Wednesday, August 25, 2004

Hunter, Part 3

I will now continue my analysis of Cornelius Hunter's essay from William Dembski's anthology Uncommon Dissent.

In the first two installments of this series (available here and here), I analyzed Hunter's arguments concerning the fossil record and the genetic code. In this, final, essay, I will discuss some of his comments about homology.

Before getting to that, let's first consider Hunter's discussion of vestigial organs:

Regarding the so-called vestigial organs, the rule-of-thumb is that sooner or later a function will be found. Robert Wiedersheim listed 86 organs in the human body that he supposed to be useless leftovers of evolution. Today, we have found functions for virtually all of them. And what about those whale bones that are thought to be vestigial? It may well be that they support the whale's reproductive organs. (P. 200)

One could hardly ask for a clearer example of the “vestigial means nonfunctional” fallacy. I addressed this point in this earlier post. Vestigial and nonfunctional are two different things. A vestigial organ is one that does not perform the function we expect it to perform from observing the same structure in other organisms, or an organ that performs a function out of all proportion to its complexity. The sightless eyes of cave-dwellling rodents may perform the function of plugging holes in the animal's head, but they are no less vestigial for that. The non-flying ostrich uses its wings to maintain balance when it is running, but the fact remains that a far less complex structure could perform the same function.

Of greater importance are Hunter's thoughts on homology:

First, there are many striking similarities shared by distinct species. In these cases, the similarities could not have evolved independently. The marsupial-placental convergence is a popular example. Over millions of years and in different corners of the earth, the marsupial and placental lineages, supposedly evolving from a mouse-like species, produced a host of similar designs. Everything from saber-toothed carnivores and wolves to flying squirrels and anteaters were produced independently. From salamanders to cacti we find striking similarites that must have arisen independently. If biology is ruled by contingency rather than necesity, then why do we find duplicated designs? When similarities are found among distant species, they are noted as cases of convergent evolution. Evolution can explain either case, but the explanations presuppose evolution. This is not powerful evidence for the theory. (P. 199-200)

Hunter is fond of the phrase “striking similarities” but it is not clear to me what it actually means. Biologists usually make a distinction between homologous characters and analogous characters; the former are the ones that are most plausibly explained via common descent. Hunter is suggesting that biologists are being arbitrary in their determinations about which characters are homologous and which are analogous.

But that is not the case. The determination that a given character is homologous in two different species is not based on some vague description of the two features as similar. Rather, it rests on a host of considerations. Mark Ridley describes many of these in his textbook on evolution. For example, homologous characteristics must have the same fundamental structure. The bone structure of the forelimbs of humans, whales and cats are a good example of this. By contrast, the wings of birds and bats are superficially similar, but their internal structure is quite different. Consequently, these wings are almost certainly analogous. Second, homologies should have the same relations to surrounding characters in both organisms. Finally, homologous features should arise via the same pathway of embryonic development.

By contrast, analogies result from convergent evolution, which is the result of natural selection. Thus, characters that are adaptations to particular environments are more plausibly explained as the result of convergence than characters that are not obviously adaptations. The streamlined shape of whales and fish is an obvious adaptation to a life spent propelling oneself through water. Consequently, convergence is a plausible explanation. On the other hand, the identical bone structure of mammalian forelimbs does not appear to be the result of natural selection crafting similar solutions to similar problems.

Picking up where the last paragraph left off, Hunter discusses the point about embryonic development:

Even the similarities claimed by evolutionists are often ambiguous, for they do not share the same developmental pattern. For example, two closely related species of frog, Rana fusca and Rana esculents, have eye lenses that are similar but they form very differently in embryological development. Did these two similar species evolve their eye lenses independently? There are many such similarites that develop differently or arise from different genes, and they seriosly challenge the claim that they could have arisen through common descent. (P. 200)

I'm afraid frog embryology is a bit far removed from my fields of expertise, but Hunter's argument here is far too vague to be replied to in any case. He points to two different species of frogs and asserts (without providing a reference) that their eye lenses are “similar” despite forming “differently” in embryological development. Sorry, but how similar and how different? Do the lenses of these frogs satisfy the other tests for homology that I described above? How different are their patterns of embryological development. Without this sort of information, it's hard to respond to this assertion.

Hunter also gives some brief consideration to molecular homologies:

Molecular comparisons, in spite of what evolutionists report, are equally ambiguous. It is not surprising that molecular comparisons are generally consistent with morphological comparisons. After all, molecular and morphological features are all part of the same organism. Only the extreme view, that there is no necessity in biology and that contingency is utterly dominant, would so divorce molecules from morphology. Yes, nature does reveal different solutions to similar design problems, but this does not mean that designs are random. If two automobiles are similar in appearance and function, are we surprised when their gears are also similar? Of course not. (P. 200)

Again, this is too vague to be responded to. Consider the structure of hemoglobin in different animal species. Hemoglobin is the molecule responsible for transporting oxygen to the various tissues of the body. That is it's function wherever it is found. It's precise structure varies from species to species, and the pattern of these differences is entirely consistent with the patterns of descent inferred from morphology and paleontology. Thus, human hemoglobin is far more similar to chimpanzee and gorilla hemoglobin than it is to dog hemoglobin, for example. Is Hunter suggetsing that the differences between human and dog hemoglobin are the necessary consequences of the differences in the morphology of humans and dogs? If that is his suggestion, then I'd appreciate it if he'd tell us what that functional necessity is.

I'm also not sure about the point of Hunter's analogy. Presumably the automobiles that are similar in design and function are the different species of animals while the gears are the underlying genes and molecules. But what we're interested in when we speak of molecular evidence for evolution is the pattern of differences in the molecules of multiple species. We are not simply considering the similarities in the molecules between two species.

It is conceivable that there is some deeply concealed functional reason for explaining this pattern of differences. But it is safe to say that no one has a plausible suggestion for what that functional reason might be. It is also safe to say that had these patterns not confirmed the phylogenies arrived at independently of them, it would have been a blow for evolution.

And Hunter doesn't even consider what to my mind is the most compelling sort of molecular evidence for common descent: patterns of similarities in the noncoding portions of the genome. Since these genes do not code for proteins their similarities can not be explained functionally (unless, of course, the noncoding DNA also serves some deeply concealed function that demands very specific sequences of nucleotides). And the sheer quanitity of these similarities makes it implausible to attribute them to chance alone. I can't imagine how you explain these similarities except via common descent.

Ed Brayton provides some additional thoughts on this subject here.

It seems that Hunter's main rhetorical trick is to consider each piece of evidence for evolution independently of the others, and then offer some other non-evolutionary, non-logically-impossible explanation for each piece. Those of his arguments that are not flatly wrong sound more like the wishful thinking of someone who desperately wants evolution to be false than the reasoned arguments of a dedicated truth-seeker.