REVIEW OF “EVOLUTION: A THEORY IN CRISIS” BY MICHAEL DENTON
Keith B. Miller
Department of Geology
Kansas State University
Manhattan, KS 66506-3201
There is much confusion in the popularized literature about the evidence for macroevolutionary change in the fossil record. Unfortunately, the discussion of evolution within certain parts of the Christian community has been greatly influenced by inaccurate presentations of the fossil data and of the methods of classification. Denton’s (1985) widely read critique of evolution entitled Evolution: A Theory in Crisis , and Darwin on Trial by Johnson (1991) which draws extensively on Denton’s book, contains serious misrepresentations of the available fossil evidence for macroevolutionary transitions and of the science of evolutionary paleontology. Furthermore, Denton does not understand the nature and interrpretation of historical data.
“NATURAL” VERSUS “SUPERNATURAL” CAUSE
Denton’s writing consistently presents “natural processes” and “supernatural” cause as mutually exclusive. He similarly sees “random” and “purpose” or “meaning” as antithetical ideas. For example: on page 54 he states “…the elimination of meaning and purpose from human existence, which was the inescapable conclusion of his position…”; and on page 66 “…the fact is that no biblically derived religion can really be compromised with the fundamental assertion of Darwinian theory.” This view of natural and supernatural cause as alternatives, widely held by both Christians and non-Christians, has, I believe, done great harm to the Church and to science. It has generated the mythology of the “warfare” of science and faith, and unnecessarily placed many Christians in a defensive position with regard to scientific knowledge. A “god of the gaps” mentality has developed in which God’s action is perceived only in the absence of natural cause-and-effect descriptions. Unfortunately, Denton’s book simply reinforces this false dichotomy.
A brief study of the writings of evangelical Christian theologians and scientists reveals that the great majority see no inherent conflict between evolution, whether Darwinian or modern, and scripture. Some of Darwin’s most vocal supporters were evangelical Christians, probably the most prominent of which was Asa Gray (Livingstone, 1987). Good historical scholarship does much to counter the warfare metaphor, so common in the popular mind (Moore, 1979; Numbers, 1992).
In its most general sense, “evolution” simply means progressive change through time, and in a scientific context implies a “natural” agent of change. As long as “natural” causes are proposed to explain transitions between taxa, even saltational changes, the theory would still be a theory of evolution. However, natural cause-and-effect descriptions of events or processes cannot exclude the existence of a divine cause. A thoroughly orthodox understanding of the Bible is that all natural events are contingent on God’s will and providential activity in the physical universe. Evolution is simply our scientific way of talking about God’s creative activity.
With respect to the teleological implications of random processes, the word “random” is really only a statistical term. It has no a priori implications about direction or purpose. The flipping of a coin is analyzed statistically because the causal factors involved are so complex that they defy anaysis or description. Interestingly the Bible contains many accounts of God accomplishing or revealing His will through random or chance processes.
Denton consistently fails to define his terms, and jumps erratically between different time scales and different levels of the biologic heirarchy. This results in confused and misleading arguments. I can’t emphasize enough how important clear definitions are in this discussion. Without them statements are often devoid of meaning, and incapable of falsification.
Throughout the book, Denton fails to define “type”, “group”, “class”, “intermediate”, or “transitional form”. Without definition, how can any of his statements be judged? Without an objective statement of what is required of a “type” or “transitional form”, the discovery of any intermediate form can either be considered (in retrospect) just an addition to the variation within a “type”, or as simply generating two new gaps. I know of no publication which provides a definition of “type”, and, I believe, this is simply because no objective definition can be given. The term is therefore of no scientific value. It has the same status as the word “kind” favored by the young Earth creationists. The word “class” does have a specific scientific meaning; however it is not at all clear that Denton’s meaning is the same when he uses the word.
Denton frequently does not make clear what level of evolutionary change he is referring to. On page 87, he states “…the differences among the fruit flies of Hawaii, for example, are utterly trivial compared with the differences between a mouse and an elephant, or an octopus and a bee.” Obviously a mouse did not evolve into an elephant, or an octopus into a bee!! But if not, he has made no comparison! This statement might have a lot of literary punch, but it is totally devoid of scientific meaning.
A major overall problem with the book is that Denton often fails to make the object of his criticism clear. Is he directing his arguments against Darwinism, the Modern Synthesis, or evolution in its most general sense? The answer to this is uncertain, as Denton repeatedly jumps from criticisms of one to another without telling the reader what he is doing. He continually equates “evolution” with a very gradualistic view. Statements or evidence which may cast doubt on that view of evolution, with its very slow and constant rate of change, are seen by Denton as evidence against “evolution” (see his use of a quote by Simpson on P.165). Virtually no one today holds the hypergradualistic views of Darwin or Lyell. Evolutionary theory has matured greatly since then, as any fresh new scientific theory must. Recognizing that Darwin’s very incomplete theory was probably wrong in several aspects is hardly a radical new idea! I don’t see people pointing to the limits and failures of Newtonian physics as the basis for calling modern quantum theory or astronomy into question.
Denton must grapple with the multitude and complexity of modern evolutionary theories and scenarios. Beating on a paper Darwinian tiger accomplishes nothing. Denton often generalizes his criticisms of Darwinism to “evolution” thus misrepresenting the present status of evolutionary theory. At the bottom of page 74, for example, he talks as though “Darwinian theory” is the preeminent theory of evolution today, and nothing has changed in 100 years. Some of Darwin’s ideas have been discarded, and the others were swallowed up as only a part of the much more encompassing “Modern Synthesis” which developed in the1960’s. In addition, there has been much recent debate on the relative importance of natural selection and “non-Darwinian” evolutionary mechanisms in the history of life. Both the mode and rate of evolution at various hierarchical levels are highly contested issues. Chapter 3 reads as though debate has been silenced within the scientific community. Denton is either ignorant of, or fails to communicate the dynamic nature of evolutionary theory today. “Evolution” simply cannot be attacked by simplistic generalizations as though it were some single monolithic entity.
THE STATUS OF SPECIATION
Denton’s statements on species transitions are very interesting given his critique of evolutionary theory. On page 81, he states “… the fact of speciation in nature can hardly be doubted.” Denton states that speciation is a “fact”, when its mechanism is a matter of great speculation! The same type of evidence from the fossil record, that of spatial and temporal patterns of morphological variation, is used to support the “general theory” of evolution at a much greater temporal scale. Thus, evolution across higher taxonomic levels is accepted while the precise mechanisms remain a matter of ongoing debate. On page 84, Denton further states, “…modern evolutionary biology has also been able to provide a thoroughly worked out model showing, step by step, precisely how species formation occurs in nature.” I have rarely read such catagorical statements about evolution in the evolutionary literature! This statement is highly misleading. If Denton means a widely applicable and detailed model for speciation, biology has done nothing of the sort. The view he presents on pages 84-85 is highly debated and contested and in no way qualifies as a universally accepted model of speciation. The theoretical problems of speciation are significant, particularly the mechanism for generating reproductive isolation.
Another incongruity is found on page 85 where Denton accepts the possibility of macromutational quantum speciation! In this Denton gives mutations more credit than most evolutionary theorists. How can he do that, and later argue in Chapter 13 that the complexity and integration of the genome prohibits major mutational change? Denton appears unaware that one implication of common descent is that the origin of all higher taxonomic groups must have ultimately involved speciation events. This is recognized by all evolutionary paleobiologists, even those who stress the significance of the origin of phyla and classes (Valentine, 1992). It is speciation, which Denton appears to accept, that provides the basis for macroevolutionary change and the diversification of life.
A final note on this general topic. At several places in the text (pages 91, 103) Denton makes the claim that there is “a distinct barrier beyond which further change is impossible” in breeding experiments. However, he fails to present anywhere a single example or reference to substantiate this absolute statement! I know of no such evidence, and would challenge him to provide it.
THE USE OF LANGUAGE AS A MODEL FOR LIVING SYSTEMS
In Chapters 4 and 13, Denton uses language as a model for “living systems”. He seems to imply he is using sentences as analogues of DNA sequences (particularly in Ch. 13) though this is not made explicit. What do the letters, words and sentences correspond to in the genetic code? In fact, the absence of any detailed comparison reduces his arguments to some general appeal to common sense. But nature often defies common sense and continually provides totally unexpected surprises!
The analogy Denton makes with language and information storage and mutation in DNA is very superficial. His discussion of word substitution in sentences on pages 88-90 fails to make a persuasive argument. In organisms, the effects of single mutations could range from zero to the alteration of entire developmental patterns. The genome is hierarchical and possesses multiple feedback systems and regulatory genes. It is also redundant and possesses a significant portion of nonsense code which is cut out during the transcription process. The genetic code is based on triplets of nucleotide base pairs, the alteration of which may or may not alter their meaning. Denton never discusses how these features might be modelled by English sentences.
A feature of language not discussed is that considerable error (substituted letters, words, omitted letters, transposed words, etc.) can be present in a sentence and the meaning still be deciphered. Also a particular meaning can be communicated by a vast number of possible wordings. Incorporation of these aspects of language and the complexities of the activation and translation of DNA are required to make any meaningful comparison between language and the genome.
The type of statistical demonstration demanded by Denton at the bottom of page 61 is impossible. He states, “The only way Darwin could have countered these doubts would have been by the provision of rigorous quantitative evidence in the form of probability estimates to show that the routes to such seemingly remarkable ends could have been found by chance in the time available.” Even if such a test could be done, it would show nothing. It is equivalent to determining the probability that I would be typing this sentence, by going back to my graduation from college and determining every possible alternative to every minute decision I could have made in my life for the past 20 years. Any change and the present end would not have occurred. The probability would therefore approach zero – but here I am! The obvious problem is that I do not know, nor can I ever know, what possible alternatives I may have had or what different routes my life may have taken if I did not make the decisions I did. This kind of retrospective statistics simply cannot be done!
“Such a [typological] model of biological classes completely excluded any sort of significant sequential order to the pattern of nature…..Typology implied that intermediates were impossible, that there were complete discontinuities between each type” (p. 96). According to Denton’s own writing, typology excludes transitions by definition! Denton appeals to this typological model as a basis for arguing that it represents an objectively accurate picture of biological reality. He fails to see that typology imposes its own order on the natural variation of the biological world and is not objectively descriptive of it. To use an illustration given by Denton (p. 96), triangles and quadrilaterals have clear typological definitions and are easily separated into two classes of geometric shapes. Now, if one side of a quadrilateral were reduced in length by infinitesimal amounts until it was only two geometric points wide, it would still be a quadrilateral by definition although absolutely indistinguishable from a triangle. The most conceivably gradual transition has been made, yet typologically there were no intermediates! For Denton to apply this typology to living organisms and the fossil record, and then claim the absence of intermediates is without meaning. Such arguments hinge on our human constructions and categories, not on the reality of variation in the biological world.
By Denton’s own admission (P. 98-99), the typology concept is by definition opposed to evolutionary thinking. That the dominant scientific and philosophical paradigm of the last century was typological (which he stresses on page 100) is irrelevant to the validity of the present paradigm which must stand or fall on the basis of the data presently available. Denton’s line of argument is like arguing for a return to a geocentric theory of the Earth because the great thinkers of the day held it. He seems to imply throughout this discussion that no significant data has come to light since the mid 1800’s!
Denton states that the typological model “holds almost universally” at levels above the species (p.105). However, it does at the species level as well, since they also have defining characteristics! It was at the species level that Cuvier and the other anatomists defended their typologic classification. But if Denton tried to argue it at this level his argument would loose its force. Even so, he never specifies at which higher taxonomic level typological discontinuity is “almost universal”. He accepts the likelihood of the horse series at the genus level even though it still conforms to typology (p.182), and examples can be given of likely transitions at the family, order, class, and even possibly phylum level. Since typology holds universally by definition, the only basis for accepting some transitions and not others is by appeal to some criterion outside of typology which Denton never specifies.
WHAT IS TRANSITIONAL?
On page 109, and in subsequent discussion in this chapter and in chapter 8, Denton rejects character mosaics as intermediate. He won’t accept species with a mixture of traits characteristic of different higher taxa as intermediate, even though he just spent several pages stating that taxonomic groups were characterized by features completely absent in species outside that group (see bottom of page 105)! A mosaic is precisely the only kind of intermediate which the typologic view will allow. Remember that typology defines taxonomic characters in such a way as to emphasize discontinuity. Also many taxonomic characters are incremental, or present/absent by nature. For example: one of the major defining characters of mammals is a jaw joint with the dentary bone of the jaw articulating the squamosal bone of the brain case. Despite how close they may be to touching, the articulation is either present or it is not.
Most paleontologists predict just the type of intermediates Denton describes – those with character mosaics! Having a mosaic of characters does not necessarily imply that a species lay on the direct line to the next higher taxa, but to a priori deny its intermediate character is absurd. What is an intermediate if not something which possesses a morphology intermediate between two other forms?
THE VALUE OF THE FOSSIL RECORD
Denton focuses on living organisms, and their internal anatomy and cellular organization, when discussing the issue of transitional forms in chapter 5. However, a focus on the present will automatically result in the diversity of life appearing more discontinuous. Once a lineage is split, its branches continue to evolve and diverge such that their morphological (and genetic) distance increases and they become more readily distinguished taxonomic entities. When the time dimension is added (ie. the fossil record), it is found that representatives of different higher level taxa become more “primitive”, that is have fewer derived characters, and appear more like the primitive members of other closely related taxa. As a result, species which lived nearer the presumed branching points become increasingly difficult to place in a higher taxon. Similarly, for lineages with a good fossil record, the appearance of a new higher taxon is associated with the occurrence of species whose taxonomic identity is uncertain or converges closely on that of the new higher taxon. Such patterns are found repeatedly by paleontologists. For example, some species occurring near the appearance of unquestioned reptiles have been at times placed within both the amphibian and reptile classes, and certain reptile species near the appearance of unquestioned mammals possess nearly all the characteristics of mammals (Benton, 1991; Desui, 1991; Hopson, 1994). (Note that the very uncertainty in the placement of a species in a higher taxonomic group suggests that “types” are not so clear-cut as Denton claims.) The most primitive ungulates (Condylarths) and carnivores (Creodonts, Miacids) are very similar to each other in morphology, and some taxa have had their assignments to these orders changed. The Miacids in turn are very similar to the earliest representatives of the Family Canidae (dogs) and Mustelidae (weasels), both of Superfamily Arctoidea, and the Family Viverridae (civets) of the Superfamily Aeluroidea. As Romer (1966) states in Vertebrate Paleontology (p.232), “Were we living at the beginning of the Oligocene, we should probably consider all these small carnivores as members of a single family.” This statement also illustrates the point that the erection of a higher taxon is done in retrospect, after sufficient divergence has occurred to give particular traits significance.
Though the fossil record is obviously of critical importance when discussing transitional forms, Denton effectively eliminates it from consideration. Though not specifically directed toward the fossil record, he states on page 117, “Even if a number of species were known to biology which were indeed perfectly intermediate, possessing organ systems that were unarguably transitional in the sense required by evolution, this would certainly not be sufficient to validate the evolutionary model of nature. To refute typology and securely validate evolutionary claims would necessitate hundreds or even thousands of different species, all unambiguously intermediate in terms of their overall biology and in the physiology and anatomy of all their organ systems.” Firstly, this would not be the expected evolutionary pattern among living organisms for the reasons given at the beginning of this section. Secondly, what Denton is demanding the fossil record cannot possibly supply. His position is completely unfalsifiable!! That this is so is made clearer by his statements on pages 176-177. After describing clear intermediate forms from the fossil record he states, “.. the point at issue here is that skeletal characteristics alone are insufficient for designating a particular organism or species as intermediate.” And “…it is not sufficient to find in the fossil record one or two types of organisms of doubtful affinity which might be placed on skeletal grounds in a relatively intermediate position between other groups.” Since the only evidence available from the fossil record is skeletal, except in rare circumstances, no fossil can be designated an intermediate form by his definition regardless of its intermediate skeletal character!! Though much about internal anatomy and physiology can be inferred from skeletal evidence, it is circumstantial, and Denton could find cause to ignore it. Denton has now eliminated intermediate forms by definition in three separate ways – by typology, by excluding character mosaics, and by eliminating the fossil record.
Denton also dismisses the significance of intermediate forms (such as mammal-like reptiles) in the fossil record by suggesting they may represent “merely convergence” (see page 181). This is in error in several ways. First, the evolution toward mammalian characters in reptiles preceeded in time the appearance of true mammals! Denton has entirely ignored the temporal aspect of the identification of transitional forms. As with most transitions between higher taxonomic categories, there is more than one lineage that possesses intermediate morphologies. Again, this is consistent with both the expectations of evolutionary theory, and the nature of the fossil record. The prediction would be for a bush of many lineages, many of which would be dead ends.
FOSSIL EXAMPLES CITED BY DENTON
Denton demonstrates a lack of current knowledge of the fossil record, which is not unexpected considering his area of specialty, and his low view of the value of fossil evidence. His statement that the mammalian characteristics listed in Figure 5.1 and on pages 105-106 are “…not found even in a rudimentary form in any species ouside that class” is demonstrably incorrect. Exceptional fossils of a mammal-like reptile show glandular skin associated with the presence of hair (Hotton, 1991), and a pterosaur shows hair impressions! The absence of lumbar ribs in some advanced mammal-like reptiles (therapsids) is characteric of mammals and related to the presence of a diaphragm. The presence or absence of the rest of the characters Denton lists cannot be directly determined from the fossil record. However, some workers argue persuasively that therapsids were endothermic and may have secreted milk in a manner similar to living monotremes (deRicqlés, 1974; Bakker, R.T., 1975; McNab, 1978).
In skeletal features the approach to the mammalian condition was almost complete (Hopson, 1994). The following mammalian characteristics were possessed by therapsids: 1) enlarged temporal openings with the loss of the post-orbital bar, 2) absence of the pineal eye, 3) differentiation of teeth, with front nipping teeth, canines, and molar-like back teeth, 4) a secondary palate permitting respiration while chewing, 5) a double occipital condyle which enlarges the hole for the spinal cord, 6) absence of lumbar ribs, 7) a nearly erect stance, and 8) an enlarged dentary bone in the lower jaw with an extremely close approach to the mammalian jaw articulation. The latter character is particularly illustrative. In reptiles, the lower jaw contains several bones, and the articular bone at the back of the jaw articulates with the quadrate bone of the skull. In mammals, the lower jaw has only one bone, the dentary, and it articulates with the squamosal bone of the skull. Within the therapsid lineage, the dentary bone becomes progressively larger and the other bones, including the articular, are reduced to nubs at the back. In one group of advanced mammal-like reptiles, the dentary bone has been brought nearly into contact with the squamosal, and in another, a secondary articulation exists between the surangular (another small bone at the back of the jaw) and squamosal (Hopson, 1991). A better intermediate character could not be imagined! The earliest known mammals, the morganucodonts, still retain the vestigial lower jaw bones of the reptiles. These reptilian jaw elements were subsequently detached completely from the jaw to become the mammalian middle ear (Crompton& Parker, 1978).
I am amazed at the ease with which Denton dismisses the fossil series from “Eohippus” (Hyracotherium) to Equus as trivial (p.57 and 184-186). He sees it as trivial only because the intermediate forms are known. Without them, the morphologic distance would appear great. “Eohippus” was a very small (some species only 18 inches long) and generalized herbivore (probably a browser). In addition to the well known difference in toe number (4 toes at front, 3 at back), “Eohippus” had a narrow elongate skull with a relatively small brain and eyes forward in the skull. It possessed small canine teeth, premolars, and low-crowned simple molars. Through the fossil series the skull becomes much deeper, the eyes move back, and the brain becomes larger. The incisors are widened, premolars are altered to molars, and the molars become very high crowned with a highly complex folding of the enamel (Evander, 1989; McFadden, 1988).
The significance of the horse series becomes clearer when it is compared with similar series of the other members of the Order Perissodactyla. The fossil series of the titanotheres is also very good, and the earliest representatives of this group are very similar to “Eohippus” (Stanley, 1974; Mader, 1989). Likewise, the earliest members of the tapirs and rhinos were very “Eohippus”-like. Thus, the different perissodactyl groups can be traced back to a group of very similar small generalized ungulates (Radinsky, 1979; Prothero, et al., 1989; Prothero & Schoch, 1989). But this is not all; the most primitive ungulates are the condylarths, which are an assemblage of forms transitional in character between the insectivores and true ungulates. Some of the genera and families of the condylarths have been previously assigned to the Insectivora, Carnivora, and Primates (Romer, 1966). The condylarths in turn are difficult to distinguish from the primitive groups of carnivores, the creodonts and miacids. As can be seen, the farther you go back in the fossil record, the more difficult it is to place species in their “correct” higher taxonomic group. The boundaries of taxa become blurred.
Denton’s discussion of Archeopteryx is also not fair to the fossil data. His diagram on page 168 shows a triassic thecodont as the accepted ancestor to birds. However, all but a few workers now consider the theropod dinosaurs as the most likely ancestors. On pages 175-176 only three reptilian features of Archeopteryx are listed: teeth, a long tail, and claws on its wings. Ostrum has described over 20 shared characteristics between Archeopteryx and coelurosaur theropods. Among these are: a theropod-like pelvis, the close similarities of the bones of the forelimbs including a swivel wrist joint, and the similarity of the hind limbs and feet with the presence of a reversed first toe (Hecht, et al., 1985; Dodson, 1985; Ostrom, 1994). The similarities to ceolurosaurs is great enough that a complete specimen of Archeopteryx, but without feather impressions, was mislabelled as Compsognathus, a coelurosaur theropod, for over twenty years in a museum! The similarities of Archeopteryx to theropod dinosaurs such as Velociraptor and Deinonychus are especially strong, and a newly discovered dinosaur called Unenlagia has features of the limbs and pelvis that are the most bird-like yet known (Novas & Puerta, 1997). As interesting as the similarities with the theropods are, the differences between Archeopteryx and all other birds are also significant: its long tail is unique, a sternum is absent, its vertebrae are not fused together over the pelvis to form a synsacrum, and air ducts are absent in its long bones. In most respects, Archeopteryx is more of a flying feathered dinosaur than a bird. In the last several years the discovery of new fossil birds from the Cretaceous has led to the erection of a whole new subclass of primitive birds called the enantiornithes. This new group includes several fossil species previously identified as theropod dinosaurs (eg. Ornithomimus)! There are also some newly discovered fossils whose classification as theropod or bird is in dispute (Chiappe, 1995). The recent discovery in China of a theropod dinosaur with the possible preservation of fine feathers, even suggests that feathers may not be exlusively characteristic of birds (Morell, 1997). This again illustrates the taxonomic uncertainties that surround transitional forms.
When arguing for the distinctiveness of the invertebrate classes on page 162, Denton chooses the molluscs as an illustration. Ironically, the molluscs provide one of the best examples of likely transitions between classes of an invertebrate phylum. Recent discoveries have provided possible links between nearly all the classes of the Mollusca by Early and Middle Cambrian fossil forms. The Cephalopoda, Bivalvia, Gastropoda, and Scaphopoda appear to be linked by peculiar fossil forms belonging to the Monoplacophora and the Rostrochoncha (Pojeta et al., 1987; Peel, 1991). The monoplacophoran to gastropod transition is especially well documented. In addition, the polyplacophoran and aplacophoran molluscs may be linked to a group of unusual scale-bearing organisms called the Machaeridia, appearing in the Early Cambrian (Bergström, 1989; Bengston, 1992; Dzik, 1993).
THE COMPLETENESS OF THE FOSSIL RECORD
There are two opposite errors which need to be countered about the fossil record: 1) that it is so incomplete to be of no value in interpreting patterns and trends in the history of life, and 2) that it is so good that we should expect a relatively complete record of the details of evolutionary transitions within most lineages. Denton makes the second of these errors. The effects of insufficient search and the incompleteness of the fossil record can be easily illustrated. Consider that Archaeopteryx is represented by only seven known specimens, of which only two are essentially complete, and these are likely from more than one species! Considering how many individuals of this genus probably lived and died over the thousands or millions of years of its existence, these few known specimens give some feeling for how few individuals are actually preserved as fossils and subsequently discovered. Yet this example actually represents an unusual wealth of material. Many, many vertebrate fossil species are represented by single specimens, and these are usually only very fragmentary! Complete skeletons are exceptionally rare. For many fossil taxa, particularly small mammals, the only fossils are teeth and jaw fragments. If so many fossil vertebrate species are represented by single or fragmentary specimens, the number of completely unknown species must be enormous!
The situation for soft-bodied animals is even worse. The inadequacy of the fossil record to preserve with any completeness the evolutionary history of soft-bodied organisms can be illustrated by the Conodonta. Conodonts are a very important group of marine fossils for paleontologists. This group is represented by tiny tooth-like phosphatic fossils which are very abundant in sedimentary rocks recording about 300 million years of Earth history, and have a worldwide distribution. Yet, until only very recently the organism to which they belonged was completely unknown. Specimens of the worm-like conodont animal have now been discovered in Carboniferous, Ordovician and Silurian rocks (Briggs et al., 1983; Mikulic et al., 1985; Aldridge & Purnell, 1996). Only a handfull of specimens are now known of a very large and diverse group of marine animals known to be extremely abundant and widespread over a tremendous length of time! Again this is not atypical. The discovery of new soft-bodied fossil localities is always met with great enthusiasm. As Denton correctly states, these localities turn up entirely unknown body plans, and new higher taxa are erected on the basis of a few specimens! Such localities are also erratically and widely spaced in geologic time between which essentially no soft-bodied fossil record exists at all. Only a small fraction of living soft-bodied metazoa have any fossil record at all!
The other issue raised by Denton on page 186 is insufficient search. A person need only visit a systematically excavated fossil site to appreciate how little of the stratigraphic record has been sampled with anything approaching thoroughness. For example, Walcott’s famous Burgess Shale quarry covered perhaps the area of a football field. New excavations just a few hundred feet higher in the section have revealed another assemblage of peculiar undescribed animals. The Burgess Shale is hundreds of feet thick and stretches across an entire range of mountains. Obviously even this presumably well sampled interval may well have many more surprises hidden in its rocks. Several years ago, I assisted for a summer at an even smaller excavation in the upper Pennsylvanian of Kansas. Nearly every plant and animal species collected was unique to this one small sampled locality! The flurry of new significant fossil discoveries is testimony to how much remains to be discovered.
Denton argues on page 189 (and in Figure 8.5) that the high percentage of living familes of terrestrial vertebrates found as fossils shows the relative completeness of the fossil record. However this data does not give any indication of the number of genera and species preserved within each family. Even several fossil representatives of each living family will not be able to generate the evolutionary sequence of species and genera which Denton demands. It will result in exactly the pattern we see: a few well preserved series, but for the most part only an outline of the general direction of evolutionary change. Also, by restricting the anaysis to only living families, his results are biased by the “pull of the recent.” Because the completeness of the fossil record increases toward the recent (if for no other reason than increased outcrop exposure), the fossil record of more recent taxa will be much better than for ancient ones. Finally, comparing specific faunas (see bottom of page189) is not indicative of the fossil record as a whole. Often for preservational reasons, availability of outcrop exposure, or collector bias, the fossil faunas of certain time periods at specific localities can be surprisingly well known. However, such well sampled, well preserved faunas are separated by long periods for which little or nothing is known of animals inhabiting those environments.
In pages 187-188 he specifically addresses reasons for present gaps in our knowledge of the Precambrian metazoan radiations, which he refers to unjustifiably as “rationalizations”. The common ancestors of the phyla will most likely be found among soft-bodied animals. From the discussion above, it is apparent that our knowledge of the soft-bodied organisms which had lived in the geologic past is trivial. Furthermore, most living worm phyla are unknown from the fossil record until well into the Phanerozoic (Benton, 1993). We thus probably know only a tiny fraction of the major body plans of the Late Precambrian and Cambrian, let alone their inter-relationships.
Despite the poor record of soft-bodied forms, much has been learned of the metazoan life in the Precambrian. Metazoan-produced burrows are known in late Precambrian rocks extending back to 850 my ago or more, and show a pattern of increasing complexity and diversity with time (Crimes, 1992a, 1992b). These burrows thus indicate an increase in both the diversity of burrowing metazoans and in their behavioral complexity during the late Precambrian. The oldest body fossils of metazoans have been identified in rocks from China dated to about 740 my, where fossils of annulated worms have been described in association with shallow burrows (Chen, 1988). Simple disks of possible metazoan origin occur in rocks in northwest Canada with ages around 640-620 my (Narbonne et al., 1994). The well-known Late Precambrian Ediacaran fossils are dominated by solitary and colonial coelenterates that may have included all four living cnidarian classes (Conway Morris, 1993). Worms may be represented by the calcareous-walled tubes of Cloudina and by the impressions of multi-segmented forms such as Dickinsonia (Jenkins, 1992). Another living phylum, that of the echinoderms, may also be present in the Ediacaran. A recently described fossil, Arkarua, bears the characteristic five-fold symmetry of this phylum and is very similar to a Cambrian group of echinoderms called edrioasteroids (Gehling, 1987).
Our knowledge of the life of the early Cambrian has increased dramatically in recent years. One group of organisms has drawn particular attention. Important new discoveries of exceptionally well-preserved fossils of soft-bodies organisms in China (Ramsköld & Xianguang, 1991; Chen & Erdtmann, 1991; Ramsköld, 1992a, 1992b), and the redescription of previously known specimens has resulted in the recognition of a diverse and widespread group of organisms called lobopods. These caterpillar-like organisms walked on fleshy legs and bore plate-like or spine-like mineralized structures on their dorsal sides. Although these small plates and spines were previously recognized as part of the Early Cambrian “small shelly fauna,” their biological affinities were unknown until these recent discoveries. Hallucigenia , one of the most enigmatic and bizarre fossils of the Burgess Shale is now recognized as a member of this group. The Cambrian lobopods occupy a transitional morphological position between several living phyla. The oldest known lobopod from the Early Cambrian is Xenusion. This organism bears similarities to both palaeoscolecid worms (Fig. 5) and to living onychophorans and tardigrads (Dzik & Krumbiegel, 1989). Furthermore, lobopods also have morphological features in common with the arthropods, particularly with forms such as Opabinia and Anomalocaris (Dzik & Krumbiegel, 1989; Dzik, 1993). The recent discovery of a Cambrian gill-bearing lobopod reinforces this comparison (Budd, 1993).
Another very important group of Early Cambrian fossils is represented by a wide variety of tiny cap-shaped and scale-like skeletal elements. It is now known that many of these belonged to slug-like animals that bore these hollow mineralized structures like a dermal armor. Two well-known, and well-preserved, examples of this group of organisms are Wiwaxia and Halkieria. Referred to as the Machaeridia, these organisms are mosaics of phylum-level characteristics, and their taxonomic affinity is a matter of present debate. A strong case can be made for the assignment of at least some of the machaeridians to the Mollusca (Bengston, 1992; Dzik, 1993). However, a relationship to the polychaete annelid worms is also strongly suggested by some workers, as in the case of Wiwaxia (Butterfield, 1990). The taxonomic confusion associated with these scale-bearing slug-like animals, and with the lobopods, is consistent with their stratigraphic position at the base of the Cambrian metazoan radiation.
The base of the Cambrian was originally defined as the first appearance of hard parts. As a result, the base of the Cambrian has moved down from its original position at the first appearance of trilobites. A new interval called the Tommotian lies at the base of the Cambrian, before the appearance of trilobites and the typical Cambrian taxa. This interval is characterized by the “small shelly fauna” of isolated spicules, tubes, and tiny conical and scale-like fossils. Among these are tiny monoplacophorans, armored lobopods, and the machaeridia mentioned above as transitional forms.
I believe that Denton has totally misconstrued the objectives and methods of cladism in his discussion of Chapter 6. Cladistics claims that it is impossible to know if a given taxa is directly ancestral to another or not, and therefore takes the opposite approach that ancestors be represented by branching points on a diagram rather than by actually known taxa. This is not to say they are denying evolutionary relationships, which Denton implies. On the contrary, they are attempting to construct an absolutely rigorous evolutionary classification which more closely reflects true evolutionary relationships! Also contrary to Denton’s understanding, their classification is not based on morphologic similarity but on presumed evolutionary relationship. Species are placed within higher taxa not based on their similarity, but on the presumed recency of their common ancestor! The cladists criticize the present system for being pre-Darwinian and based on similarity, not ancestry. The objective is to eliminate polyphyletic taxa (ie. those groups that contain species descending from more that one ancestor). The result is a total overhaul of the present classification system. For example, the group of fishes from which terrestral tetrapods evolved would be placed in the same higher taxon as a cow, while the rest of the fish would be in a separate taxon of equal rank!
Cladism is based on certain evolutionary assumptions and rules for the construction of cladograms. It considers all speciation events to have been by the splitting of lineages, and ignores completely the existence of subsequent phyletic divergence. Secondly, it supposes all evolutionary branches are dichotomous and that the parent species does not persist after the branching, but either becomes extinct or is transformed into one of the new species. Because of the limits of the fossil record, some cladists also ignore the temporal relationships of fossil taxa and do not use fossil taxa to establish the branching patterns of their cladograms. However, more and more the critical importance of the fossil record for establishing the branching points of the tree of life is being recognized by cladists.
At the bottom of page 108, Denton cites the uniformity of the genetic code as evidence of typology. What it really is evidence for is common descent! The genetic code would have been established at the very inception of life. The origin of the code is a question related to the origin of life, not its subsequent evolution. It is not unreasonable to presume that several different genetic codes may have arisen independently, and that the less efficient ones disappeared. The presence of more than one code today would provide unquestionable evidence of the polyphyletic origin of life. But such is not the case.
Denton’s conclusions about his Figure12.1 and 12.2 make no sense to me. The table of 12.1 is perfectly consistent with presumed evolutionary relationships and with taxonomy. On page 276, he states that “closely related species had closely related sequences.” This is illustrated by the data of figure 12.1. Is not this precisely what would be expected from evolution? The data of 12.2 says nothing about the relationship of the base pair sequences or the amino acid sequences between the organisms listed. To determine evolutionary sequences you must know which amino acid substitutions were made in which order. Percent divergence alone is useless to construct a possible sequence of transitions. This situation results because there are only 20 amino acids coded by triplets of nucleotide bases (codons). As a result, the substitution of a given amino acid at a given site can be generated by a great variety of mutation pathways.
Denton’s interpretations of the comparisons of percent divergence in figure 12.2, and in all the subsequent diagrams of this chapter, are based on incorrect assumptions. For example: the data of 12.2 compares bacteria with eucaryotes and shows very similar percent divergences between bacteria and all the organisms listed. However, the amount of genetic change would in general be proportional to the length of time since the divergence of the various lineages indicated. The time of divergence of bacteria from all the different eucaryotes would be the same! The result shown is perfectly consistent with presumed evolutionary relationships. This is the case for all of Denton’s comparisons. By always picking the most primitive organism to compare with all the others within a group, the resulting percent divergence will always give approximately the same values! If Denton would have chosen the most “advanced” organism as the basis of comparison, the pattern would be entirely different, and would reflect the evolutionary relationships (as seen in Figure 12.1).
Denton repeatedly states that no classes are intermediate between others in percent divergence of cytochrome C. Mammals and birds are closer than mammals and fish; isn’t this intermediate? If Denton does not accept this, I have no idea what he means by intermediate in this context. On page 291, Denton states “If the sequence of vertebrate proteins could have been arranged like the letter strings below…then this would have provided powerful confirmation of the traditional sequential interpretation of the vertebrate classes.” But Denton has not once discussed sequences like those he illustrates on this page!! Such sequences do exist, and determining them is the basis of the whole field of molecular genetics! Denton’s statement is completely unjustified.
At the bottom of page 291, Denton states “The only way to save evolution in the face of these discoveries is to make the ad hoc assumption that the degree of biochemical isolation of the major groups was far less in the past.” But this is precisely the prediction of evolutionary theory! Just as lineages become more and more generalized going back in time, and the distinction between different higher taxa become blurred such that classification becomes uncertain, so their genomes would be expected to become less and less distinct.
Beginning on page 294 and in his subsequent discussion, Denton presents the assumption that mutation rates (and therefore amino-acid substitutions) are constant as a last ditch effort to salvage evolution. The assumption of constant mutation rate is why the term “molecular clock” was coined. It is not ad hoc but based on the universal assumption that mutations are random events, and subject to statistical modelling and description. Denton himself makes the assumption of randomness in his arguments. The random nature of mutation was assumed before the advent of molecular genetics. The statistical consequence of a random process is a constant rate! It is exactly analogous to the decay rates of radioactive isotopes which are also random. The molecular clock concept is not a tautology as Denton claims. It is subject to test, and ironically, the data Denton provides is the best evidence I have seen for constant rates of mutation! Also implied by a constant mutation rate is that there is a large amount of genetic material which is redundant, unnecessary, or does not have to be highly specific (i.e. is flexible). It is already known that this is, at least to some degree, the case. A significant amount of genetic change can occur, therefore, without important phenotypic effects. The genome of a given taxon may also become more highly tuned with time so that its biochemistry becomes more efficient and also less flexible. This fine tuning could proceed with little morphologic change.
Denton finds it highly unlikely that different proteins have different mutation rates (see page 296). But this also is analogous to the different decay rates of different radioactive isotopes. Different proteins or genes apparently have different susceptibilities to mutation. Recent genetic work has suggested that genes may actually exert control over their degree of mutability. Such genes may themselves be under the control of selection. This idea, I believe, has been discussed by Gould in connection with his concept of species selection.
Mutation rates are not presumed constant per generation as Denton states on page 297, but per unit time. Selection is indeed dependent on generation length, but mutation is not. The accumulating point mutations are largely not subject to selection, but are neutral. In a species with a short generation length, fewer amino acid substitutions will occur per generation, relative to a species with a longer generation length. I was surprised to find Denton stating on page 304 that uniform mutation rates in non-functional sequences are the most difficult to explain. It seems to me the exact opposite is the case. It is within such segments of DNA that mutations can accumulate totally independent of selection. The example Denton gives is actually a strong argument in favor of a uniform decay rate model!
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