Punctuated Equilibrium and Patterns from the Fossil Record

by Casey Luskin

Introduction:
Evolution postulates that new species are descended from other pre-existing species, and that populations of the pre-existing species changed into the new species over periods of time. Evolution thus predicts that there were organisms that existed at transitional stages while one form turned into another. It is possible that evidence of these evolutionary transformations may be found in the fossil record. This was noted in Origin of Species, by the famous British naturalist Charles Darwin:

The number of intermediate varieties, which have formerly existed on the earth, [must] be truly enormous.¹ However, Darwin recognized that the fossil record did not contain fossils of these "intermediate" forms of life:

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.¹ To be sure, it should be acknolwedged from the outset that there are various fossils evolutionists have claimed are examples of intermediates, including Acanthostega (an amphibian), Archaeopteryx (a bird), and Ambulocetus (a land mammal). Discussing whether or not these alleged transitional fossils truly represent transitional intermediates is beyond the scope of this paper here. The purpose here is to understand the general pattern found in the fossil record and ask if it supports evolutionary theory.

Two types of Evolution
The term "evolution" simply means "change through time," there are two types of evolution: macroevolution and microevolution. Microevolution is "slight, short-term evolutionary changes within species."² For example, within humans, there are different eye colors, hair colors, and skin colors. These are the result of microevolution. In contrast, macroevolution is "the origin and diversification of higher taxa"² or, "evolutionary change on a grand scale, encompassing [among other things] the origin of novel designs…"³ There is thus a fundamental difference between microevolution and macroevolution.

Understanding Evidence:
When scientists propose hypotheses, they first make observations of the evidence. Sometimes they then make inferences based upon the evidence to create a hypothesis about what happened. When it comes to the fossil record, out of thousands of species known, only a few are claimed to be Darwin's intermediate forms. Fossil evidence of intermediates are generally absent, as paleontologist Stephen Jay Gould explains:

"The absence of fossil evidence for intermediary 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."⁴ Is long, slow, "gradual" evolution (see Figure 1) an inference from the evidence, or was it assumed simply because of naturalism and what was predicted by Darwin's theory? Gould explains it is inference: "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."⁵



Appealing to Incompleteness:
Darwin saved his gradual theory of evolution by claiming that intermediate fossils are not found because "[t]he geological record is extremely imperfect"¹ and thus it just so happened that the intermediate links were not the ones fossilized. Gould noted in 1977 that Darwin's argument that the fossil record is imperfect "still persists as the favored escape of most paleontologists from the embarrassment of a record that seems to show so little of evolution directly."²¹ Paleontologists emphasized that the "jumps" between species without transitional forms, were not simply the result of an incomplete record: "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."⁶ Eventually most biologists accepted that the fossil record did not contain Darwin's predicted transitional forms. One recent study found that, "if scaled to the … taxonomic level of the family, the past 540 million years of the fossil record provide uniformly good documentation of the life of the past."⁷ One biologist who studied under Gould concluded: "Evolutionary biologists can no longer ignore the fossil record on the ground that it is imperfect."⁸ Some might claim that ideas about the lack of transitional forms in the fossil record are out-dated, and that new finds confirm that the fossil record is somehow loaded with transitional forms. Yet consider this quote from Gould’s colleague, Niles Eldrege, who wrote in 1995: "No wonder paleontologists shied away from evolution for so long. It never seemed to happen. Assiduous collecting up cliff faces yields … 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!"⁹ In 1997, vertebrate paleontologist Robert Carroll wrote: "Fossils would be expected to show a continuous progression of slightly different forms linking all species and all major groups with one another in a nearly unbroken spectrum. In fact, most well-preserved fossils are as readily classified in a relatively small number of major groups…"¹⁰ In 1999, writing in Nature, Oxford zoologist Mark Pagel stated while reviewing a book by Niles Eldredge: Palaeobiologists flocked to these scientific visions of a world in a constant state of flux and admixture. But instead of finding the slow, smooth and progressive changes Lyell and Darwin had expected, they saw in the fossil records rapid bursts of change, new species appearing seemingly out of nowhere and then remaining unchanged for millions of years-patterns hauntingly reminiscent of creation.¹¹ Finally, in 2001, evolutionary biologist Ernst Mayr wrote: "Wherever we look at the living biota … discontinuities are overwhelmingly frequent…The discontinuities are even more striking in the fossil record. New species usually appear in the fossil record suddenly, not connected with their ancestors by a series of intermediates."¹² It was thus confirmed that the fossil record did not contain the pattern of transitional forms predicted by Darwin. Paleontologists had to accept this fact in order to preserve evolutionary theory, and came up with new ideas about how evolution worked.

Quote Mining and Cherry Picking?
Some might claim that citing these authorities over the lack of transitional forms misrepresents their views or constitutes "mining" their writings for quotes that fit this argument, while excluding portions that don't fit. Stephen Jay Gould complains creationists misuse his quotes about the lack of transitions: "[t]his quotation, although accurate as a partial citation, is dishonest in leaving out the following explanatory material showing my true purpose--to discuss rates of evolutionary change, not to deny the fact of evolution."¹³ It should thus be acknowledged that each author cited thus far is an unswerving evolutionist. Ernst Mayr, proclaims his belief that "[e]volution is no longer a theory, it is simply a fact."¹² But these convictions do not invalidate the fact that the fossil record generally lacks plausible candidates for transitional forms. As explained next, many evolutionists have tried to accommodate the lack of transitions into evolutionary theory by claiming the lack of transitionals tells more about the rate at which evolution occurred rather than whether or not evolution happened.

Punctuated Equilibrium
Because the fossil record did not exhibit Darwin's predicted slow and gradual evolution with transitional forms, some paleontologists sought to find a theory of evolution where, "changes in populations might occur too rapidly to leave many transitional fossils"¹³ (see Figure 2--modeled after figure 8 from Gould and Eldredge 1977 (see ref 17).



In 1972, Gould and Eldredge proposed the theory of "punctuated equilibrium" where most evolution takes place in small populations over relatively rapid geological time periods. By reducing the numerical size of the transitional population and the number of years for which it exists, punctuated equilibrium greatly limits the number of organisms bearing transitional characteristics. Since many organisms are not fossilized, this increases the likelihood that transitional forms would not be fossilized. One strength of this theory is that Gould and Eldredge claim it is predicted by population genetics. But what are the implications of punctuated equilibrium?

Under punctuated equilibrium, species usually change little as, "gradual change is not the normal state of a species."⁴ Large populations may experience, "minor adaptive modifications of fluctuating effect through time" but will "rarely transform in toto to something fundamentally new."⁴ This is called "stasis." But small "peripheral" populations may allow for more change at a quicker rate. Gould argued that most macroevolutionary change takes place in such populations during "speciation" such that there is insufficient time for the transitional forms to be fossilized: "Speciation, the process of macroevolution, is a process of branching. And this branching … is so rapid in geological translation (thousands of years at most compared with millions for the duration of most fossil species) that its results should generally lie on a bedding plane, not through the thick sedimentary sequence of a long hillslope."⁴ Is Punctuated Equilibrium a Scientific Theory?
Ernst Mayr wrote, "[a] primary tool used in all scientific activity is testing."¹³ Theories are testable when they make predictions which scientists can in principle observe. Though philosophers of science do not universally agree upon a definition of science,¹⁴ many have stated that scientific theories must be based upon repeatable observations,¹⁵ subject to testing,^{13, 16} and "falsifiable," as observations could disconfirm the theory.¹³

But what hard evidence does punctuated equilibrium predict? With respect to finding fossil evidence of the transitional stages of evolutionary change, punctuated equilibrium predicts that direct fossil evidence of these transitional morphologies will tend to not be found. The theory of punctuated equilibrium may still predict certain patterns which may be found in the fossil record, but as Steven Stanley explains, the theory is untestable against special creation and leaves scant trace of Darwin's mechanism at work: "Any claim that natural selection operated with great effect exactly where it was least likely to be documented--in small, localized, transitory populations--would seem to render Darwin's new theory untestable against special creation, and perhaps almost preposterous as a scientific proposition."¹⁷ Though it does make certain predictions, punctuated equilibrium provides a poor vehicle for validating Darwin's theory as the mechanism of evolution and confirming Darwin's strong prediction that transitional stages of morphology existed.

Punctuated Equilibrium and Genetics
Gould and Eldredge justify their model of rapid speciation by claiming that it is predicted by population genetics. According to "allopatric speciation" biological change is most likely to occur in small populations which are "reproductively isolated" or separated from the main gene pool of a population. In these cases, there is a stoppage of gene flow from one population to the other, and thus variation in the smaller population can persist, and undergo further change without being "drowned out" by the gene pool of the larger population.

Essentially, there has been a recognition that speciation, which purports to explain the rapid and large morphological jumps in the fossil record, may require too much biological change in too little time. Gould and Eldredge write that criticisms of punctuated equilibrium come from evolutionary biologists who have recognized that it requires large rapid genetic changes: "Evolutionary biologists have raised a number of theoretical issues from their domain of microevolution … [and] continued unhappiness … focuses on claims that speciation causes significant morphological change."¹⁸ The question must be asked if a punctuated equilibria model requires too much genetic change too fast. One glory of Darwin's theory was long time periods available for the origin of biological complexity. Furthermore, Darwin imagined that evolving populations might be quite large, increasing the chances that favorable mutations would happen. As Darwin wrote: "For forms existing in larger numbers will always have a better chance, within any given period, of presenting further favourable variations for natural selection to seize on, than will the rarer forms which exist in lesser numbers."³⁷ Yet punctuated equilibrium denies evolutionary change both these advantages as it compress the vast majority of origin of macroevolutionary change and biological novelty into the vast minority of the time, and small populations.¹⁹ Too few "roles of the dice" are allowed for the variation to arise.

Gould and Eldredge counter that variation could exist in the larger population before reproductive isolation was achieved and speciation could take place: "[M]orphological change may accumulate anywhere along the geological trajectory of a species. But unless that change be "locked up" by acquisition of reproductive isolation (that is speciation), it cannot persist or accumulate…¹⁸ But such a claim implies that transitional variation might pre-date the speciation event, and would seem to negate the reason why punctuated equilibrium supposedly explains why transitional forms did not fossilize in the first place. (See Figure 3.) If the transitional morphological variation predated the reproductive isolation, why is there again apparently no fossil record of that pre-existing transitional variation in the large population before the speciation event?

Figure 3. In each transition, the following occurs: where some members of the initial parent (upper) population ultimately evolve into into the descendant (lower) population of . An intermediate form is . Explanation. This diagram illustrates difficulties faced by a punctuated equilibrium (punc eq) model. Three speciation events are shown where two members of the upper population (encircled) become reproductively isolated, and undergo allopatric speciation. Transition A: there are no forms of in the initial population, and thus it requires significant genetic change to go from to during speciation. This shows how punc eq permits no transitional forms ( ) to be found. Geneticists have complained this requires too much biological change during speciation. Gould and Eldredge replied that the transitional form, ( ), could have existed in the initial population ( ), thus lessening the amount of change required during the speciation event. Transition B: transitional form exists in the initial population, requiring smaller amounts of change during the speciation event. However, the paleontological argument for why no transitional forms were fossilized is weakened, because transitional morphology is required to pre-exist for long periods of time in the initial upper population. Transition C: One might reply that this model represents a compromise, where transitional form is rare in the initial population (rare enough to not be fossilized), but still exists. It luckily gets caught in the reproductively isolated segment of the initial population, so it contributes its genes during the speciation event. This might be possible, but illustrates the tradeoff punc eq faces: as transitional morphology gets rarer in the initial population (decreasing the probability the transitional form will be fossilized), the odds of the transitional form finding itself in the reproductively isolated population, and being able to pass on its genes to a new population, also decrease significantly. Thus, as punc eq proponents argue that transitional forms are found in the initial population, chances are higher that it should have been fossilized, and punc eq loses its argument for why transitional forms are not found in the first place. Conclusion: Punc eq must walk a fine line to allow for large morphological change at an extremely rapid rate, and yet keep the transitional population small enough so that its representatives are not fossilized. Can punc eq have it both ways? Is it likely that this model of evolutionary change would predominate the history of life, as would be required by the lack of transitional forms in the fossil record? Perhaps punc eq represents special pleading and requires an unlikely mechanism with many weaknesses.

Based upon the fossil record, and following the traditional mode of explanation under punctuated equilibrium, the variation must originate during a speciation event. But how fast did Gould and Eldredge claim speciation occurred? Punctuated equilibrium accepts the conventional idea that species form over hundreds or thousands of generations and through a series of intermediate stages.¹³ (Gould)

"Geographic isolation leading to reproductive isolation need not take long to occur: our estimate was from five thousand to fifty thousand years.²⁰ (Eldredge)

"I'd be happy to see speciation taking place over, say, 50,000 years…²¹ (Gould)

Is 50,000 years (at the most) enough time for speciation? What is the maximum amount of genetic change possible in 50,000 years? Using a mutation rate of one point mutation per 10^-9 loci per year, this means that the maximum amount of change in 50,000 years is:

10^-9 mutation / loci / year x 50,000 years = 0.00005 mutation / loci

This implies that in 50,000 years, a species can undergo at the very maximum a 0.005% total change in its DNA through the traditional point mutation mechanism of genetic change during a speciation. Given that genetic variation within a species can range much higher than 1%, this very small amount of genetic change possible during a speciation event seems insufficient to justify the many rapid and large transitionless morphological jumps in the fossil record which punctuated equilibrium purports are possible during speciation. Though some evolutionary biologists believed the rapid appearance of species could be accounted for by neo-Darwinian population genetics, they were critical of punctuated equilibrium: "[S]ome of the genetic mechanisms that have been proposed [by proponents of punctuated equilibrium] to explain the abrupt appearance and prolonged stasis of many species are conspicuously lacking in empirical support. Thus, we do not feel logically compelled to abandon neo-Darwinism in favor of the theory of punctuated equilibria."²² Gould would probably claim he was not abandoning neo-Darwinism, but yet he did propose some new forms of macro-mutations which were lacking in empirical support. Appealing to traditional neo-Darwinian mechanisms of mutation requires too much change in too little time. And in the end, fossils documenting this gradual change are generally conspicuously absent. If there is no validated mutational mechanism which can account for the necessary rapid rate of biological change during speciation events, then perhaps evolution is without a reasonable explanation for the lack of transitional forms in the fossil record. Regarding these fossil jumps, Michael Denton writes: "[M]ajor discontinuities simply could not, unless we are to believe in miracles, have been crossed in geologically short periods of time through one or two transitional species occupying restricted geographical areas. Surely such transitions must have involved long lineages including many collateral lines of hundreds or probably thousands of transitional species.²³ The Origin of the Animal Phyla:
According to paleontologists, almost all of the major living animal phyla appear in the fossil record during the Cambrian Period, about 550 million years ago (see Figure 4). This takes place within a 5-10 million year period which has been called the "Cambrian Explosion." It is unlikely that any theory of evolution can account for the lack of transitional forms, because it must generate simply too much biological complexity too quickly: "It follows that 6-10 million year in the evolutionary time scale is but a blink of an eye. The Cambrian explosion denoting the almost simultaneous emergence of nearly all the extant phyla of the Kingdom Animalia within the time span of 6-10 million years can’t possibly be explained by mutational divergence of individual gene functions."²⁴ Before the Cambrian, very few fossils having anything to do with modern animal phyla are found in the fossil record: "Most of the animal phyla that are represented in the fossil record first appear, ‘fully formed’ and identifiable as to their phylum in the Cambrian some 550 million years ago...The fossil record is therefore of no help with respect to the origin and early diversification of the various animal phyla..."²⁵ Though this quote is from an invertebrate zoology textbook, it is now known that even vertebrates--fossil fish-- appear in the Cambrian Explosion.²⁶ Many have claimed the explosion is the result of a gap in the record,²⁷ however some prominent paleobiologists stated the Cambrian explosion, "is real; it is too big to be masked by flaws in the fossil record."²⁸ Some have also cited that some Pre-Cambrian fossils, such as fossil embryos, have been discovered. However, Stephen Jay Gould stated regarding the discovery of a very few number of pre-Cambrian fossils that, "[n]ew evidence of very early branching of animal phyla in no way disproves the reality of the Cambrian explosion."²⁹ Meyer, Nelson, Chien, and Ross note:

[T]he suddenness of the appearance of animal life in the Cambrian, "the Cambrian explosion" has now earned titles such as "The Big Bang of Animal Evolution" (Scientific American), "Evolution’s Big Bang" (Science), and the "Biological Big Bang" (Science News).³⁰ A Pattern of Explosions:
The Cambrian Explosion is by no means the only "explosion" in the fossil record. One evolutionist concedes that for the origin of fishes, "this is one count in the creationists' charge that can only evoke in unison from paleontologists a plea of nolo contendere [no contest]."³¹

Plant biologists have called the origin of plants an "explosion," saying, "[t]he … radiation of land [plant] biotas is the terrestrial equivalent of the much-debated Cambrian 'explosion' of marine faunas."³² Vertebrate paleontologists believe there was a mammal explosion because of the few transitional forms between major mammal groups: "There 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."³³ Another study, "Evolutionary Explosions and the Phylogenetic Fuse," found a bird (as well as a mammal) "Early Tertiary 'explosion'" because many bird and mammal groups appear in a short time period lacking immediately recognizable ancestral forms.³⁴ Finally, others have called the origin of our own genus Homo, "a genetic revolution"³⁵ where "no australopithecine [ape] species is obviously transitional"³⁵ leading one commentator to call it, like others called the Cambrian Explosion, a "big bang theory" of human evolution.³⁶ While these papers appeal to adapative radiation, niche-filling, and "genetic revolutions" as the mechanisms for these explosions, the pattern of rapid appearance of diverse morphologies without transitions remains an important pattern in the fossil record.

Conclusion:
Out of thousands of species in the fossil record, only a few are claimed to be transitional forms. This lack of transitional forms poses, as Darwin said, "the most obvious and gravest objection which can be urged against [evolutionary] theory."¹ And, at least to this point, it appears to be an objection that is unsolved by evolutionists.

References:

1. Darwin, C., The Origin of Species (1859).
2. Futuyma, D., Evolutionary Biology, pg. 447, glossary (1998).
3. Campbell, N. A., Reece, J. B., Mitchell, L. G., Biology 4th Ed, pg. G-13 (1999).
4. Gould, S. J., "Is a new and general theory of evolution emerging?" Paleobiology, vol 6(1), p. 119-130 (1980).
5. Gould, S. J., "Evolution's erratic pace," Natural History, Vol. 86, No. 5, pp.12-16, (May 1977, emphasis added).
6. Eldredge, N. and Tattersall, I., The Myths of Human Evolution, pg. 59 (1982).
7. Benton, M. J., Wills M. A., and Hitchin, R., "Quality of the fossil record through time," Nature Vol 403:534-536 (February 3, 2000).
8. David S. Woodruff, Science, pg.717 (May 16, 1980).
9. Eldredge, N., Reinventing Darwin, p. 95 (1995).
10. Carroll, R., Patterns and Processes of Vertebrate Evolution, pgs. 8-10 (1997).
11. Pagel M., "Happy accidents?," Nature, Vol 397, pg. 665 (February 25, 1999).
12. Mayr, E., What is Evolution, pg. 189 (2001).
13. See Teaching About Evolution and the Nature of Science (National Academy Press, 1998), pg. 43, 56, 57.
14. Essays by Larry Lauden in But Is It Science? Edited by Michael Ruse. (Buffalo, N.Y.: Prometheus Books, 1988).
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16. K. Popper, Conjectures and Refutations, p. 257 (1963).
17. Stanley, S., Macroevolution, pg. 93.
It should be noted that in Gould and Eldredge's 1977 paper, "Punctuated equilibria: the tempo and mode of evolution reconsidered," (Paleobiology 3:115-151), they do give specific data in favor of punctuated equilibrium. They state that "the model of punctuated equilibria is eminently testable..." (p. 120). Gould and Eldredge state find that the case of the radiolarian Pseudocobus vema provides good documentation of two very rapid periods of change. This species was put forth by a critic of punc eq which makes it significant that Gould and Eldredge can claim that it supports their theory. The diagram documenting this change through time is pictured below (his figure 3):

My comments are in red. Very rapid rates of change is a pattern, not an explanation or a process. But what is the mechanism of these "very rapid rates of change?" Why must it be Darwin's theory of mutation - and selection when this is perhaps equally consistent with the rapid infusion of information into the biosphere by an intelligent agent. If an intelligent agent were to infuse large amounts of information into the biosphere, is this not exactly the sort of pattern we might expect to find? Additionally, Gould and Eldredge find other cases in favor of the "punc-eq" pattern, the radiolarian Eucyrtidium calvertense, the history of the trilobite Placoparia, and even cited that punc eq has been used to explain the origin of Homo sapiens. Devonian brachiopods are also cited as evidenc of punc eq, where one commentator wrote, "In twenty yeasr work on the Mesozoic Brachiopods, I have found plenty of relationships, but few if any evolving lineages ... What it seems to mean is that evolution did not noramlly proceed by a process of gradual change of one species into another over long periods of time..." Though many have tried to give mechanisms, punc eq could be seen more fundamentally as simply a pattern in the fossil record with rapid morphological change followed by periods of stasis. But a pattern is not a process--what is the process which can account for this pattern? Is this pattern more consistent with the process of intelligent design, or Darwinian evolution. On its face, intelligent design is capable of infusing large amounts of information into the biosphere rapidly, which could result in this rapid morphological change. In contrast, on its face, Darwinian evolution requires forms go through intermediate stages. Thus, Gould has appealed to developmental mutations (see ref. 4) to account for rapid changes and hold faithful to neo-Darwinian mechanisms of change. But at what point does this become special pleading where we must always invoke special developmental mutations to create rapid morphological change which is somehow both functional and advantageous? On its face, neo-Darwinism would not predict a pattern of punc eq in the fossil record, implying that punc eq is a poor vehicle for testing if stepwise mutations occurred.
18. Gould, S. J., Eldredge, N., "Punctuated equilibrium comes of age," Nature Vol 366:223-227 (November 18, 1993).
19. See "Punctuated Equilibria: An Alternative to Phyletic Gradualism" Eldredge and Gould in T.J.M. Schopf, ed., Models in Paleobiology. 1972, pg. 106.
20. Eldredge, N. The Triumph of Evolution and the Failure of Creationism (2000).
21. Stephen Jay Gould as quoted in Lewin, R., "Evolutionary Theory Under Fire," Science Vol 210:883-887 (November, 1980).
22. Charlesworth, B., Lande, R., and Slatkin, M., "A Neo-Darwinian Commentary on Macroevolution," Evolution, 36(3) 1982 pg. 474-498.
23. Denton, M., Evolution: A Theory in Crisis, pg. 193 (1986).
24. Ohno, S., "The notion of the Cambrian pananimalia genome," Proc. Nat. Acad. Sci. USA, Vol 93 pg. 8475-8478 (August 1996).
25. Barnes, R.S.K., P. Calow, P.J.W. Olive, D.W. Golding, J.I. Spicer. The Invertebrates: A New Synthesis 3^rd Ed. (2001).
26. "Catching the first fish" by Philippe Janvier; Nature 402:21 - 22 (1999).
27. Dawkins, R., The Blind Watchmaker, pg. 229-230 (1986).
28. James W. Valentine, Stanley W. Awramik, Philip W. Signor & Peter M. Sadler, Evolutionary Biology (1991).
29. Gould, S. J., "On Embryos and Ancestors," Natural History, July/Aug 1998, pg. 64.
30. See Meyer, Nelson, Chien, and Ross in "The Cambrian Explosion: Biology's Big Bang," in Darwinism, Design, and Public Education, edited by J. A. Campbell and S. C. Meyer, pg. 326 (reference 6) (2003).
31. Arthur Strahler's anticreationist book, Science and Earth History – The Evolution/Creation Controversy Buffalo: Prometheus Books, 1987.
32. "Early Evolution of Land Plants: Phylogeny, Physiology, and Ecology of the Primary Terrestrial Radiation" Annual Review Ecol. Syst. 1998, 29:263-292.
33. Eldredge, N., The Monkey Business: A Scientist Looks at Creationism, pg. 65-66, (1982).
34. Cooper, A., and Fortey, R. "Evolutionary Explosions and the Phylogenetic Fuse" Tree vol 13, no 4 (1998).
35. Hawks, J., Hunley, K., Sang-Hee, L., Wolpoff, M., "Population Bottlenecks and Pleistocene Evolution," J. of Mol. Biol. and Evolution, 17(1):2-22 (2000).
36. "New study suggests big bang theory of human evolution" (January 10, 2000), at http:// www.umich.edu/~newsinfo/ Releases/2000/Jan00/r011000b.html" (accessed 10/21/03).
37. Darwin, Charles, The Origin of Species available online at http://www.literature.org/authors/darwin-charles/the-origin-of-species/chapter-06.html (accessed October 6, 2004)