Wednesday, April 02, 2008

The Common Ancestry Two-Step
Darwin Catholic had an interesting post a few days back on Catholics and Evolution. Of course, the general view has always been that the Church is accepting of the theory (in fact, the Catholic Encyclopedia as far back as 1909 states this).

This is a far cry from most evangelical branches of Protestantism that reject it outright in favor of Biblical literalism. (Actually, they're not alone, there are some Catholic organizations selling this kool aid as well, but I will refrain from sending them any traffic.)

But based on what I've read on blogs ---including some of the worst science reporting in the mainstream conservative journals whenever it comes to evolution---the issue apparently is not as clear cut as it seems.

A number of Catholics, for example, stake a 'middle' position on evolution, basically claiming to accept common ancestry, while maintaining a skeptical view of the mechanisms of evolution: meaning genetic variation, genetic drift, recombination, etc., in combination with natural selection.

This may sound reasonable, at first-- but it seems to me that there is a disconnect here: a disconnect that must assume a fundamental lack of understanding of precisely what biologists mean by the term, common ancestry. At the risk of belaboring the obvious, it means more than just agreeing that, yeah, a lot of animals look related, the more recent ones do look more complex, etc: it means accepting the underlying reasons why--and these underlying reasons cannot be dissociated from variation and natural selection except by the most willful mental compartmentalization (or intellectual dishonesty if you're a paid ID flack). Herein a rather lengthy aside from Douglas Theobald:

Even Aristotle discussed the peculiar vestigial eyes of moles in the fourth century B.C. in De animalibus historiae (lib. I cap. IX), in which he identified them as "stunted in development" and "eyes not in the full sense".

As these individuals noted, vestiges can be especially puzzling features of organisms, since these "hypocritical" structures profess something that they do not do—they clearly appear designed for a certain function which they do not perform. However, common descent provides a scientific explanation for these peculiar structures. Existing species have different structures and perform different functions. If all living organisms descended from a common ancestor, then both functions and structures necessarily have been gained and lost in each lineage during macroevolutionary history. Therefore, from common descent and the constraint of gradualism, we predict that many organisms should retain vestigial structures as structural remnants of lost functions. Note that the exact evolutionary mechanism which created a vestigial structure is irrelevant as long as the mechanism is a gradual one.


[Figure2.1.2 (flightless weevil, apterocyclus_honolulensis)] [Figure2.1.2 (vestigial dandelion] [Figure2.1.2 (vestigial dandelion pollen)]

Figure 2.1.2. Various organisms displaying vestigial characters. From top to bottom: A. Apterocyclus honolulensis, a flightless weevil. The black wing covers cannot open, as they are fused, yet underneath are perfectly formed beetle wings. B. The vestigial flower of Taraxacum officinale, the common dandelion. C. A vestigial pollen grain from the dandelion.

There are many examples of rudimentary and nonfunctional vestigial characters carried by organisms, and these can very often be explained in terms of evolutionary histories. For example, from independent phylogenetic evidence, snakes are known to be the descendants of four-legged reptiles. Most pythons (which are legless snakes) carry vestigial pelvises hidden beneath their skin (Cohn 2001; Cohn and Tickle 1999). The vestigial pelvis in pythons is not attached to vertebrae (as is the normal case in most vertebrates), and it simply floats in the abdominal cavity. Some lizards carry rudimentary, vestigial legs underneath their skin, undetectable from the outside (Raynaud and Kan 1992).

Many cave dwelling animals, such as the fish Astyanax mexicanus (the Mexican tetra) and the salamander species Typhlotriton spelaeus and Proteus anguinus, are blind yet have rudimentary, vestigial eyes (Besharse and Brandon 1976; Durand et al. 1993; Jeffery 2001; Kos et al. 2001). The eyes of the Mexican tetra have a lens, a degenerate retina, a degenerate optic nerve, and a sclera, even though the tetra cannot see (Jeffery 2001). The blind salamanders have eyes with retinas and lenses, yet the eyelids grow over the eye, sealing them from outside light (Durand et al. 1993; Kos et al. 2001).

Dandelions reproduce without fertilization (a condition known as apomixis), yet they retain flowers and produce pollen (both are sexual organs normally used for sexual fertilization) (Mes et al. 2002). Flowers and pollen are thus useless characters for dandelions in terms of sexual reproduction.

There are many examples of flightless beetles (such as the weevils of the genus Lucanidae) which retain perfectly formed wings housed underneath fused wing covers. All of these examples can be explained in terms of the beneficial functions and structures of the organisms' predicted ancestors (Futuyma 1998, pp. 122-123).

The ancestors of humans are known to have been herbivorous, and molar teeth are required for chewing and grinding plant material. Over 90% of all adult humans develop third molars (otherwise known as wisdom teeth). Usually these teeth never erupt from the gums, and in one third of all individuals they are malformed and impacted (Hattab et al. 1995; Schersten et al. 1989). These useless teeth can cause significant pain, increased risk for injury, and may result in illness and even death (Litonjua 1996; Obiechina et al. 2001; Rakprasitkul 2001; Tevepaugh and Dodson 1995).

Natural selection is really what defined Darwin's breakthrough. Saying you accept common ancestry but not genetic variation and natural selection, is akin to saying, "Yes, of course your grandfather is related to you, and of course you share the same genes. Who would deny that? were adopted."

In short, to maintain that common ancestry is true, without accepting the mechanisms of evolution, by which the lineages are explained, strikes me as incoherent.

But that seems to be the position many Christians have staked out.

Steve Matheson shared some thoughts on why: "The roots of this thing are pretty deep, and the error is very seductive. Evolution is targeted for Christian opposition, uniquely among major biological theories, because it's been characterized as the final refutation of divine action in the world. This is ludicrous, of course, but combine it with the modern Christian's typical struggle with what it means to acknowledge the supernatural and/or the transcendent, and you have what looks like a frontal assault on the divine. Add to the mix the fact that evolution is a theory of the past, necessarily shrouded in a certain amount of mystery, and you have a perfect storm. Note that I haven't even mentioned concerns about the biblical narrative of creation."


SFMatheson said...

"In short, to maintain that common ancestry is true, without accepting the mechanisms of evolution, by which the lineages are explained, strikes me as incoherent."

Well said! I'm grabbing this for Friday's sampler.

John Farrell said...

Much obliged!