Why cats don’t have a sweet tooth

Cats don’t care for chocolate and other sweets. Interestingly, neither do their wild cousins, like tigers and lions. That observation has prompted speculation among biologists that perhaps the ability to taste sweets was lost early on during the evolution of this group, probably as a byproduct of natural selection for carnivory. Since carnivors don’t seek sugar-laden foods, selection maintaining that ability would be relaxed, leading to the loss of both the taste for sweet and the gene(s) that make it possible (in reverse causal order, of course).

Now, contrary to creationist claims, evolutionary biology is a legitimate science because it makes testable predictions. If the above scenario is correct, one would expect to find in cats what is called a “pseudogene,” i.e. a segment of DNA that used to code for a functional protein (in particular, the protein that makes possible to taste sweets), but is no longer working because of a damaging mutation that was not eliminated by natural selection (since the gene wasn’t necessary any more anyway). Moreover, such pseudogene should also be present in all the closely related species to cats, such as tigers and lions.

Sure enough, recent work carried out by Xia Li and a host of collaborators has found exactly what evolutionary theory would predict (their paper was published in PLoS Genetics, July 2005). The ability to taste sweets in mammals depends on the action of two genes, Tas1r2 and Tas1r3. These produce two proteins – rather unimaginatively called T1R2 and T1R3 – which have to combine with each other in order to make a receptor for sweets in the taste buds. Li and coworkers found that cats, tigers, and lions have a functional version of Tas1r3, but also that Tas1r2 cannot make a functional protein because of mutations that stop the translation and transcription processes (from DNA to RNA, and eventually to protein) too early. Interestingly, the damaged gene exists in other mammals (such as ourselves), and works normally (allowing us not only to taste sugar, but artificial sweeteners as well). In other words, Tas1r2 is a pseudogene shared by domestic and wild cats, in accordance with the predictions of evolutionary theory.

Now, I ask you, would this piece of elegant detective work have been possible based on the “theory” of Intelligent Design? What would ID “scientists” have predicted, and why? Besides, what kind of intelligent designer would deprive cats of the taste of chocolate? I’d say that comes pretty close to animal cruelty.