NATURAL DISCONTINUITIES
AND THE FOSSIL RECORD
One often encounters several objections to the notion of intelligent design.
One is that intelligent design is non-natural and hence outside the scope
of the natural sciences. Another is that intelligent design may be invoked
to explain virtually anything. Yet another is that intelligent design is
empirically ambiguous: a continuous evolutionarily developed cosmos may
be as intelligently designed as a discontinuous one in which God periodically
infused new information. Henry Morris and Howard Van Till, for example,
both claim to believe in intelligent design yet hold to radically different
views on natural history and God's means of creation.
These apparent disadvantages can be turned into a tremendous advantage
for an open-minded scientist (or honest lawyer, for that matter). Obviously,
one of the greatest advantages of an intelligent design paradigm over a
naturalistic one is that it is open to more empirical possibilities.
Philosophical naturalism requires that nature be fully continuous. The
history of life must be represented by a tree. All life must have a common
ancestor. All genetic change must ultimately be the result of purely unguided,
materialistic processes. Theism or intelligent design, on the other hand,
is much less restraining. Life may be either continuous or discontinuous.
It follows that life on earth may be modeled as a either a tree or a forest.
Graphics
"If any event in life's history resembles man's creation
myths, it is this sudden diversification of marine life when multicellular
organisms took over as the dominant actors in ecology and evolution. Baffling
(and embarrassing) to Darwin, this event still dazzles us and stands as
a major biological revolution on a par with the invention of self-replication
and the origin of the eukariotic cell. The animal phyla emerged out of the
Precambrian mists with most of the attributes of their modern descendants."
- Bengston, Stefan (1990)
Nature 345:765
... The number of intermediate varieties, which have formerly
existed on the earth, (must) be truly enormous. Why then is not every geological
formation and every stratum full of such intermediate links? Geology assuredly
does not reveal any such finely graduated organic chain; and this, perhaps,
is the most obvious and gravest objection which can be urged against my
theory.
- Darwin, C. (1859)
The Origin of Species (Reprint of the first edition)
Avenel Books, Crown Publishers, New York, 1979, p. 292
"It is still, as it was in Darwin's day, overwhelmingly
true that the first representatives of all the major classes of organisms
known to biology are already highly characteristic of their class when they
make their initial appearance in the fossil record. This phenomenon is particularly
obvious in the case of the invertebrate fossil record. At its first appearance
in the ancient paleozoic seas, invertebrate life was already divided into
practically all the major groups with which we are familiar today.
- Denton, Michael (1986)
Evolution: A Theory in Crisis
Bethesda, Maryland, Adler & Adler, Pub., p.162
Biology in the early decades of the nineteeth century was dominated by
the idea that the organic world was a fundamentally discontinuous system
in which all the major groups of organisms were unique and isolated and
unlinked by transitional forms. ... Where there was variation, it was only
trivial variation within the clearly defined limits of the species or type.
Thus to the naturalists of the nineteenth century the basic order of nature
was static and discontinuous, very different from the dynamic continuous
model which was later to become axiomatic for most biologists after 1859.
(p. 18)
Before 1859 it was fashionable and intellectually respectable to view
the organic world as a discontinuous system-- the result of successive creation
interventions in the history of the world. After 1859 it became intellectually
respectable to view life as the natural product of purely natural processes
operating over long periods of time. Changing one's interpretation of the
world is not, however, the same as establishing a new fact. The facts were
the same in 1850 as they were in 1870, only the perception of them had changed.
(p. 73)
The so-called typological model of nature adhered to by biologists early
in the (19th) century was not without a considerable degree of empirical
support. ... [T]he work of the great nineteenth century comparative anatomists
such as Cuvier and, later, Owen had shown that the living world could be
considered divided into distinct types or phyla and that organisms clearly
intermediate between different classes were virtually unknown.
Comparative anatomy had also revealed that organisms were integrated
wholes in which all the components were coadapted to function together;
and this seemed to many to preclude any sort of major evolutionary transformation.
As William Coleman, an authority on Georges Cuvier, points out:
The organism, being a functionally integrated whole each part of which
stood in close relation to every other part, could not, under pain of almost
immediate extinction, depart significantly from the norms established for
the species by the first anatomical rule.
A major change, for example, a sharp increase in the heart beat or the
diminution by half of the kidney and thus a reduction in renal secretion,
would by itself have wrought havoc with the general constitution of the
animal. In order that an animal might persist after a change of this magnitude
it would be necessry that the other organs of the body be also proportionally
modified. In other words, an organism must change en bloc or not at all.
Only saltatory modification could occur, and this idea was to Cuvier, as
it is to most modern zoologists, but for very different reasons, unverified
and basically absurd. Transmutation by the accumulation of alterations,
great or small, would thus be impossible.
Coleman, W. (1964)
Georges Cuvier: Zoologist
Harvard University Press
Cambridge, Mass, pp 172-73
(Denton, 1986, p. 18)
When the appeal of the scientific paradigm and the natural desire of
the scientific community to extend the range of scientific explanation are
taken in conjunction with all the various intellectual trends and fashions
of the later Victorian era, it is in retrospect perfectly easy to understand
how Darwin's theory proved irresistible even though, as Darwin himself admitted,
the actual empirical evidence was insufficient, and there was absolutely
no evidence that any of the major divisions of nature had been crossed in
a gradual manner. If nature was to be explained by natural processes, she
had to be continuous. (p. 73)
As the years passed after the Darwinian revolution, and as evolution
became more and more consolidated into dogma, the gestalt of continuity
imposed itself on every facet of biology. The discontinuities of nature
could no longer be perceived. (p. 74)
- Denton, Michael (1986)
Evolution: A Theory in Crisis
Bethesda, Maryland, Adler & Adler, Pub.
No wonder paleontologists shied away from evolution for
so long. It never seemed to happen. Assiduous collecting up cliff faces
yields zigzags, minor oscillations, and the very occasional slight accumulation
of change--over millions of years, at a rate too slow to account for all
the prodigious change that has occurred in evolutionary history. When we
do see the introduction of evolutionary novelty, it usually shows up with
a bang, and often with no firm evidence that the fossils did not evolve
elsewhere! Evolution cannot forever be going on somewhere else. Yet that's
how the fossil record has struck many a forlorn paleontologist looking to
learn something about evolution.
- Eldredge, N., 1995
Reinventing Darwin
Wiley, New York, p. 95
Most families, orders, classes, and phyla appear rather
suddenly in the fossil record, often without anatomically intermediate forms
smoothly interlinking evolutionarily derived descendant taxa with their
presumed ancestors.
- Eldredge, N., 1989
Macro-Evolutionary Dynamics: Species, Niches, and Adaptive Peaks
McGraw-Hill Publishing Company, New York, p. 22
[T]here are all sorts of gaps: absence of gradationally intermediate
'transitional' forms between species, but also between larger groups --
between, say, families of carnivores, or the orders of mammals. In fact,
the higher up the Linnaean hierarchy you look, the fewer transitional forms
there seem to be.
- Eldredge, N., 1982
The Monkey Business: A Scientist Looks at Creationism
Washington Square Press, pp. 65-66
We are faced more with a great leap of faith -- that gradual,
progressive adaptive change underlies the general pattern of evolutionary
change we see in the rocks -- than any hard evidence.
- Eldredge, N. and Tattersall, I. (1982)
The Myths of Human Evolution
Columbia University Press, p. 57
The record jumps, and all the evidence shows that the
record is real: the gaps we see reflect real events in life's history --
not the artifact of a poor fossil record.
- Eldredge, N. and Tattersall, I. (1982)
The Myths of Human Evolution
Columbia University Press, p. 59
The fossil record flatly fails to substantiate this expectation
of finely graded change.
- Eldredge, N. and Tattersall, I. (1982)
The Myths of Human Evolution
Columbia University Press, p. 163
The fossil record suggests that the major pulse of diversification
of phyla occurs before that of classes, classes before that of orders, and
orders before families. This is not to say that each higher taxon originated
before species (each phylum, class, or order contained at least one species,
genus, family, etc. upon appearance), but the higher taxa do not seem to
have diverged through an accumulation of lower taxa.
- Erwin, D., Valentine, J., and Sepkoski, J. (1988)
"A Comparative Study of Diversification Events"
Evolution, vol. 41, p. 1183
"The fossil record pertaining to man is still so
sparsely known that those who insist on positive declarations can do nothing
more than jump from one hazardous surmise to another and hope that the next
dramatic discovery does not make them utter fools... Clearly, some people
refuse to learn from this. As we have seen, there are numerous scientists
and popularizers today who have the temerity to tell us that there is 'no
doubt' how man originated. If only they had the evidence..."
- Fix, William R. (1984)
The Bone Peddlers
New York: Macmillan Publishing Company, p.150
“To take a line of fossils and claim that they represent a lineage is not a scientific hypothesis that can be tested, but an assertion that carries the same validity as a bedtime story — amusing, perhaps even instructive, but not scientific.”
- Gee, Henry
In Search of Deep Time: Beyond the Fossil Record to a New History of Life
New York: The Free Press, 1999, pp. 113, 116-117.
To be sure there are still major groups whose origins remain
enigmatic. Bats, for example, have the poorest fossil record of all major
vertebrate groups despite their numerical abundance in the world today.
... There are some remarkably well preserved early Tertiary fossil bats,
such as Icaronycteris index, but Icaronycteris tells us nothing about the
evolution of flight in bats because it was a perfectly good flying bat.
- Godfrey, L. R.,
"Creationism and Gaps in the Fossil Record"
Scientists Confront Creationism
W. W. Norton and Company, 1983, p. 199
"Moreover, within the slowly evolving series, like
the famous horse series, the decisive steps are abrupt and without transition."
- Goldschmidt, Richard B. (1952)
"Evolution, As Viewed By One Geneticist"
American Scientist, Vol. 40, No. 1, pp. 84-94
The history of most fossil species include two features
particularly inconsistent with gradualism:
1) Stasis - most species exhibit no directional change during their tenure
on earth. They appear in the fossil record looking much the same as when
they disappear; morphological change is usually limited and directionless;
2) Sudden appearance - in any local area, a species does not arise gradually
by the steady transformation of its ancestors; it appears all at once and
'fully formed'.
- Gould, S.J. (1977)
"Evolution's Erratic Pace"
Natural History, vol. 86, May
The Meanings of Diversity and Disparity
"I must introduce at this point an important distinction that should allay a classic source of confusion. Biologists use the vernacular term diversity in several different technical senses. They may talk about "diversity" as number of distinct species in a group: among mammals, rodent diversity is high, more than 1,500 separate species; horse diversity is low, since zebras, donkeys, and true horses come in fewer than ten species. But biologists also speak of "diversity" as difference in body plans. Three blind mice of differing species do not make a diverse fauna, but an elephant, a tree, and an ant do -- even though each assemblage contains just three species.
The revision of the Burgess Shale rests upon its diversity in this second sense of disparity in anatomical plans. Measured as number of species, Burgess diversity is not high. This fact embodies a central paradox of early life. How could so much disparity in body plans evolve in the apparent absence of substantial diversity in number of species? -- for the two are correlated, more or less in lockstep, by the iconography of the cone (see figure 1.16). ...
Several of my colleagues (Jaanusson, 1981; Runnegar, 1987) have siggested that we eliminate the confusion about diversity by restricting this vernacular term to the first sense -- number of species. The second sense--- difference in body plansshould then be called disparity. Using this terminology, we may acknowledge a central and surprising fact of life's history -- marked decrease in disparity followed by an outstanding increase in diversity within the few surviving designs."
- Stephen Jay Gould (1989)
Wonderful Life: The Burgess Shale and the Nature of History
W. W. Norton & Company, New York, London, p. 49
Writing on Darwin's decision to portray evolution as a gradual and stately
process, Gould states, "I do not know why Darwin chose to follow Lyell
and the gradualists so strictly, but I am certain of one thing: preference
for one view or the other had nothing to do with superior perception of
empirical information. On this question, nature spoke (and continues to
speak) ambiguously and multifariously. Cultural and methodological preferences
had as much influence upon any decision as the actual data."
...
"... in defending gradualism as a nearly universal tempo, Darwin
had to use Lyell's most characteristic method of argument -- he had to reject
literal appearance and common sense for an underlying "reality."
(Contrary to popular myths, Darwin and Lyell were not the heroes of true
science, defending objectivity against the the theological fantasies of
such "catastrophists" as Cuvier and Buckland. Catastrophists were
as committed to science as any gradualist; in fact, they adopted the more
"objective" view that one should believe what one sees and not
interpolate missing bits of a gradual record into a literal tale of rapid
change."
...
"The extreme rarity of transitional forms in the fossil record persists
as the trade secret of paleontology. The evolutionary trees that adorn our
textbooks have data only at the tips and nodes of their branches; the rest
is inference, however reasonable, not the evidence of fossils. Yet Darwin
was so wedded to gradualism that he wagered his entire theory on a denial
of this literal record:
The geological record is extremely imperfect and this fact will to a
large extent explain why we do not find interminable varieties, connecting
together all the extinct and existing forms of life by the finest graduated
steps. He who rejects these views on the nature of the geological record,
will rightly reject my whole theory.
Darwin's argument still persists as the favored escape of most paleontologists
from the embarrassment of a record that seems to show so little of evolution.
In exposing the its cultural and methodological roots, I wish in no way
to impugn the potential validity of gradualism (for all general views have
similar roots). I wish only to point out that it was never "seen"
in the rocks.
Paleontologists have paid an exorbitant price for Darwin's argument.
We fancy ourselves as the only true students of life's history, yet to preserve
our favored account of evolution by natural selection we view our data as
so bad that we never see the very process we profess to study."
...
Comment: Gould goes on to explain that Darwinian process do not require
slow gradual change and that a model of punctuated equilibrium can explain
the pattern of sudden appearance and stasis in the fossil record. "Eldredge
and I believe that speciation is responsible for almost all evolutionary
change." The problem is complicated, however, by the fact that species
diversity is the one feature conspiculously absent upon the appearance of
most phyla. See Valentine, J., and Erwin, D. (1985) "Interpreting Great
Developmental Experiments: The Fossil Record", Development as an
Evolutionary Process.
...
"The history of most fossil species include two features particularly
inconsistent with gradualism:
1) Stasis - most species exhibit no directional change during their tenure
on earth. They appear in the fossil record looking much the same as when
they disappear; morphological change is usually limited and directionless;
2) Sudden appearance - in any local area, a species does not arise gradually
by the steady transformation of its ancestors; it appears all at once and
'fully formed'."
- Gould, S.J. (1977)
"Evolution's Erratic Pace"
Natural History, vol. 86, May
Gould honestly admits that the neo-Darwinian synthesis
is not supported by the fossil evidence and "is effectively dead, despite
its persistence as textbook orthodoxy."
- Gould, S. J. (1980)
"Is a new and general theory of evolution emerging?"
Paleobiology, 6(1), p. 120
[T]he absence of fossil evidence for intermediate stages
between major transitions in organic design, indeed our inability, even
in our imagination, to construct functional intermediates in many cases,
has been a persistent and nagging problem for gradualistic accounts of evolution.
- Gould, S.J., 1982
"Is a new and general theory of evolution emerging?"
Evolution Now: A Century After Darwin
Maynard Smith, J. (editor)
W. H. Freeman and Co. in association with Nature, p. 140
Indeed, it is the chief frustration of the fossil record
that we do not have empirical evidence for sustained trends in the evolution
of most complex morphological adaptations.
- Gould, S. J. and Eldredge, N., 1988
"Species selection: its range and power"
Scientific correspondence in Nature, Vol. 334, p. 19
"As is now well known, most fossil species appear
instantaneously in the fossil record."
- Kemp, Tom (1985)
"A Fresh Look at the Fossil Record"
New Scientist, Vol. 108, No. 1485, December 5, 1985), p. 66
(Dr. Tom Kemp is Curator of Zoological Collections at the Oxford University
Museum.)
Described recently as "the most important evolutionary
event during the entire history of the Metazoa," the Cambrian explosion
established virtually all the major animal body forms -- Bauplane or phyla
-- that would exist thereafter, including many that were 'weeded out' and
became extinct. Compared with the 30 or so extant phyla, some people estimate
that the Cambrian explosion may have generated as many as 100. The evolutionary
innovation of the Precambrian/Cambrian boundary had clearly been extremely
broad: "unprecedented and unsurpassed," as James Valentine of
the University of California, Santa Barbara, recently put it (Lewin, 1988).
Lewin then asked the all important question:
"Why, in subsequent periods of great evolutionary activity when countless
species, genera, and families arose, have there been no new animal body
plans produced, no new phyla?"
- Lewin, R. (1988)
Science, vol. 241, 15 July, p. 291
Paleontologists had long been aware of a seeming contradiction
between Darwin's postulate of gradualism ... and the actual findings of
paleontology. Following phyletic lines through time seemed to reveal only
minimal gradual changes but no clear evidence for any change of a species
into a different genus or for the gradual origin of an evolutionary novelty.
Anything truly novel always seemed to appear quite abruptly in the fossil
record.
- Mayr, E., 1991
One Long Argument: Charles Darwin and the Genesis of Modern Evolutionary
Thought
Harvard University Press, Cambridge, p. 138
What one actually found was nothing but discontinuities.
All species are separated from each other by bridgeless gaps; intermediates
between species are not observed. ... The problem was even more serious
at the level of the higher categories.
- Mayr, E., 1982
The Growth of Biological Thought: Diversity, Evolution, and Inheritance
The Belknap Press of Harvard University Press, p. 524
[W]e have so many gaps in the evolutionary history of life,
gaps in such key areas as the origin of the multicellular organisms, the
origin of the vertebrates, not to mention the origins of most invertebrate
groups.
- McGowan, C., 1984
In the Beginning... A Scientist Shows Why the Creationists are Wrong
Prometheus Books, p. 95
With the benefit of hindsight, it is amazing that palaeontologists
could have accepted gradual evolution as a universal pattern on the basis
of a handful of supposedly well-documented lineages (e.g. Gryphaea, Micraster,
Zaphrentis) none of which actually withstands close scrutiny.
- Paul, C. R. C., 1989
"Patterns of Evolution and Extinction in Invertebrates"
Allen, K. C. and Briggs, D. E. G. (editors),
Evolution and the Fossil Record
Smithsonian Institution Press, Washington, D. C., 1989, p. 105
[T]ransitions between major groups of organisms ... are difficult
to establish in the fossil record.
- Padian, K., 1991
"The Origin of Turtles: One Fewer Problem for Creationists"
National Center for Science Education Reports
Vol. 11, No. 2, Summer, p. 18
[G]aps between higher taxonomic levels are general and
large.
- Raff, R. A. and Kaufman, T. C., 1991
Embryos, Genes, and Evolution: The Developmental-Genetic Basis of Evolutionary
Change
Indiana University Press, p. 35
Well, we are now about 120 years after Darwin, and knowledge
of the fossil record has been greatly expanded ... ironically, we have even
fewer examples of evolutionary transition than we had in Darwin's time.
By this I mean that some of the classic cases of darwinian change in the
fossil record, such as the evolution of the horse in North America, have
had to be discarded or modified as a result of more detailed information
....
- Raup, D. (1979)
"Conflicts Between Darwin and Paleontology"
Field Museum of Natural History Bulletin, vol. 50 (1), p. 24, 25
"A large number of well-trained scientists outside
of evolutionary biology and paleontology have unfortunately gotten the idea
that the fossil record is far more Darwinian than it is. This probably comes
from the oversimplification inevitable in secondary sources: low-level textbooks,
semipopular articles, and so on. Also, there is probably some wishful thinking
involved. In the years after Darwin, his advocates hoped to find predictable
progressions. In general these have not been found yet the optimism has
died hard, and some pure fantasy has crept into textbooks."
Science
July 17, 1981, p. 289
The known fossil record is not, and has never has been, in
accord with gradualism. What is remarkable is that, through a variety of
historical circumstances, even the history of opposition has been obscured.
... 'The majority of paleontologists felt their evidence simply contradicted
Darwin's stress on minute, slow, and cumulative changes leading to species
transformation.' ... their story has been suppressed.
- Stanley, S. M., 1981
The New Evolutionary Timetable: Fossils, Genes, and the Origin of Species
Basic Books, Inc., Publishers, N.Y., p. 71
Evidence of gradualism between phyla, classes and even
orders is either non-existent or is much disputed. Certainly, no pervasive
pattern of gradualism exists. George Gaylord Simpson acknowledged this decades
ago as he described the situation in these terms:
"This is true of all thirty-two orders of mammals...The earliest and
most primitive known members of every order already have the basic ordinal
characters, and in no case is an approximately continuous sequence from
one order to another known. In most cases the break is so sharp and the
gap so large that the origin of the order is speculative and much disputed...
This regular absence of transitional forms is not confined to mammals, but
is an almost universal phenomenon, as has long been noted by paleontologists.
It is true of almost all classes of animals, both vertebrate and invertebrate...it
is true of the classes, and of the major animal phyla, and it is apparently
also true of analogous categories of plants."
- Simpson, G. G. (1944)
Tempo and Mode in Evolution
Columbia University Press, New York, p. 105, 107
"It remains true, as every paleontologist knows,
that most new species, genera, and families, and that nearly all categories
above the level of families, appear in the [fossil] record suddenly, and
are not led up to by gradual, completely continuous transitional sequences"
- Simpson, George Gaylord (1953)
The Major Features of Evolution
New York: Columbia University Press, p. 360
[F]or more than a century biologists have portrayed the evolution
of life as a gradual unfolding ... Today the fossil record ... is forcing
us to revise this conventional view.
- Stanley, S. M., 1981
The New Evolutionary Timetable: Fossils, Genes, and the Origin of Species
Basic Books, Inc., Publishers, N.Y., p.3
[T]he fossil record itself provided no documentation of
continuity -- of gradual transitions from one kind of animal or plant to
another of quite different form.
- Stanley, S. M., 1981
The New Evolutionary Timetable: Fossils, Genes, and the Origin of Species
Basic Books, Inc., Publishers, N.Y., p. 40
Since the time of Darwin, paleontologists have found themselves
confronted with evidence that conflicts with gradualism, yet the message
of the fossil record has been ignored. This strange circumstance constitutes
a remarkable chapter in the history of science, and one that gives students
of the fossil record cause for concern.
- Stanley, S. M., 1981
The New Evolutionary Timetable: Fossils, Genes, and the Origin of Species
Basic Books, Inc., Publishers, N.Y., p. 101
The success of Darwinism was accompanied by a decline in
scientific integrity. This is already evident in the reckless statements
of Haeckel and in the shifty, devious and histrionic argumentation of T.
H. Huxley...
To establish the continuity required by the theory, historical arguments
are invoked even though historical evidence is lacking. Thus are engendered
those fragile towers of hypotheses based on hypotheses, where fact and fiction
intermingle in an inextricable confusion.
- Thompson, W. R. (1956)
Introduction to The Origin of Species, (Reprint of the first edition),
Charles Darwin, Everyman Library, no. 811, Dent
The gaps in the fossil record are real, however. The absence
of a record of any important branching is quite phenomenal. Species are
usually static, or nearly so, for long periods, species seldom and genera
never show evolution into new species or genera but replacement of one by
another, and change is more or less abrupt.
- Wesson, R., 1991
Beyond Natural Selection
MIT Press, Cambridge, MA, p. 45
[T]he origin of no innovation of large evolutionary significance
is known.
- Wesson, R., 1991
- Beyond Natural Selection
- MIT Press, Cambridge, MA, p. 45
[L]arge evolutionary innovations are not well understood.
None has ever been observed, and we have no idea whether any may be in progress.
There is no good fossil record of any.
- Wesson, R., 1991
Beyond Natural Selection
MIT Press, Cambridge, MA, p. 206
Taxa recognized as orders during the (Precambrian-Cambrian)
transition chiefly appear without connection to an ancestral clade via a
fossil intermediate. This situation is in fact true of most invertebrate
orders during the remaining Phanerozoic as well. There are no chains of
taxa leading gradually from an ancestral condition to the new ordinal body
type. Orders thus appear as rather distinctive subdivisions of classes rather
than as being segments in some sort of morphological continuum.
- Valentine, J.W., Awramik, S.M., Signor, P.W., and Sadler, P.M. (1991)
"The Biological Explosion at the Precambrian-Cambrian Boundary"
Evolutionary Biology, Vol. 25, Max K. Hecht, editor, Plenum Press,
New York and London, p.284
Valentine and Erwin review hypotheses as to the mode
of origin of animal body plans for consistency with the fossil evidence.
They conclude that both Darwinian gradualism and punctuated equilibrium
are inadequate to account for the appearance of invertebrate body plans
and their major modifications:
"The models we consider are of three sorts: those that extrapolate
processes of speciation to account for higher taxa via divergence, those
that invoke selection among species, and those that emphasize that many
higher taxa originated as novel lineages in their own right, not only as
a consequence of species-level processes. It is in this latter class of
model that we believe the record favors." (Valentine and Erwin, 1985,
p. 71)
If large populations have gradually evolved there should be unmistakable
evidence in the fossil record, yet it is simply not found.
"... many of the large populations should have been preserved, yet
we simply do not find them. Small populations are called for, then, but
there are difficulties here also. The populations must remain small (and
undetected) and evolve steadily and consistently toward the body plan that
comprises the basis of a new phylum (or class). This is asking a lot. Deleterious
mutations would tend to accumulate in small populations to form genetic
loads that selection might not be able to handle. Stable intermediate adaptive
modes cannot be invoked as a regular feature, since we are then again faced
with the problem of just where their remains are. We might imagine vast
arrays of such small populations fanning continually and incessantly into
adaptive space. Vast arrays should have produced at least some fossil remains
also. Perhaps an even greater difficulty is the requirement that these arrays
of lineages change along a rather straight and true course --- morphological
side trips or detours of any frequency should lengthen the time of origin
of higher taxa beyond what appears to be available. Why should an opportunistic,
tinkering process set on such a course and hold it for so long successfully
among so many lineages?
We conclude that the extrapolation of microevolutionary rates to explain
the origin of new body plans is possible, but does not accord with the primary
evidence." (Valentine and Erwin, 1985, pp. 95, 96)
The model of punctuated equilibrium or species selection attempts to
account for the lack of evidence by relying primarily on the evolution of
small isolated populations which would have a diminished chance of leaving
a fossil record. This scenario has its difficulties, however, as Valentine
and Erwin point out:
"The required rapidity of the change implies either a few large steps
or many and exceedingly rapid smaller ones. Large steps are tantamount to
saltations and raise the problems of fitness barriers; small steps must
be numerous and entail the problems discussed under microevolution. The
periods of stasis raise the possibility that the lineage would enter the
fossil record, and we reiterate that we can identify none of the postulated
intermediate forms. Finally, the large numbers of species that must be generated
so as to form a pool from which the successful lineage is selected are nowhere
to be found. We conclude that the probability that species selection is
a general solution to the origin of higher taxa is not great, and that neither
of the contending theories of evolutionary change at the species level,
phyletic gradualism or punctuated equilibrium, seem applicable to the origin
of new body plans." (p. 96)
- Valentine, J., and Erwin, D. (1985)
"Interpreting Great Developmental Experiments: The Fossil Record"
Development as an Evolutionary Process
Rudolf A. Raff and Elizabeth C. Raff, Editors
Alan R. Liss, Inc., New York, pp. 71, 95, 96