normal gene, has no discernible effect (it will be remembered that
genes operate in pairs, each gene in the pair being derived from one
parent). But if two of these mutant genes are paired in the fertilised
egg, the offspring will be an eyeless fly. If now a pure stock of
eyeless flies is made to inbreed, then the whole stock will have only
the 'eyeless' mutant gene, because no normal gene can enter the stock to
bring light into their darkness. Nevertheless, within a few generations,
flies appear in the inbred 'eyeless' stock with eyes that are perfectly
normal
. The traditional explanation of this remarkable phenomenon
is that the other members of the gene-complex have been 'reshuffled
and recombined in such a way that they deputise for the missing normal
eye-forming gene'. [6] Now re-shuffling, as every poker player knows,
is a randomising process. No biologist would be so perverse as to suggest
that the new insect-eye evolved by pure chance, thus repeating within a
few generations an evolutionary process which took hundreds of millions
of years. Nor does the concept of natural selection provide the slightest
help in this case. The re-combination of genes to deputise for the missing
gene must have been co-ordinated according to some overall plan
which
includes
the rules of genetic self-repair after certain types of damage
by deleterious mutations. But such co-ordinative controls can only operate
on levds higher than that of individual genes. Once more we are driven
to the conclusion that the genetic code is not an architect's blueprint;
that the gene-complex and its internal environment form a remarkably
stable, closely knit, self-regulating micro-hierarchy; and that mutated
genes in any of its holons are liable to cause corresponding reactions
in others, co-ordinated by higher levels. This micro-hierarchy controls
the prenatal skills of the embryo, which enable it to reach its goal,
regardless of the hazards it may encounter during development. But
phylogeny is a sequence of ontogenies, and thus we are confronted with
the profound question: is the mechanism of phylogeny also endowed with
some kind of evolutionary instruction-booklet? Is there a strategy of
the evolutionary process comparable to the 'strategy of the genes' --
to the 'directiveness' of ontogeny (as E.S. Russell has called it)?

 

 

Let me recapitulate. The eyes in the normal fruit fly, and the eyes
which suddenly appear in the 'eyeless stock', are homologous organs,
identical in appearance, and yet produced by a different combination of
genes; and this is only one of many similar phenomena. Genetic atomism is
dead. Hereditary stability and hereditary change are both based, not on a
mosaic of genes, but on the action of the gene-complex 'as a whole'. But
this face-saving expression -- which is now coming into increased
use -- is empty, like so many other holistic formulations, unless we
interpolate between the gene-complex as a whole, and the individual
gene, a hierarchy of genetic subassemblies -- self-regulating holons of
heredity, which control the development of organs, and
also control their
possible evolutionary modifications
, by canalising the effects of random
mutations. A hierarchy with its built-in, self-regulatory safeguards is
a stable affair. It cannot be pulled in here, pulled out there, like
Patou belabouring his model. It is capable of variation and change,
but only in co-ordinated ways
and only in limited directions
. Can we
say anything about the general principles which determine that direction?

 

 

 

The Puzzle of Homology

 

 

The most fundamental principle of evolutionary strategy, related to the
watchmakers' parable, is the
standardisation
of sub-assemblies.
But since most of us have no very clear idea of the mechanism of our
time-pieces, we might look under the hood of a motor car instead. Here
the sub-assemblies are easily named: chassis, engine, battery,
steering, brakes, differential, and so on to the distributor and heating
system. Each of these component parts is a more or less self-contained
unit, a mechanical holon in its own right. A V8 engine, or a standard
battery, can be taken out of the car and made to function by itself,
like an organ in vitro. It can be transferred to another type of car,
and even to a different species of machine, such as a motor boat. But
how do automobiles
evolve
?

 

 

The manufacturers know that it does not pay to design a new model from
scratch, by starting on the level of elementary components; they make
use of already existing, standard components -- chassis, brakes, etc. --
each of which has developed out of long previous experience, and then
proceed by relatively small improvements or modifications of some of
these -- for instance, by re-designing the body-line, or improving the
cooling system, or introducing bucket seats.

 

 

Similar restraints can be shown to operate in biological evolution. Compare
the front wheels of the latest model with those of a pre-war vintage car --
they are based on the same principles. Compare the structure of the
forelimbs in man, dog, bird and whale, and you find that evolution has
retained the same basic design:

 

 

 

 

The human arm and the bird's wing are called homologous organs because
they show the same structural design -- of bones, muscles, blood vessels
and nerves -- and are descended from the same ancestral organ. The
functions of arm and wing are so different that it would be logical
to expect each to have a quite different design. In fact, evolution
proceeded, just as car manufacturers do, by merely modifying an already
existing component (the forelimb of the reptilian ancestor, from which
birds and mammalians branched out more than two hundred million years
ago), instead of starting from scratch. Once Nature has taken out a
patent for manufacturing a component organ, it sticks to it tenaciously:
the organ or device has become a stable evolutionary holon.

 

 

This principle holds all along the line, from the sub-cellular level to
the 'wiring diagram' of the primate brain. The same make of organelles
functions in the cells of mice and men; the same make of contractile
protein serves the motion of amoeba and of the pianist's fingers; the
same four chemical units constitute the alphabet of heredity throughout
the animal and plant kingdoms -- only the words are different for every
creature. The proverbial lavishness of Nature is compensated by its less
obvious conservatism and parsimony -- one might almost call it stinginess
-- of basic homologous designs, from organelles to brain structures. 'This
concept of homology', wrote Sir Alister Hardy, 'is absolutely fundamental
to what we are talking about when we speak of evolution. Yet in truth',
he added wistfully, 'we cannot explain it at all in terms of present-day
biological theory'. [7]

 

 

The reason for this failure is, as we have seen, that the orthodox theory
assumed homologous structures in different species to be due to the same
'atomic' genes inherited from the common ancestor (though modified by
mutation in the course of their long descent); whereas there is now
ample evidence that homologous structures can be produced by the action
of quite different genes. The only way out of this cul-de-sac seems to be
to substitute for genetic atomism, which has so drastically broken down,
the concept of the genetic micro-hierarchy, with its own built-in rules,
that permit a great amount of variation, but only in limited directions
on a limited number of themes. This really amounts to the revival of an
ancient idea which goes back to Goethe -- and even further to Plato. The
point is worth a short historical digression -- which may make it clear
why the concept of homology has such great importance not only for the
biologist, but also for the philosopher.

 

 

 

Archetypes in Biology

 

 

Long before Darwin, naturalists were divided into evolutionists (Buffon,
Lamarck, St. Hilaire, etc.), and anti-evolutionists who believed that the
Creator had put down the first giraffe, mosquito and walrus simultaneously
as ready-made products on the earth. But both pro- and anti-evolutionists
were equally struck by the similarity of organs and designs in otherwise
widely different species. The term 'homologue organ' was actually coined
by Geoffroy St. Hilaire. His
Philosophie Anatomique
, published in 1818,
starts with the question: ' . . . Is it not generally acknowledged that
vertebrates are built up on one uniform plan -- e.g., the forelimb may be
modified for running, climbing, swimming or flying, yet the arrangement
of bones remains the same . . .?' [8]

 

 

Goethe had become an evolutionist long before that, through his studies
of the morphology (a term which he coined) of plants and animals. In his
Metamorphosis of Plants
, published in 1790, he postulated that all
existing plants could be derived from a common ancestor, the
Urpflanze
or arche-plant; and that all the organs of plants are homologous*
modifications of a single structure, expressed in its simplest form
in the leaf. Though Goethe was already at the height of his fame, the
Metamorphosis
had a hostile reception (incredible as it seems, his own
publisher in Leipzig rejected it, and he had to go to Cotta in Gotha);
but it had considerable influence on the German
Naturphilosophen
,
who combined comparative anatomy with transcendental mysticism. These
men were not evolutionists, but they were fascinated by the universal
recurrence of the same basic patterns in the design of animals and plants;
they called them 'archetypes' and thought that they constituted the key
to the Lord's design of creation.

 

* Though he does not use the word.

 

The idea that all the existing flowers, trees, vegetables and so on are
derived from a single ancestral plant seems to have occurred to Goethe
during his sojourn in Sicily, where he had spent most of his time in
botanising. After his return, in 1787, he confided to Herder:

 

I have seen the main point, the core of the problem, clearly and
beyond doubt; everything else I can already see, too, as a whole,
and only a few details need working out. The ancestral plant will
turn out to be the most wondrous creation of the world, for which
Nature herself shall envy me. With the aid of this model and the
key to unlock it, one can then invent further plants ad infinitum
which, however, must be consistent; that is to say, plants which,
if they do not exist, yet could exist; which, far from being shadows
or glosses of the poet's or painter's fancy, must possess an inherent
rightness and necessity.  The same law applies to all the remaining
domains of the living. [9]

 

The conditions of 'inherent rightness and necessity', to which all
existing and possible forms of life must conform, Goethe was of course
unable to define; but his intuition told him that they could not include
fanciful, arbitrary patterns created by the unbridled imagination of
painters -- or science-fiction writers. They must conform to certain
archetypal patterns,
limited in their range by the basic structure
and chemistry of organic matter
. Evolution cannot be a random process,
pulling in bits here, pulling out bits there. It must conform to some
orderly design, like 'the stern, eternal laws which guide the wandering
planets in their orbits'.*

 

* Faust, Prologue.

 

Goethe's German followers, the Naturphilosophen, took up his concept of
archetypes, but not his belief in evolution. They regarded the archetypes
not, as he did, as ancestral forms out of which the homologue organs had
evolved, but as patterns of divine design -- leitmotivs which, together
with all their possible variations, have coexisted since the day of
Creation. Much the same beliefs were shared by some great anatomists in
Europe at that time, among them Richard Owen. It was Owen who defined
'homologous organs' as 'the
same
organ in different animals under
every variety of form and function'. While he tirelessly demonstrated
the multitude of such organs in the animal kingdom, he attributed them
to the parsimony of the divine Designer -- just as Kepler had attributed
his planetary laws to the ingenuity of the divine Mathematician.