Nothing in Biology Makes
Sense Except in the Light of Evolution
Theodosius
Dobzhansky,
"Nothing in
biology makes sense except in the light of evolution." The American
Biology Teacher, March 1973
As recently as 1966, sheik Abd el Aziz bin Baz asked
the king of Saudi Arabia to suppress a heresy that was spreading in his land.
Wrote the sheik:
"The Holy Koran, the Prophet's teachings, the
majority of Islamic scientists, and the actual facts all prove that the sun is
running in its orbit... and that the earth is fixed and stable, spread out by
God for his mankind.... Anyone who professed otherwise would utter a charge of
falsehood toward God, the Koran, and the Prophet."
The good sheik evidently holds the Copernican theory
to be a "mere theory," not a "fact." In this he is
technically correct. A theory can be verified by a mass of facts, but it
becomes a proven theory, not a fact. The sheik was perhaps unaware that the
Space Age had begun before he asked the king to suppress the Copernican heresy.
The sphericity of the earth has been seen by astronauts, and even by many
earth-bound people on their television screens. Perhaps the sheik could retort that
those who venture beyond the confines of God's earth suffer hallucinations, and
that the earth is really flat.
Parts of the Copernican world model, such as the
contention that the earth rotates around the sun, and not vice versa, have not
been verified by direct observations even to the extent the sphericity of the
earth has been. Yet scientists accept the model as an accurate representation
of reality. Why? Because it makes sense of a multitude of facts which are
otherwise meaningless or extravagant. To non-specialists most of these facts
are unfamiliar. Why then do we accept the "mere theory" that the
earth is a sphere revolving around a spherical sun? Are we simply submitting to
authority? Not quite: we know that those who took the time to study the evidence
found it convincing.
The good sheik is probably ignorant of the evidence.
Even more likely, he is so hopelessly biased that no amount of evidence would
impress him. Anyway, it would be sheer waste of time to attempt to convince
him. The Koran and the Bible do not contradict Copernicus, nor does Copernicus
contradict them. It is ludicrous to mistake the Bible and the Koran for primers
of natural science. They treat of matters even more important: the meaning of
man and his relations to God. They are written in poetic symbols that were
understandable to people of the age when they were written, as well as to
peoples of all other ages. The king of Arabia did not comply with the sheik's
demand. He knew that some people fear enlightenment, because enlightenment
threatens their vested interests. Education is not to be used to promote
obscurantism.
The earth is not the geometric center of the universe,
although it may be its spiritual center. It is a mere speck of dust in the
cosmic spaces. Contrary to Bishop Ussher's calculations, the world did not
appear in approximately its present state in 4004 BC. The estimates of the age
of the universe given by modern cosmologists are still only rough
approximations, which are revised (usually upward) as the methods of estimation
are refined. Some cosmologists take the universe to be about 10 billion years
old; others suppose that it may have existed, and will continue to exist,
eternally. The origin of life on earth is dated tentatively between 3 and 5
billion years ago; manlike beings appeared relatively quite recently, between 2
and 4 million years ago. The estimates of the age of the earth, of the duration
of the geologic and paleontologic eras, and of the antiquity of man's ancestors
are now based mainly on radiometric evidence the proportions of isotopes of
certain chemical elements in rocks suitable for such studies.
Shiek bin Baz and his like refuse to accept the
radiometric evidence, because it is a "mere theory." What is the
alternative? One can suppose that the Creator saw fit to play deceitful tricks
on geologists and biologists. He carefully arranged to have various rocks
provided with isotope ratios just right to mislead us into thinking that
certain rocks are 2 billion years old, others 2 million, which in fact they are
only some 6,000 years old. This kind of pseudo-explanation is not very new. One
of the early anti-evolutionists, P. H. Gosse, published a book entitled Omphalos
("the Navel"). The gist of this amazing book is that Adam, though he
had no mother, was created with a navel, and that fossils were placed by the
Creator where we find them now -- a deliberate act on His part, to give the
appearance of great antiquity and geologic upheavals. It is easy to see the
fatal flaw in all such notions. They are blasphemies, accusing God of absurd
deceitfulness. This is as revolting as it is uncalled for.
Diversity of Living Beings
The diversity and the unity of life are equally
striking and meaningful aspects of the living world. Between 1.5 and 2 million
species of animals and plants have been described and studied; the number yet
to be described is probably as great. The diversity of sizes, structures, and
ways of life is staggering but fascinating. Here are just a few examples.
The foot-and-mouth disease virus is a sphere 8-12 mm
in diameter. The blue whale reaches 30 m in length and 135 t in weight. The
simplest viruses are parasites in cells of other organisms, reduced to barest
essentials minute amounts of DNA or RNA, which subvert the biochemical
machinery of the host cells to replicate their genetic information, rather than
that of the host.
It is a matter of opinion, or of definition, whether
viruses are considered living organisms or peculiar chemical substances. The
fact that such differences of opinion can exist is in itself highly
significant. It means that the borderline between living and inanimate matter
is obliterated. At the opposite end of the simplicity complexity spectrum you
have vertebrate animals, including man. The human brain has some 12 billion
neurons; the synapses between the neurons are perhaps a thousand times
numerous.
Some organisms live in a great variety of
environments. Man is at the top of the scale in this respect. He is not only a
truly cosmopolitan species but, owing to his technologic achievements, can
survive for at least a limited time on the surface of the moon and in cosmic
spaces. By contrast, some organisms are amazingly specialized. Perhaps the
narrowest ecologic niche of all is that of a species of the fungus family
Laboulbeniaceae, which grows exclusively on the rear portion of the elytra of
the beetle Aphenops cronei, which is found only in some limestone caves
in southern France. Larvae of the fly Psilopa petrolei develop in
seepages of crude oil in California oilfields; as far as is known they occur
nowhere else. This is the only insect able to live and feed in oil, and its
adult can walk on the surface of the oil only as long as no body part other
than the tarsi are in contact with the oil. Larvae of the fly Drosophila
carciniphila develop only in the nephric grooves beneath the flaps of the
third maxilliped of the land crab Geocarcinus ruricola, which is
restricted to certain islands in the Caribbean.
Is there an explanation, to make intelligible to reason
this colossal diversity of living beings? Whence came these extraordinary,
seemingly whimsical and superfluous creatures, like the fungus Laboulbenia,
the beetle Aphenops cronei, the flies Psilopa petrolei and Drosophila
carciniphila, and many, many more apparent biologic curiosities? The only
explanation that makes sense is that the organic diversity has evolved in
response to the diversity of environment on the planet earth. No single
species, however perfect and however versatile, could exploit all the
opportunities for living. Every one of the millions of species has its own way
of living and of getting sustenance from the environment. There are doubtless
many other possible ways of living as yet unexploited by any existing species;
but one thing is clear: with less organic diversity, some opportunities for
living would remain unexploited. The evolutionary process tends to fill up the
available ecologic niches. It does not do so consciously or deliberately; the
relations between evolution and environment are more subtle and more
interesting than that. The environment does not impose evolutionary changes on
its inhabitants, as postulated by the now abandoned neo-Lamarckian theories.
The best way to envisage the situation is as follows: the environment presents
challenges to living species, to which the later may respond by adaptive
genetic changes.
An unoccupied ecologic niche, an unexploited
opportunity for living, is a challenge. So is an environmental change, such as
the Ice Age climate giving place to a warmer climate. Natural selection may
cause a living species to respond to the challenge by adaptive genetic changes.
These changes may enable the species to occupy the formerly empty ecologic
niche as a new opportunity for living, or to resist the environmental change if
it is unfavorable. But the response may or may not be successful. This depends
on many factors, the chief of which is the genetic composition of the
responding species at the time the response is called for. Lack of successful
response may cause the species to become extinct. The evidence of fossils shows
clearly that the eventual end of most evolutionary lines is extinction.
Organisms now living are successful descendants of only a minority of the
species that lived in the past and of smaller and smaller minorities the
farther back you look. Nevertheless, the number of living species has not
dwindled; indeed, it has probably grown with time. All this is understandable
in the light of evolution theory; but what a senseless operation it would have
been, on God's part, to fabricate a multitude of species ex nihilo and then let
most of them die out!
There is, of course, nothing conscious or intentional
in the action of natural selection. A biologic species does not say to itself,
"Let me try tomorrow (or a million years from now) to grow in a different
soil, or use a different food, or subsist on a different body part of a
different crab." Only a human being could make such conscious decisions.
This is why the species Homo sapiens is the apex of evolution. Natural
selection is at one and the same time a blind and creative process. Only a
creative and blind process could produce, on the one hand, the tremendous
biologic success that is the human species and, on the other, forms of adaptedness
as narrow and as constraining as those of the overspecialized fungus, beetle,
and flies mentioned above.
Anti-evolutionists fail to understand how natural
selection operates. They fancy that all existing species were generated by
supernatural fiat a few thousand years ago, pretty much as we find them today.
But what is the sense of having as many as 2 or 3 million species living on
earth? If natural selection is the main factor that brings evolution about, any
number of species is understandable: natural selection does not work according
to a foreordained plan, and species are produced not because they are needed
for some purpose but simply because there is an environmental opportunity and
genetic wherewithal to make them possible. Was the Creator in a jocular mood
when he made Psilopa petrolei for California oil fields and species of Drosophila
to live exclusively on some body-parts of certain land crabs on only certain
islands in the Caribbean? The organic diversity becomes, however, reasonable
and understandable if the Creator has created the living world not by caprice
but by evolution propelled by natural selection. It is wrong to hold creation
and evolution as mutually exclusive alternatives. I am a creationist and an
evolutionist. Evolution is God's, or Nature's method of creation. Creation is
not an event that happened in 4004 BC; it is a process that began some 10
billion years ago and is still under way.
Unity of Life
The unity of life is no less remarkable than its
diversity. Most forms of life are similar in many respects. The universal
biologic similarities are particularly striking in the biochemical dimension.
From viruses to man, heredity is coded in just two, chemically related
substances: DNA and RNA. The genetic code is as simple as it is universal.
There are only four genetic "letters" in DNA: adenine, guanine,
thymine, and cytosine. Uracil replaces thymine in RNA. The entire evolutionary
development of the living world has taken place not by invention of new
"letters" in the genetic "alphabet" but by elaboration of
ever-new combinations of these letters.
Not only is the DNA-RNA genetic code universal, but so
is the method of translation of the sequences of the "letters" in
DNA-RNA into sequences of amino acids in proteins. The same 20 amino acids
compose countless different proteins in all, or at least in most, organisms.
Different amino acids are coded by one to six nucleotide triplets in DNA and
RNA. And the biochemical universals extend beyond the genetic code and its
translation into proteins: striking uniformities prevail in the cellular
metabolism of the most diverse living beings. Adenosine triphosphate, biotin,
riboflavin, hemes, pyridoxin, vitamins K and B12, and folic acid implement
metabolic processes everywhere.
What do these biochemical or biologic universals mean?
They suggest that life arose from inanimate matter only once and that all
organisms, no matter now diverse, in other respects, conserve the basic
features of the primordial life. (It is also possible that there were several,
or even many, origins of life; if so, the progeny of only one of them has
survived and inherited the earth.) But what if there was no evolution and every
one of the millions of species were created by separate fiat? However offensive
the notion may be to religious feeling and to reason, the anti-evolutionists
must again accuse the Creator of cheating. They must insist that He
deliberately arranged things exactly as if his method of creation was
evolution, intentionally to mislead sincere seekers of truth.
The remarkable advances of molecular biology in recent
years have made it possible to understand how it is that diverse organisms are
constructed from such monotonously similar materials: proteins composed of only
20 kinds of amino acids and coded only by DNA and RNA, each with only four
kinds of nucleotides. The method is astonishingly simple. All English words,
sentences, chapters, and books are made up of sequences of 26 letters of the
alphabet. (They can be represented also by only three signs of the Morse code:
dot, dash, and gap.) The meaning of a word or a sentence is defined not so much
by what letters it contains as by the sequences of these letters. It is the
same with heredity: it is coded by the sequences of the genetic
"letters" the nucleotides in the DNA. They are translated into the
sequences of amino acids in the proteins.
Molecular studies have made possible an approach to
exact measurements of degrees of biochemical similarities and differences among
organisms. Some kinds of enzymes and other proteins are quasi-universal, or at
any rate widespread, in the living world. They are functionally similar in
different living beings, in that they catalyze similar chemical reactions. But
when such proteins are isolated and their structures determined chemically,
they are often found to contain more or less different sequences of amino acids
in different organisms. For example, the so-called alpha chains of hemoglobin
have identical sequences of amino acids in man and the chimpanzee, but they
differ in a single amino acid (out of 141) in the gorilla. Alpha chains of
human hemoglobin differ from cattle hemoglobin in 17 amino acid substitutions,
18 from horse, 20 from donkey, 25 from rabbit, and 71 from fish (carp).
Cytochrome C is an enzyme that plays an important role
in the metabolism of aerobic cells. It is found in the most diverse organisms,
from man to molds. E. Margoliash, W. M. Fitch, and others have compared the
amino acid sequences in cytochrome C in different branches of the living world.
Most significant similarities as well as differences have been brought to
light. The cytochrome C of different orders of mammals and birds differ in 2 to
17 amino acids, classes of vertebrates in 7 to 38, and vertebrates and insects
in 23 to 41; and animals differ from yeasts and molds in 56 to 72 amino acids.
Fitch and Margoliash prefer to express their findings in what are called
"minimal mutational distances." It has been mentioned above that
different amino acids are coded by different triplets of nucleotides in DNA of
the genes; this code is now known. Most mutations involve substitutions of
single nucleotides somewhere in the DNA chain coding for a given protein.
Therefore, one can calculate the minimum numbers of single mutations needed to
change the cytochrome C of one organism into that of another. Minimal
mutational distances between human cytochrome C and the cytochrome C of other
living beings are as follows:
Monkey
|
1
|
Chicken
|
18
|
Dog
|
13
|
Penguin
|
18
|
Horse
|
17
|
Turtle
|
19
|
Donkey
|
16
|
Rattlesnake
|
20
|
Pig
|
13
|
Fish (tuna)
|
31
|
Rabbit
|
12
|
Fly
|
33
|
Kangaroo
|
12
|
Moth
|
36
|
Duck
|
17
|
Mold
|
63
|
Pigeon
|
16
|
Yeast
|
56
|
It is important to note that amino acid sequences in a
given kind of protein vary within a species as well as from species to species.
It is evident that the differences among proteins at the level of species,
genus, family, order, class, and phylum are compounded of elements that vary
also among individuals within a species. Individual and group differences are
only quantitatively, not qualitatively, different. Evidence supporting the
above propositions is ample and is growing rapidly. Much work has been done in
recent years on individual variations in amino acid sequences of hemoglobin of
human blood. More that 100 variants have been detected. Most of them involve
substitutions of single amino acids - substitutions that have arisen by genetic
mutations in the persons in whom they are discovered or in their ancestors. As expected,
some of these mutations are deleterious to their carriers, but others
apparently are neutral or even favorable in certain environments. Some mutant
hemoglobins have been found only in one person or in one family; others are
discovered repeatedly among inhabitants of different parts of the world. I
submit that all these remarkable findings make sense in the light of evolution:
they are nonsense otherwise.
Comparative Anatomy and Embryology
The biochemical universals are the most impressive and
the most recently discovered, but certainly they are not the only vestiges of
creation by means of evolution. Comparative anatomy and embryology proclaim the
evolutionary origins of the present inhabitants of the world. In 1555 Pierre
Belon established the presence of homologous bones in the superficially very
different skeletons of man and bird. Later anatomists traced the homologies in
the skeletons, as well as in other organs, of all vertebrates. Homologies are
also traceable in the external skeletons of arthropods as seemingly unlike as a
lobster, a fly, and a butterfly. Examples of homologies can be multiplied
indefinitely.
Embryos of apparently quite diverse animals often
exhibit striking similarities. A century ago these similarities led some
biologists (notably the German zoologist Ernst Haeckel) to be carried by their
enthusiasm as far as to interpret the embryonic similarities as meaning that
the embryo repeats in its development the evolutionary history of its species:
it was said to pass through stages in which it resembles its remote ancestors.
In other words, early-day biologists supposed that by studying embryonic
development one can, as it were, read off the stages through which the
evolutionary development had passed. This so-called biogenetic law is no longer
credited in its original form. And yet embryonic similarities are undeniable
impressive and significant.
Probably everybody knows the sedentary barnacles which
seem to have no similarity to free-swimming crustaceans, such as the copepods.
How remarkable that barnacles pass through a free-swimming larval stage, the
nauplius! At that stage of its development a barnacle and a Cyclops look
unmistakably similar. They are evidently relatives. The presence of gill slits
in human embryos and in embryos of other terrestrial vertebrates is another
famous example. Of course, at no stage of its development is a human embryo a
fish, nor does it ever have functioning gills. But why should it have
unmistakable gill slits unless its remote ancestors did respire with the aid of
gills? It is the Creator again playing practical jokes?
Adaptive radiation: Hawaii's Flies
There are about 2,000 species of drosophilid flies in
the world as a whole. About a quarter of them occur in Hawaii, although the
total area of the archipelago is only about that of the state of New Jersey.
All but 17 of the species in Hawaii are endemic (found nowhere else).
Furthermore, a great majority of the Hawaiian endemics do not occur throughout
the archipelago: they are restricted to single islands or even to a part of an
island. What is the explanation of this extraordinary proliferation of
drosophilid species in so small a territory? Recent work of H. L. Carson, H. T.
Spieth, D. E. Hardy, and others makes the situation understandable.
The Hawaiian Islands are of volcanic origin; they were
never parts of any continent. Their ages are between 5.6 and 0.7 million years.
Before man came there inhabitants were descendants of immigrants that had been
transported across the ocean by air currents and other accidental means. A
single drosophilid species, which arrived in Hawaii first, before there were
numerous competitors, faced the challenge of an abundance of many unoccupied
ecologic niches. Its descendants responded to this challenge by evolutionary
adaptive radiation, the products of which are the remarkable Hawaiian
drosophilids of today. To forestall a possible misunderstanding, let it be made
clear that the Hawaiian endemics are by no means so similar to each other that
they could be mistaken for variants of the same species; if anything, they are
more diversified than are drosophilids elsewhere. The largest and the smallest
drosophilid species are both Hawaiian. They exhibit an astonishing variety of
behavior patterns. Some of them have become adapted to ways of life quite
extraordinary for a drosophilid fly, such as being parasites in egg cocoons of
spiders.
Oceanic islands other than Hawaii, scattered over the
wide Pacific Ocean, are not conspicuously rich in endemic species of drosophilids.
The most probable explanation of this fact is that these other islands were
colonized by drosophilid after most ecologic niches had already been filled by
earlier arrivals. This surely is a hypothesis, but it is a reasonable one.
Anti-evolutionists might perhaps suggest an alternative hypothesis: in a fit of
absentmindedness, the Creator went on manufacturing more and more drosophilid
species for Hawaii, until there was an extravagant surfeit of them in this
archipelago. I leave it up to you to decide which hypothesis makes sense.
Strength and Acceptance of the Theory
Seen in the light of evolution, biology is, perhaps,
intellectually the most satisfying and inspiring science. Without that light it
becomes a pile of sundry facts some of them interesting or curious but making
no meaningful picture as a whole.
This is not to imply that we know everything that can
and should be known about biology and about evolution. Any competent biologist
is aware of a multitude of problems yet unresolved and of questions yet
unanswered. After all, biologic research shows no sign of approaching
completion; quite the opposite is true. Disagreements and clashes of opinion
are rife among biologists, as they should be in a living and growing science.
Anti-evolutionists mistake, or pretend to mistake, these disagreements as
indications of dubiousness of the entire doctrine of evolution. Their favorite
sport is stringing together quotations, carefully and sometimes expertly taken
out of context, to show that nothing is really established or agreed upon among
evolutionists. Some of my colleagues and myself have been amused and amazed to
read ourselves quoted in a way showing that we are really anti-evolutionists
under the skin.
Let me try to make crystal clear what is established
beyond reasonable doubt, and what needs further study, about evolution.
Evolution as a process that has always gone on in the history of the earth can
be doubted only by those who are ignorant of the evidence or are resistant to
evidence, owing to emotional blocks or to plain bigotry. By contrast, the
mechanisms that bring evolution about certainly need study and clarification.
There are no alternatives to evolution as history that can withstand critical
examination. Yet we are constantly learning new and important facts about
evolutionary mechanisms.
It is remarkable that more than a century ago Darwin
was able to discern so much about evolution without having available to him the
key facts discovered since. The development of genetics after 1900 especially
of molecular genetics, in the last two decades has provided information
essential to the understanding of evolutionary mechanisms. But much is in doubt
and much remains to be learned. This is heartening and inspiring for any
scientist worth his salt. Imagine that everything is completely known and that
science has nothing more to discover: what a nightmare!
Does the evolutionary doctrine clash with religious
faith? It does not. It is a blunder to mistake the Holy Scriptures for
elementary textbooks of astronomy, geology, biology, and anthropology. Only if
symbols are construed to mean what they are not intended to mean can there
arise imaginary, insoluble conflicts. As pointed out above, the blunder leads
to blasphemy: the Creator is accused of systematic deceitfulness.
One of the great thinkers of our
age, Pierre Teilhard de Chardin, wrote the following: "Is evolution a
theory, a system, or a hypothesis? It is much more it is a general postulate to
which all theories, all hypotheses, all systems much henceforward bow and which
they must satisfy in order to be thinkable and true. Evolution is a light which
illuminates all facts, a trajectory which all lines of though must follow this
is what evolution is. Of course, some scientists, as well as some philosophers
and theologians, disagree with some parts of Teilhard's teachings; the
acceptance of his worldview falls short of universal. But there is no doubt at
all that Teilhard was a truly and deeply religious man and that Christianity
was the cornerstone of his worldview. Moreover, in his worldview science and
faith were not segregated in watertight compartments, as they are with so many
people. They were harmoniously fitting parts of his worldview. Teilhard was a creationist,
but one who understood that the Creation is realized in this world by means of
evolution.
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