chapter ]

THE TIME SCALE
AND SOME
EVOLUTIONARY PRINCIPLES

I. most of us, paleontology is the name of a sort of
genteel outdoor science concerned with the collection and gross de-
scription of old bones and hardened mud blocks containing preserved
animal tracks. To the paleontologist and, for that matter, to any
novice who has had the good fortune to pass through what might be
called the “Darwin-to-Simpson reading stage,” no definition could be
further from the truth. Just as history, to the historian, is alive and
a part of the continuing pageant of human experience, so is the study
of the life of the past a living science to its devotees.

The study of fossils cannot tell us a great deal about the natural
forces that shape the evolutionary process, but it does furnish us
with guidelines for the consideration of information derived from
other sciences. As G. S. Carter’ has put it, “The part of paleontology
in the study of evolutionary theory resembles that of natural selection
in the process of evolution; it serves to remove the inefficient but
cannot itself initiate.” It is clear that we can, and should, present
only the most superficial survey of the fossil record and its interpre-
 

tation in the present volume. For our purposes here we need only

 

 

 

 

 

 

Life Scale

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fe arrive at some general appreciation of the arbitrary divisions of geo-
- afl. z2 logical time and outline the phylogenetic relationships that exist be-
23| ue =| 3.3 > tween the various living and extinct forms of life.
@e|8= #8 BE a aoa Measurements of the extent of decay of long-lived radioactive ele-
“*| $s a2|856 Ezs = ments in the rock strata of the earth’s crust enable us to make reason-
#2 aie . 252-4 able estimates of the ages of various strata. Utilizing such data as
LY check points, but relying mostly on time estimates arrived at by
- e| lelelol 2) classical geological methods, the paleontologist can arrange the
3 8/5] 8/8 /8|2)8 fossilized remnants of life in a consecutive order with reasonable
mi as |* ie iz)" accuracy. He can also, in many cases, make certain deductions con-
% cerning the relation of specific upheavals and rearrangements of the
_ lelslelelZléglelelels - 2 6 earth’s surface to the changing patterns in the nature and distribution
8 2385 Sif e 2/212) 2) 2 8 s & g of life as it was in the past.
& |22=/ 5/2/32) 8 g 3 firis 8 - 3 § For the purposes of those interested in the earth sciences, time
3 may be expressed perfectly well on a linear scale, as shown on the
ef lt lole ale nlelele ts left of Figure 1. Such a scale serves to emphasize the relatively
$3 8-(5/2)/8)8/8/8/8) 2/2 (5) esis 3 small fraction of global time during which life has existed on the
a earth. The biologist is, however, more naturally preoccupied with
& 2 2 3 4 “protoplasmic” time and must magnify the portion of the time scale
5 3 g g ® that has to do with living things. The right half of Figure 1 is more
a . = 3 useful to the biologist and lists some of the landmarks in evolution,
« assigned to their proper paleontological time period.
/ 3 The earliest fossils that occur in any abundance may be assigned
2 * os : :
\ / 2 to the Cambrian and Ordovician periods and include a large propor-
/ é tion of the basic types of aquatic animals and the possible beginnings
/ of the vertebrates. The record for the Pre-Cambrian period is ex-
7 - tremely sparse and is represented mostly by the relatively primitive
/ 3 plants, the algae. At the end of the Pre-Cambrian, most of the in-
/ iz vertebrate phyla were relatively well differentiated, although the ab-
2 / sence in most instances of structural elements that could survive as
a / fossils makes the reconstruction of their phylogenetic tree somewhat

controversial. One scheme is presented in Figure 2. This arrange-

 

Earth Scale
Era

ment of the phyla, which includes the higher vertebrate forms for
comparison is, according to its author, L. H. Hyman, not to be taken
literally but is only suggestive. It is based on an arrangement of
animals in order of structural complexity, without separation of the

 

Years
x1976

 

 

L- 4000

   
  

allied phyla. The bacteria, yeasts, ete., are not shown, for they
branched off at some early point in time when the momentous biologi-
cal accident occurred which led to the establishment of plant and

8

_

t~ 3000
[- 2000

bupiiiu

 

animal kingdoms.
Another way of looking at the phyla is shown in Figure 3, taken

THE MOLECULAR BASIS OF EVOLUTION THE TIME SCALE AND SOME EVOLUTIONARY PRINCIPLES 3
 

 

    
      
        
  
 

Mollusca

DEUTEROSTOMIA

Echinodermata

 

 

Primitive
acoel
flatworms

 

BILATERIA

 

 

 

  

Cnidaria
Primitive
medusa

ee_)

Giher Protoz0®

 

 

 

 

 

 
 

Flagetlata

Figure 2. Relationships of the phyla of the animal kingdom. The arrangement
here is based on the scheme given by L. H. Hyman in The Invertebrates, volume
1, McGraw-Hill Book Company, p. 38, 1940.

4 THE MOLECULAR BASIS OF EVOLUTION

from George Gaylord Simpson’s fascinating book, The Meaning of
Evolution. Here we see the major phyla, as they have existed through
most of biological time, in terms of their relative abundances. We
can observe here some of the correlations between geology and
biology which the paleontologist is able to make. For example, the
distinct contractions in the abundances of almost all the phyla in the
Permian and Triassic periods and the actual extinction of the
Graptolithina correlate well with the geological evidence for great
mountain building and climatic rigor during these times.

A final illustration for this phylogenetic orientation is given in
Figure 4, in which the vertebrates are arranged in their proper as-
cendency (to use an “anthropophilic” expression). In our discussions
of the relations between the biochemistry and genetics of various or-
ganisms we shall refer from time to time to the contents of these
figures. We shall be interested, for example, in the structure of pro-
teins as they occur in various species and in the possibilities of mak-
ing some crude estimates from such data of the rates at which specific
genes have become modified.

The basic characteristics of the evolutionary process vary consid-
erably, depending on the level of evolution which is being considered.
Evolutionary change, measured broadly in terms of the origin of new
systems of animal organization, is an expression of average change.

 
  
   

   

 

    
  

 

 

Tertiary Arthropoda ee
, =: Chordata
Cretaceous
Jurassic
a Worm-like
Triassic phyla TK
Permian. —— (especially
annelida)
Pennsylvanian
Mississippian Mollusca
Devonian
Silurian
Ordovician
Cambrian -Graptolithina Relative variety in fossil record

 

 

Duration and diversity of the principal groups of animals (based mainly on counts of genera and higher groups)

Figure 3. A schematic diagram of the history of life. The various phyla of
animals are represented by paths, the widths of which are proportional to the
known variety of each phylum during the various biological periods. Redrawn
from G. G. Simpson, The Meaning of Evolution, 1950, by permission of Yale
University Press.

THE TIME SCALE AND SOME EVOLUTIONARY PRINCIPLES 5
Recent

Tertiary

      
 

Cretaceous

Cartilage
. fishes
Triassic '
Permian

'

I

|

|

|
Jurassic !
|

|

|

|
Pennsylvanian =|
|

Mississippian =|
Devonian

Jawless

Silurian :
fishes ”

Based on numbers of known genera

Ordovician

Figure 4. A schematic diagram of the history of the vertebrates. The widths of
the pattern for each vertebrate class is proportional to the known varieties of the
class in each of the geological periods. Redrawn from G. G. Simpson, The Mean-
ing of Evolution, 1950, by permission of Yale University Press.

As Simpson has put it, “It is populations, not individuals, that
evolve.” As we approach the level of immediate cause and effect,
however, certain aspects of evolution become more highly significant,
and when we consider a small experimental population of the fruit
fly, Drosophila, we must become more concerned with individual
mutations and their contribution to the survival or death of these
specific flies than with theoretical, infinitely large populations. This
example is obviously not evolution in the grand sense. It emphasizes
“the survival of the fittest,” a phrase which, in the light of modern
ideas, we know must be replaced with “the survival of the branch
of a population which is adapted well enough to its environment to
live to procreate.” Nevertheless, all evolutionists will agree that the
basic cause of change must be gene mutation (although some authors
will hold out for the additional involvement of something more
ethereal in the way of causation, variously termed “aristogenesis,”
“élan vital,” “entelechy,” among other names—we shall return briefly
to these terms later in this chapter).

As our store of information concerning species variations in bio-
chemical properties, and specifically in protein structure, increases,
we will do well to have before us, as a constant frame of reference,
a clear picture of the phylogenetic relationships between various forms
of life and, particularly, of the time, in terms of numbers of genera-
tions, required to accomplish these variations.

6 THE MOLECULAR BASIS OF EVOLUTION

To develop some appreciation of the magnitude of time involved
in the differentiation of a species in relation to that required for a
more sweeping phylogenetic change, let us briefly examine those
divisions of the process called micro-, macro-, and megaevolution.
To quote the capsule summary given by Carter,’ “There is, first, the
origin of the smallest evolutionary differences as seen in continuous
series of strata; secondly, there is the differentiation of the members
of a group in adaptive radiation; and thirdly, the evolution of a new
type of animal organization from its predecessors.”

Microevolution

In certain favorable instances, when geological processes have re-
sulted in the formation of a continuous local succession of strata,
paleontologists have been able to reconstruct the morphological pro-
gression of a species as it took place over many hundreds of thousands
of years. An elegant example of such a reconstruction is the work
of Trueman? and his collaborators on the evolution of the coiled
lamellibranch, Gryphaea, a mollusk derived from oysters of the genus
Ostrea which is frequently found in the strata of the Mesozoic era.
Mollusks of the genus Gryphaea arose frequently and independently
from flat-shelled predecessors, presumably in response to the neces-
sity for raising the mouth of the shell above its muddy environment.
Four stages in the progressive development of a line of Gryphaea are
shown in Figure 5. During the evolution from Ostrea irregularis to
Gryphaea incurva a number of morphological characters were modi-
fied, and each of these was changed at different rates. Any one of
these characters may be used as a measure of rate of change; in Fig-
ure 6 is shown a plot of the variations in one of these, the number
of whorls in the shell, as a function of the vertical location of the
sample studied within the superimposed strata. The populations ex-
amined by Trueman from any given stratum gave a unimodal distribu-
tion curve, strongly suggesting that the population was single and
was not a mixture of independent populations.

In a case such as this there is little question that microevolution
has occurred without any large and sudden changes (saltations).
The general characteristics of the evolution typified by the Gryphaea,
with its succession of imperceptible gradations and with its uni-
formity around a mean, led Trueman to suggest that “such an evolv-
ing stock must be regarded as a ‘plexus’ or ‘bundle of anastomosing
lineages.” The example has been presented here mainly to illustrate

THE TIME SCALE AND SOME EVOLUTIONARY PRINCIPLES 7
 

About

6x 108 <
years

 

 

Gryphaea aff incurva

Figure 5. Four stages in the evolution of Gryphaea from its oyster-like ancestor.
Redrawn from A. E. Trueman, Biol. Revs. Biol. Proc., Cambridge Phil. Soc., 5,
296 (1930).

8 THE MOLECULAR BASIS OF EVOLUTION

|
|
Gryphaea incurva. {
? Gmuendense |

zone “vay
/ | \ |
/ \ |
7 | os
Y | K,
Gryphaea sp. -” | J SAK
Rotiformis | |
zone ‘IN | |
About 4) \.) Gryphaea obliquata

6x 106 < / | \\ ?Rotiformis zone
years a | \ { |
| ~~ {

|

   

[|
Gryphaea sp.
Vermiceras
zone

  

\ Ostrea irregularis
\ Lower angulata zone

PMN 1 | tt tit

l
4 ¢ #¢ 2 YF 1

Number of whorls

 

 

Nhe

Figure 6. Distribution curves showing the variation in the number of whorls of
the shells of successive populations of evolving Gryphaea. Redrawn from A. E.
Trueman, Biol. Revs. Biol. Proc. Cambridge Phil. Soc., 5, 296 (1930).

that microevolution is a population phenomenon and that the sep-
arate development of radiating lines becomes almost impossible in a
restricted population since continual interbreeding prevents the suc-
cessful rise of deviant groups.

Macro- and Megaevolution

When a mutation confers some benefit on an organism within the
framework of the environmental restrictions on the population to
which it belongs, the characteristic controlled by the mutant gene
may ultimately become firmly entrenched in the heredity of the en-
tire group. However, a limited horizon, such as that available to
the Gryphaea, permits only a limited phenotypic change. Thus, even
though a few “advanced” Gryphaea might have appeared which were

THE TIME SCALE AND SOME EVOLUTIONARY PRINCIPLES 9
endowed with some unique and specially favorable character, they
would not be likely to be perpetuated as a unique strain because of
their random interbreeding with the standard average organism.

The major factor responsible for the larger changes in evolution
that lead to distinct new specializations, or to new systems of animal
or plant organization, is adaptive radiation. Adaptive radiation is the
term used by evolutionists to describe the separation of populations
into smaller groups having different natural histories. The more
mobile the group and the more demanding the environmental changes
to which adaptation must be made, the greater the diversity of form
(and the number of unsuccessful “experiments”!) that results. This
diversity and mobility, together with the concomitant high rate of
evolutionary change, make the fossil record scattered and incomplete
as opposed to the situation for the sedentary Gryphaea. Neverthe-
less, paleontologists have been able to reconstruct the phylogeny of
numerous lines with great success, and certain distinct parameters of
macroevolution are well delineated.

The macroevolution of a particular population of organisms leads
to great complexity of form, most of the examples of which are false
starts and become extinct after a relatively short time (paleontologi-
cally speaking). For an evolutionary development to be successful,
all the various morphological parts must change in a correlated way
to insure survival. The evolutionists can express such correlations in
relative growth and development of parts by means of double
logarithmic plots, as shown in Figure 7. Here are represented the
relations between the heights of the paracones (a cusp of the molar
teeth) and the lengths of the ectolophs (the ridge on the outer
border of the crown of the same tooth) of the teeth of horses during
their progression from the primitive Eohippus to the modern animal.
Characters that may be related by such straight-line plots (of the
general form Y = bX*) are said to be undergoing allometric change,
and the slopes of the lines (k) give a measure of the relative rates
at which two specific bodily characters are changing.

A sudden modification in the slope of the plot relating two allo-
metric changes indicates a sudden shift in evolutionary direction.
For example, such an indication is given several times during the
evolution of the horse. As horses underwent adaptive radiation they
became exposed to new types of environmental opportunities in-
volving both new kinds of food and new terrain. The changes in
the position of the eye and in the structure of the foot and of
other physical characteristics have been described in a fascinating
way by Simpson in his book Horses. The modification of the molars

10 THE MOLECULAR BASIS OF EVOLUTION

 

60 L
50+ Neohipparion
occidentale
40+
Merychippus
30+ paniensis
@ 25/-
Oo
Figure 7. Changes in the structure of 8 20; x Hyohippus
the molars during the evolution of the & isk osborni
horses. After G. G. Simpson, Tempo S
and Mode in Evolution, 1944, by per- & 10b
mission of Columbia University Press. z 2 _ Mesohippus bairdi
7 =
6 b—
5 i x Eohippus borealis
4t it bo a

 

8910 15 20 2530
Length of ectoloph

during equine evolution is particularly instructive in connection with
our present consideration of sharp changes in evolutionary direction.
As ecological conditions made browsing more favorable than grazing,
the whole plan of the molar was modified by natural selection in a
direction compatible with the abrasive action of hard grasses. Thus
the crown of the molar became thicker and, together with the de-
velopment of cement, permitted the animals to enjoy a fertile life
span in spite of the erosive nature of their natural food supply. A
schematic representation of the adaptive radiation of horses is shown
in Figure 8. This figure shows the eating habits of the various suc-
cessive members on the main evolutionary line. The correlative plot
of two allometric structural features of the molars, the size of the
tooth and its height, shows that there occurred an abrupt increase in
the relative height of the tooth in the horses that converted to grazing,
whereas in another evolutionary offshoot, Hyohippus, which continued
to browse on soft, easily chewed plants, such a change did not occur.

Most authorities seem to agree that the evolution of a particular
line of organisms, like the horses, can be explained without compli-
cation on the basis of the selection of mutants that confer a survival]
value on the individual and on the population to which he belongs.
The occurrence of a particularly advantageous mutation has fre-
quently led to an almost explosive change in structure or habit, and
Simpson has proposed the name “quantum evolution” for such major
jumps. The view is frequently expressed, however, that the process
of natural selection might still be an adequate explanation for these
rapid shifts. Their suddenness is perhaps overemphasized because

THE TIME SCALE AND SOME EVOLUTIONARY PRINCIPLES Bi
 

South North America Old World
America

 

Recent

Gee

 

Pleistocene

 

Pliocene

vW ww
Hipparion ,

 

 

Miocene

   

Oligocene

we +
ww Grazing horses

Mesohippus

5 .
or? Browsing horses

— %

 

 

Epihippus &

. theres,
& Orohippua Palaeotheres,

Eocene

 

 

&
Hyracotherium

Figure 8. The evolution of the horses. The diagram shows the geographic dis-
tribution of the various forms and indicates their manner of securing food by
browsing or by grazing. Redrawn from G. G. Simpson, Horses, 1951, by per-
mission of Oxford University Press.

12 THE MOLECULAR BASIS OF EVOLUTION

of gaps in the fossil record that resulted from the rapidity of the
changes and the limited geographical region in which they occurred.

In discussions of those portions of evolution in which whole new
systems of biological organization arose, the terminology and interpre-
tations of experts becomes varied and, sometimes, delightfully mysti-
cal, at least to window-shoppers such as myself. We have already
mentioned the terms entelechy, élan vital, and aristogenesis. Such
terms have been coined to explain (explain away, perhaps) the fre-
quent, puzzling phenomena in which new structures and physiologies
have arisen in the absence of obvious adaptive value or selective in-
fluence. During the evolution of reptiles, for example, there occurred
a simplification of the jaw structure which made superfluous the
quadrate and articular bones of the reptilian jaw. Ultimately, mil-
lions of years later, these “liberated” units became involved in a major
change in the structure of the middle ear and made possible the
chain of small bones which is characteristic of this organ in mammals.
This “aristogenic” change, leading to an entirely new anatomical or-
ganization at a much later time, is not easy to explain on the basis
of selection and adaptation alone. The phenomenon has implied to
some that the evolutionary process has, built into it, some knowledge
of the future and that temporarily useless structures may be stored
away for later use according to some master plan.

From the standpoint of maintaining a more unified picture of the
evolutionary process, “aristogenesis” and the “preadaptation” of an
organism for some subsequent evolutionary event do not appear to
be necessary concepts. Simpson has pointed out that, in small pop-
ulations, a mutation which confers no adaptive value (or, indeed,
which may be detrimental) can become established, although “al-
most always this would lead to extinction.” In those rare cases when
the word “almost” applies, a change in the natural history of the or-
ganism might then cause an enormously rapid and major evolutionary
modification owing to the sudden usefulness of this otherwise dis-
advantageous gene, fortuitously harbored in the heredity of the strain.
From this point of view we may explain the whole of evolution, from
the localized, sedentary sort of microevolution to the dramatic ap-
pearance of new phyla, on the basis of mutation and selection alone.

As we shall discuss in a later chapter, certain structural parts of
biologically active proteins appear to be superfluous from the stand-
point of function. A tendency to assume that such parts are non-
essential might simply reflect the fact that we have not yet developed
sufficiently sensitive methods for the detection of subtle, second-order
relationships between structure and function. On the other hand,

THE TIME SCALE AND SOME EVOLUTIONARY PRINCIPLES 13
certain structural configurations may now actually be unessential and
may have been preserved as chemical vestiges of earlier molecules,
much as the bones of the mammalian ear were retained from the re-
arranging components of the reptilian jaw.

The information available to us on proteins and other chemical
components of protoplasm is, of course, insufficient to permit a
rational choice between these alternatives at the present time. We
can only hope, in analogy to the paleontologist and his “fossil record,”
that as the “protein record” relating the proteins of various species
to one another becomes more complete, some basic ground plan for
phylogenesis and speciation may begin to emerge at a molecular level
of understanding.

REFERENCES

1. G. S. Carter, Animal Evolution; a Study of Recent Views of Its Causes,
Sidgwick & Jackson, Ltd., London, 1951.
9, A, E. Trueman, Geol. Mag., 61, 360 (1924).

SUGGESTIONS FOR FURTHER READING

Colbert, E. H., Evolution of the Vertebrates, John Wiley & Sons, New York,
1955.

Huxley, J. S., Evolution, the Modern Synthesis, Allen & Unwin, London, 1942.

Oparin, A. L, The Origin of Life on the Earth, translated from the Russian by
Ann Synge, Academic Press, New York, third edition, 1957.

Simpson, G. G., The Meaning of Evolution, Yale University Press, New Haven,
third printing, 1950.

Simpson, G. G., Horses, Oxford University Press, New York, 1951.

Simpson, G. G., Life of the Past, Yale University Press, New Haven, 1953.

Smith, H. W., From Fish to Philosopher, Little, Brown & Company, Boston,
1953.

14 THE MOLECULAR BASIS OF EVOLUTION