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July 27, 2005

Pharyngula::Cats, candy, and evolution

P.Z. Myers on the non-sweet tooth of cats:

Pharyngula::Cats, candy, and evolution: What this protein does is detect sugar, and then instruct your taste buds to start sending nerve impulses up to your brain.... Cats don't get to experience that... their TAS1R2 gene carries a substantial mutation that destroys its function.... There is a small deletion near the beginning of the sequence that chops out 247 base pairs. This deletion puts the remainder of the sequence out of register... turning it into non-functional nonsense, and also generating multiple stop sequences.... Poor kitties. They don't even know what they are missing.

It's nice to have an explanation for why cats prefer fish to candy bars, but there's more to the story than that. It's also another piece of evidence for evolution. The cat TAS1R2 gene has been thoroughly blasted into uselessness, but there is obviously more than one way to do that. A larger deletion that took out the whole gene would be just as effective, as would a 1 base pair deletion at the beginning of the sequence. Any random scrambling would do. So how do you explain this?

The sequence was analyzed in house cats, but the gene was also examined in samples taken from a tiger and a cheetah. They have exactly the same mutation... "plagiarized errors ", a phenomenon that is most simply explained by common descent. The last common ancestor of house cats, tigers, and cheetahs had this mutation, and passed it on to all of its progeny.

We can also make an evolutionary prediction: I expect that lions, leopards, and lynxes will also have the same 247 base pair deletion... the scar of this ancient gouge in their DNA will be present in all cats...

One question: is there any sense in which it is... adaptive for cats to not like fruit? Fruit is, after all, a quick source of energy--easily digested calories. One would think that a cat that liked fruit would have more energy for the hunt.

Or is this just an example of genetic drift at work? The few animals that speciated into the ancestral cat lineage got this mutation, and it wasn't (very) harmful, so they didn't become extinct. And thereafter there was no way to undo it.

Posted by DeLong at July 27, 2005 01:03 PM