The Molecular Clock Hypothesis, Part I

By Jon Covey, BA, MT(ASCP)
Edited by Anita K. Millen, MD, MPH, MA

Often, an evolutionist will refer to molecular biology and particularly to the molecular clock hypothesis and its concomitant neutral theory as powerful evidence for evolution. I’ll give you the basic idea of what the molecular clock hypothesis means so you can evaluate some statements from scientists working in the field of molecular biology who are especially looking for evidence of molecular clocks in their research. Their remarks concerning molecular clocks are damning to the hypothesis they are trying to substantiate.

Cytochrome c is often referred to when speaking of molecular clocks because it is so widespread in nature. It is found in bats, cats, rats, and bacteria. The structure and function of cytochrome c are not important to our discussion. It is simply one of those universal, recognizable molecules that has pretty much the same role in every living thing. The important issue is the gist of the molecular clock hypothesis. Molecular clocks are often based upon the amino acid or nucleic acid sequence of cytochrome c or its gene.

Michael Denton, a physician and geneticist, explained the molecular clock hypothesis in his book Evolution: A Theory in Crisis (pp. 295-96):

"We see then that the highly ordered pattern of cytochrome diversity could only have been generated if the overall net rate of sequential divergence had been constant with respect to absolute time in all the diverse branches of every class since their common evolutionary origin. Moreover, only if such a strange rule had been repeated over and over again, throughout eucaryotic evolution, following each evolutionary divergence, could it have generated the highly ordered pattern and uniform isolation of each class of eucaryotic cytochrome sequences.

"Only if the degree of evolution in a family of molecules such as the cytochromes had been constrained by some kind of time constant mechanism, so that in any one class the degree of change which occurs is always proportional to the lapse of absolute time, can the ordered pattern of molecular diversity be explained. This remarkable concept is widely known as that of the ‘molecular clock hypothesis’. But although such a clock is perfectly capable of accounting for the observed equal divergence of, say, all vertebrate cytochromes from those of insects, no one has been able to explain in precise terms exactly how such a time constant process could work. Rather than being a true explanation, the hypothesis of the molecular clock is really a tautology, no more than a restatement of the fact that at a molecular level the representatives of any one class are equally isolated from the representatives of another class. The tautological nature of the molecular clock hypothesis is reminiscent of the explanations of the gaps in the fossil record. The proposal put forward to save evolution in the face of the missing links–that connecting links are missing from the fossil record because transitional species are very rare–is essentially tautological. If evolution is true then indeed the intermediates must be very rare. But unfortunately we can only know that evolution is true after we have found the transitional types! The explanation relies on belief in evolution in the first place. Similarly, if evolution is true then, yes indeed, the clock hypothesis must also be true. Again the hypothesis gets us nowhere. We save evolution because we believed it in the first place.

"But there is an additional twist to the clock hypothesis. As we saw above, different proteins exhibit different degrees of interspecies variation. While haemoglobin sequences differ by fifty per cent between man and carp; cytochrome C differs by only thirteen per cent. To account for the fact that all the haemoglobin sequences of a particular group differ by fifty per cent from another group, while all the cytochrome C sequences differ by only thirteen per cent, it is necessary in evolutionary terms to presume that the molecular clock has ticked at a faster rate in the case of haemoglobin than in the case of cytochrome C; in other words, to propose two molecular clocks ticking at a different rate, one for the haemoglobin family and one for the cytochrome family. However, as there are hundreds of different families of proteins and each family exhibits its own unique degree of interspecies variation, some greater than haemoglobin, some far less than the cytochromes, then it is necessary to propose not just two clocks but one for each of the several hundred protein families, each ticking at its own unique and highly specific rate."

The clock for each protein ticks at different rates in different organisms. Clearly said, molecular clocks are faster in some animals than others. The "time" the clocks tell vary from one species to another. There is nothing regular about the clocks except the attempt to manipulate the data mathematically to fit preconceived beliefs. Evolutionists are so committed to the "fact" of evolution, they are trying to get the data to conform to the fact, having forgotten that the "fact" is the thing they are trying to prove.

According to Simon Easteal, [Easteal, p. 415] the molecular clock hypothesis says that particular nucleotide and amino acid sequences accumulate mutational changes at a stochastically uniform rate (Stochastic: A family of random variables dependent upon a parameter which usually denotes time. Also known as random process. Dictionary of Scientific and Technical Terms, 5^th ed.). He says that the hypothesis is of interest for two principal reasons. One reason is that a molecular clock is predicted by the neutral theory of molecular evolution. That is to say, the theory that balancing and directional natural selection have little or no influence on the evolutionary changes occurring in genes and other regions of DNA and the proteins they code for. He says that evidence for and against a molecular clock has sometimes been construed as evidence for and against the neutral theory (see below). The other reason for this interest in the hypothesis is that a molecular clock would help trace the evolutionary history of DNA and validate the theory of evolution in some measure.

Easteal remarks:

"Many studies of the molecular clock have focused on mammals, largely because of the prior availability of sequence and other molecular data. However, despite being investigated extensively, the issue of whether or not DNA and proteins evolve in a clock-like fashion in mammalian lineages remains controversial more than 25 years after the molecular clock was first proposed by Zuckerkandl and Pauling. Results continue to be interpreted as both supporting and refuting the existence of a molecular clock. Differing conclusions are reached not so much because of differences in the data being analyzed, although these exist, but more because of differences in the assumptions made in the analysis."

The neutral theory prediction of a molecular clock, according to Easteal, is based on the assumption that mutation is a stochastic process and that there is no systematic variation in mutation rate among lineages. Random mutations will result in a variable rate at which mutations accumulate.

Easteal says that the neutral theory does not preclude the occurrence of purifying selection–the loss of deleterious or bad mutants. Purifying selection can only take place in regions of DNA having functional importance. There are portions of genes referred to as introns that seem to have no coding significance according to evolutionary molecular biologists. I doubt this, but I’ll not argue against it because I want to get to the heart of the molecular clock hypothesis. This hypothesis is based on the "fact of evolution." All the statistical methods developed to make sense of the variant DNA sequences among divergent organisms, e.g. squirrel, squid, and sapsucker are simply manipulations of the data. If there were no evolutionary presupposition, these mathematical techniques would not be used. These statistical methods are purposely set up to detect clock-like mutational changes within the genomes of living organisms, but even with this feverish intent, by and large, they have failed to establish the validity of the molecular clock hypothesis. There is not a single clock, based on one or another gene, that tells the correct time.

The following conclusions drawn by researchers searching for molecular clocks illustrate the controversy over this questionable hypothesis. For instance, Milinkovitch tells us that the order Cetacea is classified into two highly distinct suborders: the toothed whales such as the dolphins, porpoises, and sperm whales, and the filter-feeding baleen whales [Milinkovitch]. He says his research on the DNA sequencing of two mitochondrial ribosomal gene segments and the mitochondrial cytochrome b gene for these two major groups of whales contradicts this classification scheme.

Actually, this could be helpful in giving us a better way to classify the whales, but it is not healthy for the molecular clock hypothesis. He says that his finding challenges the conventional scenario of a long, independent evolutionary history of toothed and filter-feeding whales. He says that the Amazon River dolphin is genetically more divergent from the marine dolphins of similar appearance than the sperm whales are from the dissimilar baleen whales. He remarks that, based on molecular clock assumptions, the DNA data suggests a more recent origin of baleen whales (about 25 million years) than the previously assumed 40+ million years. Does this mean the molecular data should be accepted over the previously established relationship between radiometry and biostratigraphy? If we give greater credence to molecular data, we would have to reject radiometric dating methods or assume these methods have still not been perfected. Of course, isotope geologists might say that the assumptions of molecular biologists must be wrong, and vice versa.

Let’s look at the molecular results of chloroplast DNA studies in plants. The chloroplast is where the process of converting light energy into chemical energy takes place (also known as photosynthesis). In his report, Gielly reveals that high variability exists in the divergence rates for two regions of the chloroplast genome [Gielly]. He explains that thirty-six species of plants from ten different genera were studied and the divergence rates, the mutation rates from the assumed common ancestors of these organisms, varied 5-8 fold. This means the molecular clocks in some of these plants ran 5-8 times faster than in others. He went on to say that in order to explain this great variance, three separate ad hoc hypotheses had to be proposed to explain the data: 1) sampling bias; 2) age of common ancestor; 3) variants of the molecular clock. It would have been better if he admitted there was no clock. If our clocks acted this way, we would never really know what time it was, because there would be no standard time. If your friend said, "Stay here. I’ll return in an hour," would you wait one hour or eight? You would have no way of knowing. The important point here is that the assumption of evolution as "fact" has created this difficulty.

Consider this telling abstract from the studies of Fitch and Ayala [Fitch and Ayala]:

"The Cu, Zn superoxide dismutase (SOD) was examined earlier and found to behave in a very unclocklike manner despite (accepted point mutation, or PAM) corrections for multiple replacements per site. Depending upon the time span involved, rates could differ 5-fold. We have sought to determine whether the data might be clocklike if a covarion model were used. We first determined that the number of concomitantly variable codons (covarions) in SOD is 28. With that value fixed we found that the observations for SOD could fit reasonably well a molecular clock if, given 28 covarions, (i) there are approximately six replacements every 10 million years, (ii) the total number of codons is 162, (iii) the number of codons that are permanently invariable across the range of taxa from fungi to mammals is 44, and (iv) the persistence of variability is quite low (0.01). Thus, the inconsistent number of amino acid differences between various pairs of descendent sequences could well be the result of a fairly accurate molecular clock. The general conclusion has two sides: (i) the inference that a given gene is a bad clock may sometimes arise through a failure to take all the relevant biology into account and (ii) one should examine the possibility that different subsets of amino acids are evolving at different rates, because otherwise the assumption of a clock may yield erroneous estimates of divergence times on the basis of the observed number of amino acid differences."

Points to remember:

• The validity of the molecular clock hypothesis is based on the assumption that evolution is true.
• The molecular clock runs in different organisms at a different speeds for each protein studied. Which one, if any, is correct?
• Dates based on molecular clocks do not agree with radiometric dating. Which, if any, is correct?

Molecular Clocks and Phylogenetic Analysis

I asked professors in the Department of Molecular Biology at the USC graduate school for a definitive textbook on the molecular clock hypothesis. They informed me that no book had yet been written, but they graciously referred me to two journal review articles that I have added to my growing collection on this subject.

The first paper was by Douglas Erwin. In his abstract, he wrote:

"I consider the applicability of molecular clocks and phylogenetic analysis of molecular data to the origin of metazoan phyla, and conclude that molecular information is often ambiguous or misleading. Amino acid sequences are of limited use because the redundancy of the genetic code masks patterns of descent, while in a nucleotide sequence only four potential states exist at each site (the four nucleotide bases). In each case, homoplasy may often go undetected. The application to resolve the timing of metazoan radiation is unwarranted, while molecular phylogenetic reconstruction should be approached with care." [Erwin, see the following definitions for emboldened phrases—emphases mine]

A Few Definitions

A phylogenetic analysis is an evaluation of the evolutionary or ancestral history of an organism [Parker]. To make a phylogenetic analysis, we must assume that descent from a common ancestor is true, i.e., that evolution is true. Such an assumption forces us to consider any data only in terms of evolutionary doctrine rather than in terms of what the true source of all organisms might be. It might be evolution, creation, colonization by intelligent beings from another galaxy (this might seem far-fetched, but some scientists regard this as true), or some other strange and wonderful process as yet unperceived. Our assumptions determine what kind of data we seek, how we seek it, and how we interpret it.

Biological Classification Scheme: Taxonomy

In biology, living things are classified according to a hierarchy of levels or taxa, taxon being the singular term for a category by which organisms are classified. The seven major categories or taxa in increasing order of specificity are: kingdom, phylum, class, order, family, genus, species. As an example of how taxonomy works, the table below shows how our species, Homo sapiens, is classified. The term metazoan phyla includes those phyla (plural of phylum) which are assigned to the subkingdom Metazoa (many-celled animals).

Homoplasy is the correspondence between similar structures or organs in different organisms assumed to have evolved during the course of converging or parallel evolution. Terms such as convergent and, to a lesser degree, parallel evolution, show a basic flaw in the logic of evolutionary theory. They are actually ad hoc explanations for deficiencies of the theory. For instance, in convergent evolution, two widely divergent groups of organisms, e.g., octopus and man, evolved similar eyes. The claim is that "similar selective pressures may have selected for the evolution of similar adaptations in unrelated groups [Hickman]." This is an example of the extremely flexible explanatory power of the theory. It can explain any seeming contradiction to its basic tenets, and this kind of thing often disturbed me in college because I believed evolution was true.

In the first quote, Erwin is saying that he considers the information derived from comparing the homologous DNA sequences of animals from different taxa ambiguous or misleading. Therefore, using it as a way of relating them to a common evolutionary ancestor is unwarranted. He does not like using amino acid sequences of proteins in phylogenetic analysis because amino acids can be coded for in as many as six different ways, and DNA mutations go undetected.

DNA contains the code for making proteins in the cell. The basic unit of information is a sequence of three adjacent nucleotides in DNA called a codon or a triplet that codes for a specific amino acid [Lehninger]. The redundancy of the genetic code means that more than one codon codes for the same amino acid. There are four nucleotide bases in DNA: adenine (A), cytosine (C), guanine (G), and thymine (T). The codon GGC codes for the amino acid glycine, so do GGG, GGT, and GGA. In RNA, uracil (U) substitutes for thymine. I mention this because some authors, such as Lehninger, explain redundancy (also known as degeneracy) in terms of RNA rather than DNA. Erwin gives a good example of redundancy, illustrating the codon differences between a star fish [Piaster ochraceus (Po)] and a sea urchin [Strongylocentrotus purpuratus (Sp)] by showing a segment of the codon sequences and corresponding amino acid (aa) sequences of both organisms for the H4 histone gene.

While there are seven differences in the DNA sequences, there are no differences in the protein sequences between these two organisms. He warns against using a single sequence to establish a phylogeny (ancestry), a thing which is constantly done by evolutionists. Have you heard it said that the percent difference in this or that protein between the chimpanzee (or another ape) and man is only 1 or 2%, while the percent difference in the same proteins between man and the electric eel (or some other widely disparate organism) is much greater? The interpretation is that chimp and man must be closely related by evolution. This is very misleading. Martin, Naylor and Palumbi [Martin] examined the genes for cytochrome b and cytochrome oxidase I in 13 species of shark from two orders well represented in the fossil record. They concluded that

"… it is inappropriate to use a calibration for one group to estimate divergence times of demographic parameters for another group."

They also reported that chimpanzee and human mitochondrial DNA (mtDNA) differ by 27% at silent sites (where a mutation produces a redundant codon). This is quite a difference. While Erwin and Thorp (see below) are convinced that the rate of change in organisms is clearly related to generation time and not to absolute time, Martin says that sharks and primates have similar generation times, but the differences in substitution rates between these groups is almost 10-fold. They suggest different possibilities to explain this, including DNA repair efficiency, exposure to substances that cause mutations (mutagens), and metabolic rate. They suggest that sharks might have better repair efficiency, or that we might have lost a major repair pathway. They and other investigators see a difference in the substitution rates between poikilotherms (cold-blooded) and homeotherms (warm-blooded). When substitution rates between apes and humans are compared, the assumption of a molecular clock is bewildering. On one hand, the rate of neutral mutations (redundant mutations) is lower in man than in other primates, but on the other hand, in the X chromosome, the rate is faster in man than in the apes. Go figure! [Laursen]

I have to question the remarks of Michael Denton, the renegade evolutionist who wrote Evolution: A Theory in Crisis. He devoted a large part of his book to biochemical typology, showing that all major taxonomical groups have almost the same percent divergence from one another, therefore disproving evolution, yet his opponents, such as William Thwaites see a perfect example of a phylogenetic tree in the same data. Neither idea reflects the result of the many recent studies. I tend to side with Denton, who has become a sort of strange bedfellow with creationists, concerning his typological argument that at the higher levels of biological organization, the organisms making up that group seem to be equally isolated from every other group. Both structurally and biochemically, there are no directional indicators that would justify placing one group more ancestral to the others. Denton could not bring himself to say this is what one would expect if all living things were created, but he was certain that there was no spectrum of characteristics that would identify different classes as coming from a common ancestor. According to him, there are neither structural nor biochemical transitional entities that could link groups together in terms of descent from a common ancestor but rather, they are equidistant from one another.

Based on the reports of many leading researchers in molecular biology, I could not say that they support any evolutionary explanation. They seem to deny it, although most of the investigators are deeply committed to the idea of evolution. Indeed, the purpose of their investigations is to discover molecular clocks, and the molecular clock hypothesis is based on evolution. Probably not one of them would favor any current evolutionary theory because none seems to fit. This field is topsy-turvy from an evolutionary point of view but makes sense if all life were created. Theodosius Dobzhansky once said that nothing in biology makes sense except in the light of evolution. Duane Gish retorted by saying that nothing in evolution makes sense in the light of biology.

Linus Pauling and Emile Zuckerkandl [Zuckerkandl] suggested that substitutions of amino acids within proteins via mutations have a characteristic substitution rate related to absolute time rather than generation length of the organism. Erwin reported that researchers are discovering significant deviations from the idea of rate constancy, citing research 20 years after Pauling and Zuckerkandl hatched their idea. Erwin said that application of the molecular clock to certain proteins and RNA sequences produced a date of 1000 to 900 million years ago (Ma) for the initiation of the animal phyla, also known as the "Cambrian explosion." Paleontologists put a date of 570 Ma on this dubious event. He explains that the molecular clock model can only be applied when certain assumptions are met: 1) Constancy of substitution rate for the genes under investigation within lineages, and 2) Dependence of substitution on absolute rather than generation time. Both assumptions are refuted by the discovery of highly variable substitution rates. Statistical studies have revealed deviations from rate constancy and recent studies suggest that the deviations are significant. The implication is that the clock should be rejected [Erwin, p. 252]. He concludes his paper by saying that these two fundamental assumptions are invalid and that molecular evidence does not presently allow us to discriminate between a lengthy Precambrian divergence and the rapid burst of divergence close to the Cambrian explosion when these fossils actually appeared. We’ll look more closely at the Cambrian explosion later in this program, but for now I can say that molecular studies cannot pinpoint the divergence times of the various phyla.

I wish each of you could read through some of these papers. The remarks are astounding and make me wonder why anyone would continue to propose the molecular clock. Consider Erwin’s observations as he responds to Runnegar’s calculated date for the origin of phyla:

"There are several reasons for questioning the 1000-900 million years date. First, the annelid, mollusc, and vertebrate divergence dates the actual divergence of the phyla only if molluscs, annelids and vertebrates are each others closest relatives, or if all three diverged simultaneously (perhaps along with other taxa) from a common ancestor. Otherwise, the date, if correct, may simply reflect the divergence of pre-annelid, pre-mollusc and pre-vertebrate lineages during the evolution of metazoa. In this case the date is for an earlier event in metazoan phylogeny and may not reflect the radiation of existing animal phyla. More importantly, this application of the molecular clock requires extrapolation of evolutionary rates beyond calibrations points rather than interpolation between known points (W. M. Fitch, pers. comm., 1988). Since there are no methods to test the accuracy of such extrapolation, the results of such studies are unreliable. Third, as discussed above, substitution rates appear to be highly episodic, a feature which will be masked by the sort of long-term analysis used by Runnegar. As noted, increased substitution rates are particularly common during gene duplications events. If substitution rates have varied widely, the true substitution rate is likely to have been greater than the calculated rate, and the divergence time less."[Erwin, p. 234]

In the second recommended paper, John Thorp remarks that the proposal that the changes in homologous proteins of animals diverging from a common ancestor are time dependent and can be related to evolutionary time forms the basis of the molecular clock hypothesis. He refers to many researchers who do not support the hypothesis because the accuracy, reliability, and usefulness of such clocks are controversial [Thorp].

The number of researchers who expressed doubts about the validity of the molecular clock hypothesis is impressive. Many of them tried to explain their results in terms of descent from a common ancestor (macroevolution). W.M. Fitch, a professor in the Department of Ecology & Evolutionary Biology at the University of California, Irvine, presented a paper in which he said [Fitch, 1994].

"The evolutionary tree for eukaryotic Cu/Zn superoxide dismutase (SOD) has amounts of change for the time span 1.2-0.6 billion years ago that are only twice that of the last 60 million years. This appears to imply that the evolutionary rate of change for SOD has increased fivefold in the most recent 5% of its history, a conclusion that is not likely to be believed by many biologists but that certainly suggests that the molecular clock is extraordinarily bad for SOD. I shall show that these results are based on the false assumption that all of the amino acid positions are capable of change when there is reason to believe that, in any one lineage at any one point in time, the number is only ~21 of the 159 positions."

This paper became the basis for the superoxide dismutase paper quoted above. Fitch tried to reconcile the "very unclocklike manner" in which SOD behaves by invoking a "covarion model" which considers only the 28 variable codons in SOD–a bit like stacking the deck. The invariable codons "across the range of taxa from fungi to mammals is 44." He says this fits the molecular clock. (See Ayala’s remarks on SOD below)

What is a possible creationist response to this? Most proteins, especially enzymes, have active regions that cannot be changed without disturbing their functions. When these regions are disturbed by mutation, adding, deleting, or substituting an amino acid, functionality is affected slightly to drastically depending on the substitution. Some changes are lethal, some are debilitating, and some are neutral in their effects. The invariable codons probably closely reflect the original creation sequences, and cannot be changed without producing serious deleterious effects or changes that are inimical to life and cannot be passed on.

Zuckerkandl and Pauling suggested that the enzymes responsible for the synthesis of similar polysaccharides, i.e., plant starches and animal glycogens, might be very different, and similar end products might be obtained by different biochemical pathways. Examples are the synthesis of nicotinic acid (related to vitamin B3) and that of tyrosine in bacteria versus other organisms. The existence of these molecules in bacteria and probably all other organisms in "no way points to a phylogenetic relationship between bacteria and the other organisms."[Zuckerkandl, 1965]

In part one, we saw that Easteal seemed to paint a dark picture of the molecular clock hypothesis while actually supporting it. He remarked near the end of his article that recent studies based on the relative rate approach tend to support the molecular clock, "at least as it applies to non-coding DNA in mammals." His theme was that all researchers in molecular biology have to be careful about how they interpret their data, otherwise they will end up with conflicting conclusions.

Dentist HIV Transmission Debunks Stats

I claimed that the statistical methods used to discover molecular clocks were invented to validate evolutionary assumptions and that these methods simply manipulate the data until some point of divergence from a hypothetical ancestor can be inferred (a magic word used frequently by molecular biologists). My interpretation is strongly supported by a study on the AIDS virus that a Florida dentist transmitted to some patients [Crandall]. Traditional statistical methods, based on variations of the maximum parsimony procedure are used for phylogeny reconstruction. They supposedly establish relationships among diverse organisms based on shared derived characters evolutionists believe were inherited from a common ancestor. These methods were developed for estimating higher-level systematic relationships and don’t work with HIV.

In other words, statistical methods developed to connect phyla, classes, orders and families of widely divergent organisms to ancestors predating even the Cambrian, don’t work on known microevolutionary histories. Hmmm! Maybe the assumptions are wrong.

Summary: The reliability and usefulness of the molecular clock hypothesis is controversial and does not establish the truth of evolution.

Debunking the molecular clock hypothesis will not bring the theory of evolution crashing down in flames. In 1859 Darwin established the theory of evolution based upon many lines of dubious evidence but virtually no knowledge of genetics. Gregor Mendel, the ‘father of genetics,’ had not yet published his work on garden peas (Pisum sativum, 1866). We should not expect to refute evolution by discrediting the molecular clock hypothesis. After all, Darwinists have been the strongest and most vociferous opponents of the molecular clock hypothesis although it is becoming widely accepted by them [Kimura].

It now seems that this hypothesis is held high as a banner for evolution and used as a shield against creationary critics’ fiery darts. Its proponents don’t realize that some of the most potent foes are evolutionists. It has become a cornerstone of evolutionary theory. Many who believe in evolution feel the evidence is overwhelming, citing among other things, the molecular clock hypothesis. When the deficiencies of each line of evidence are examined, one concludes that the evolutionist must exercise ‘faith’ in what he or she might have rejected based on objective science. As we have already seen, the so-called molecular clock seems to be made up of a hodgepodge of conflicting data.

Continued in Part II

References

Crandall, Keith A., 1995, "Intraspecific Phylogenetics: Support for Dental Transmission of Human Immunodeficiency Virus," J. Virology, 69(4)2351-2356.

Easteal, Simon, 1992, "A Mammalian Molecular Clock?" BioEssays, vol. 14, No. 6, p. 415.

Erwin, Douglas H., 1989, "Molecular clocks, molecular phylogenies and the origin of phyla," Lethaia, 22:251-257.

Fitch, W.M., 1994, "Molecular Clocks Are Better Than You Think," Journal of General Physiology, 102(6):A1-A1.

Fitch, W.M., Ayala, F.J., 1994, "The superoxide-dismutase molecular clock revisited," Proceedings of the National Academy of Sciences of the United States of America, vol. 91, issue 15, pp. 6802-6807.

Gielly, L., Taberlet, P. 1994, "Chlorplast DNA polymorphism at the intrageneric level and plant phylogenies," Comptes Rendus De L’Academie Des Sciences Serie III-Sciences De La Vie-Life Sciences, vol. 317, issue 7, pp. 685-692.

Hickman, Cleveland P., Larry S. Roberts, Frances M. Hickman, 1988, Integrated Principles of Zoology, Times Mirror/Mosby College Publishing, St. Louis, p. 9.

Kimura, Motoo, 1985, "The neutral theory of molecular evolution," New Scientist, 11 July, p. 42.

Laursen, H. Blegvad, A. Lund Jøgensen, Carol Jones, A. Leth Bak, 1992, "Higher rate of evolution of X chromosome a-repeat DNA in human than in the great apes," EMBO Journal, 11(7)2367-2372.

Lehninger, Albert L., 1982 Principles of Biochemistry, 5th printing, 1987, Worth Publishers, Inc., New York, p. 970.

Martin, Andrew P., Gavin J.P. Naylor, Stephen R. Palumbi, 1992, Rates of mitochondrial DNA evolution in sharks are slow compared with mammals," Nature 357:153-155.

Milinkovitch, M.C., Meyer, A., Powell, J.R., 1994, "Phylogeny of all major groups of cetaceans based on DNA-sequences from 3 mitochondrial genes," Molecular Biology and Evolution, vol. 11, issue 6, pp. 939-948.

Parker, Sybil P., 1994, Dictionary of Scientific and Technical Terms, 5th ed., S.P. Parker, editor, McGraw-Hill, New York.

Thorp, John P., 1982, The Molecular Clock Hypothesis: Biochemical Evolution, Genetic Differentiation and Systematics," Ann. Rev. Ecol. Syste., 13:139-68. (This article contains many references, which makes it more valuable.)

Zuckerkandl, Emile, and Linus Pauling, 1962, "Molecular disease, evolution and genetic heterzygosity." In Kasha, M., Pullman, B. (eds.): Horizons in Biochemistry, Academic Press, pp. 189-225.

Zuckerkandl, Emile, and Linus Pauling, 1965, "Molecules as Documents of Evolutionary History," J. Theoret. Biol. 8:357-66.