Are current critiques of the theropod origin of birds science? Rebuttal to Feduccia (2002)

Auk, The, Apr 2003 by Prum, Richard O

Feduccia specifically criticizes Wagner and Gauthier's (1999) hypothesis of a homeotic shift in the identity of the single wing digit of the kiwi (Apteryx spp.) as evolutionarily inexplicable given its reduced forelimb function. Yet, recent findings on the development of digit identity in mice document that a single digit one morphology is the phenotype of a loss of function mutant that lacks specific digit development and identity genes (Shh-/-; Gli-/ ; Litingtung et al. 2002). A similar loss of function mutation is an entirely plausible hypothesis for the mechanism of kiwi wing digit reduction, and one such mechanism produces a single digit one. This molecular evidence supports the plausibility of Wagner and Gauthier's (1999) homeotic shift hypothesis for the kiwi wing digit, and further discredits Feduccia's assumption that homeotic shifts must be accompanied by strong natural selection.

In conclusion, there are no conflicts between developmental biology and the theropod origin of birds. Rather, Wagner and Gauthier (1999) used the theropod origin hypothesis to make bold predictions about developmental biology that have been supported (Dahn and Fallon 2000).

FEATHERS

Alan Brush and I have recently reviewed the developmental, paleontological, and phylogenetic evidence on the theropod origin of feathers (Prum and Brush 2002), so I will only address a few points here.

Feduccia defended the elongate scale-based model of the origins of feathers and criticizes my alternative developmental model (Prum 1999), but he fails to understand the implications of our recent molecular developmental research on feather evolution (Harris et al. 2002). Along the way, he makes a number of inaccurate statements. First, the developmental model I proposed is not a "filament-to-feather" model. Rather, the model hypothesizes that feathers are fundamentally tubular, and that feathers evolved through a series of derived novelties in the developmental mechanisms within the tubular feather follicle and feather germ (Prum 1999). I hypothesized that the first feathers were hollow tubes, followed by a tuft of barbs, followed by doubly branched feathers, and finally by feathers with a closed permaceous vane (Fig. 2). Second, the developmental model is explicitly independent of all functional scenarios (Prum 1999), and is not wedded to the thermal insulation theory of feather origin, as Feduccia (2002) stated. However, the developmental model easily falsifies the hypothesis that feathers evolved for flight (Feduccia 1999b), because the first feather that could have had an aerodynamic function would have required many derived developmental mechanisms (Fig. 2, Stage IV). Third, our molecular developmental experiments are not based on the theropod hypothesis (Harris et al. 2002). Rather, our data consist of observations and experiments on the cellular molecular signaling mechanisms involved in feather morphogenesis. Those data are entirely independent of, and entirely congruent with, the theropod origin of birds.