This is the third of a series of posts reviewing Michael Denton’s new book Evolution: Still a Theory in Crisis.
This is a good time in our discussion to note that the title of Denton’s book has resulted in considerable unnecessary confusion, because far from believing that “evolution” as such is a theory in crisis, as we have seen, Denton is a firm believer in evolution defined as descent with modification. Denton believes that the specific evolutionary theory of Neo-Darwinism is in crisis, and he wanted to title his book “Neo-Darwinism: A Theory in Crisis,” but his publisher prevailed upon him to use the more widely used, but far less accurate, term.
In Chapter 3 Denton notes that everyone agrees that all living things (whether currently existing or extinct) can be grouped within taxa that form nested hierarchies. Prior to Darwin most biologists believed that these taxa were immutable natural “types.” These pre-Darwinian typologists have been roundly criticized for allowing their metaphysics to color their views. After Darwin everyone knows organisms are infinitely mutable, and only a credulous rube held in thrall to superstition would believe otherwise. Denton begs to differ. The existence of fixed types is not a metaphysical position; it is an empirical position based on taking the data at face value:
Today, 150 years after Darwin, [Richard] Owen’s “biological atoms” are still as distinct as ever. The vast majority of all organisms can be assigned to distinct and unique classes based on their possession of particular defining homologs or novelties which are not led up to via Darwin’s “innumerable transitional forms.”
Homology is the relationship between biological structures or sequences that are derived from an assumed common ancestor. A homologous trait is often called a homolog. Taxa are defined by “suites of homologs.” For example, the human body exhibits homologs that group it in (in descending taxonomic order): tetrapod (all terrestrial vertebrates); amniotic membrane (all higher vertebrates); diaphragm (all mammals), etc.
Denton discusses several taxa-defining homologs, including the pentadactyl limb that is shared by all extant terrestrial vertebrates; the feather shared by all modern birds; the insect body plan shared by all insects; and the flower, which is shared by all higher angiosperms (eudicots). Denton notes that these “types,” if you will, exist and there is simply no long series of adaptive transitional forms between them and their taxonomic neighbors.
Instead, the record is largely one of DISCONTINUITY between the taxonomic types, and that record is exactly the opposite of what Darwinism predicts.
Denton comes back to this theme again and again. If Darwinism were the whole story, suites of taxa-defining homologs not led up to by a long series of transitional forms should not exist. Yet this is seen over and over. Denton emphasizes that this is not a “creationist” view. He quotes Rupert Riedl, a convinced evolutionist, a world authority on marine invertebrates, and one of the foremost biological theorists in the last quarter of the twentieth century:
Although such fixation [the invariance of the taxa-defining traits] may not be self-evident to some of my colleagues, I must emphasize that in accepting the evolutionary history of taxonomic groups the fixation of homologues is a logical necessity. Thus for example, the chorda remains a chorda in all chordates from ascidians to man; the backbone remains a backbone in all vertebrates, from frog to python; and a particular digit remains the same digit in all tetrapods from horses to bats.
Denton then gets to the point of all of this:
Ironically, as Riedl argues, it is only because organisms can be classified into distinct groups on the basis of their possession of invariant unique homologs that descent with modification can be inferred in the first place. If it was not for the invariance of the homologs and the Types they define, the very notion of the common descent of all the members of a particular clade from a common ancestor would be in serious doubt. The living realm would conform to a chaotic network rather than an orderly branching tree.
Denton notes that again, his view that taxa-defining homologs define real “types” is well within the mainstream of evolutionary biology. He quotes Gareth Nelson and Norman Platnick:
Since the advent of the so-called new Systematics, it has become popular to deprecate as “essentialistic” or “typological” the notions that species (and hence groups of them) have defining characters, and that it is the business of systematics to find them… The rationale for this deprecation seems to be that if evolution occurs, the characters of species (and hence groups) may change in the future; therefore, species and groups of species cannot be permanently characterized by means of a single character or set of characters such that the character or set is necessary and sufficient for membership in the species or group. The argument seems to rest on the misleading use of character states: it assumes that when a species is modified, and acquires a new apomorphic character (state), it is no longer recognizable as having, the original plesiomorphic character (state). In other words, according to this argument, we cannot use characters (such as fins) to define groups (such as Vertebrata), because some members of those groups (such as tetrapods) may acquire apomorphies (such as limbs). If one accepts the validity of ontogeny or outgroup comparison (i.e., Parsimony) or any other possible test of hypotheses about character transformation, the argument is obviated. In this sense, systematists always have been, are, will be, and should be typologists.
Gareth Nelson and Norman Platnick, Systematics and Biogeography (New York: Columbia University Press, 1981), 328, emphasis added.
More irony. Foaming at the mouth Darwinian cladists like Nick Matzke unwittingly support the structuralist paradigm over the Darwinian functionalist paradigm by the very nature of what they do. Denton writes:
The cladistics enterprise would be impossible if different groups were not unambiguously defined by synapomorphies (homologs unique to those particular groups). Indeed, current evolutionary literature is replete with thousands of cladograms to illustrate the phylogeny of various groups of organisms and the sequence in which the various defining traits of the subgroups were acquired.
Which brings us back to where we began. Denton is not a creationist. He is not even an ID proponent, if ID is defined in a narrowly interventionist way. He firmly believes in the tree of life and common descent. If Denton does not believe the “types” arose though Darwinian processes and he does not believe they arose through interventionist acts of a designer, what does he believe? He believes the types were “prefigured into the order of things from the beginning.” From this I surmise he is a front-loading ID proponent. He summarizes his position as follows:
Although I think the evidence is consistent with most of the novelties being achieved in a relatively saltational manner typology does not demand absolute saltation, just that the Types (or more properly the homologs which define them) are a special set of robust natural forms or stable material systems, part of nature’s order from the moment of creation, to which the paths of evolution were inevitably drawn.