Of Hen's Teeth and Neutral Mutations
Evolutionists often cite an experiment which purportedly induced tooth growth in chickens, supposedly confirming that birds have genes for teeth and are descended from toothed reptilian ancestors. For example, in his book Hen's Teeth and Horse's Toes, Stephen Jay Gould discusses this experiment, and suggests that "we are seeing, in part, the actual form of a latent bird's tooth--the potential structure that chick epithelium has encoded for sixty million years but has not expressed in the absence of dentin to induce it." (p. 184) But there's a problem with Gould's argument: as Sean Carroll explains, neo-Darwinism has a 'use-it-or-lose-it' rule. According to neo-Darwinism, if a trait is not used then the DNA which encodes it will accumulate neutral mutations, and eventually the trait will be lost forever. If supposed chicken genes for producing teeth haven't been used for 60+ million years, then that would strongly suggest that neutral mutations should have long-since destroyed their ability to function. Biologist Nelson Cabej elaborates on why neo-Darwinism is incompatible with toothed chickens:
Neo-Darwinian explanation would essentially relate the loss of dentition in Aves to gradual accumulation of mutations in genes that determine odontogenesis or genes of the GRN (gene regulatory network) for odontogenesis. Accordingly, during the evolutionarily long period of time since Aves lost their dentition, natural selection could not have acted against accumulation of deleterious mutations in odontogenic genes and such harmful changes in the genes thus rendering nonfunctional the odontogenic genes in birds.A paper that backs up Cabej's argument was published by three biologists in Proceedings of the National Academy of Sciences:
Even for traits that are determined by a single gene in metazoans the maximal estimated time after which a silenced gene could be reactivated for producing the lost ancestral trait is 6 million years. Since in the development of most phenotypic traits, including teeth, a varying number of genes rather than a single gene is involved, reversion of dentition in birds would be possible only for periods of time shorter than 6 million years. Consequently, not only the natural reversion of teeth but even the experimental induction of tooth development in species of this vertebrate class would be impossible.
(Nelson R. Cabej, Epigenetic Principles of Evolution, p. 557 (Albanet, 2010).)
Dollo's law, the concept that evolution is not substantively reversible, implies that the degradation of genetic information is sufficiently fast that genes or developmental pathways released from selective pressure will rapidly become nonfunctional. Using empirical data to assess the rate of loss of coding information in genes for proteins with varying degrees of tolerance to mutational change, we show that, in fact, there is a significant probability over evolutionary time scales of 0.5-6 million years for successful reactivation of silenced genes or "lost" developmental programs. Conversely, the reactivation of long (>10 million years)-unexpressed genes and dormant developmental pathways is not possible unless function is maintained by other selective constraints; the classic example of the resurrection of "hen's teeth" is most likely an experimental artifact...Thus, according to Marshall, Raff, and Raff, genes that have gone unused for more than 10 million years will be lost forever. Something is going on with hen's teeth, but it isn't the "resurrection" of vestigial genes.
(Charles R. Marshall, Elizabeth C. Raff, and Rudolf A. Raff, "Dollo's law and the death and resurrection of genes," Proceedings of the National Academy of Sciences USA, Vol. 91: 12283-12287 (December, 1994).)