Avian digits continue to cause controversy
It’s an old controversy, and one that was hashed out fairly well when birds were claimed to be the descendents of dinosaurs, while others demurred. Well, we know which side won, but recent evidence suggests that birds (and likely the ancestral dinosaurs) are indeed missing the first digit in their “hands” (wings), which was long the argument against their being related to dinosaurs.
In short the argument was that dinosaurs have digits I, II, and III, while supposedly the birds have digits II, III, and IV. Later studies have settled the matter, and bird wings do have digits I, II, and III. Here is the abstract of the report of the research indicating that, regardless of position, development of the avian “hand” involves expression for the “thumb” (digit I).
The fact remains, however, that bird digits grow in the positions of digits II, III, and IV. So one idea has been that the digits “frameshifted” in both dinosaurs and in birds (naturally), and that is what the latest comparison with crocodilians (alligators, in this case) seems to support.
Further, the lack of expression of the HoxD-11 gene in the first finger of the wing makes it most similar to finger one (the “thumb”) of the mouse, consistent with comparative morphology. However, the mouse is only distantly related to birds; crocodilians, in turn, are bird’s closest living relatives.
To see whether the evidence from mouse HoxD-11 expression held up, Vargas and colleagues, working at the lab of Gunter Wagner at Yale, have examined the expression of this gene in alligators; they found the expression to be, as in mice, absent only in finger one (the “thumb”).
Developmental and evolutionary biologists are familiar with the phenomenon of homeotic transformations, in which one structure begins to develop at a different position within the body. A famous example is the case of the fruitfly mutant antennapaedia, which develops legs on its head instead of antennae. The new work by Vargas et al. rekindles the hypothesis that a “hometic frameshift” occurred in the evolution of the bird wing, such that fingers one, two and three began to develop from the embryological positions of fingers two, three and four.
I am not sure that the crocodilians’ closer relationship to birds than to mice is really the most important factor in their conclusions, rather the fact that two relatively independent lines show the same developmental patterns (while birds appear “frameshifted” in expression) probably matters more. To be sure, the fact that the closer relative, alligators, differ in development of digits from birds’ digit development much as mice digits does bolster the earlier conclusions even more than if the independent line were not more closely related to birds.
Anyway, it is an interesting development, because although frameshifts are known, a shift in digit framing is hardly expected. Nevertheless, both the fossil evidence and the developmental evidence points toward bird digits being numbers I, II, and III, while this and other evidence indicates that those digits develop where digits II, III, and IV develop in most animals. Hence the conclusion of a “homeotic frameshift” appears to be sound.
Just to tie this into the ID theme, of course the upshot is that ID would have no problem and no explanation for development of the bird/dinosaur hand, since IDists’ only “prediction” is that anything can happen, hence the fact that something happened is “evidence” for design. While I may have changed the wording, this is about all that ID says, for Behe’s fallback position is indeed that “we cannot know that something has not been designed.” DBB 194 It is the fact that we can say that evolution cannot be expected to cause some things that leads to research such as this.