CKing: "Therefore, like the atomic clock, the "molecular clock" really measures (albeit much less accurately than the atomic clock) roughly how long ago two species may have shared a common ancestor."

Richard Wells: Yeah, so I have been told...and this, like much of molecular biology I pretty well have to take the word of others better informed. I claim no skill or special knowledge in this obviously important area at all, yet this inadequacy has been no impediment at all to me in the identification of biological species. And indeed, I can confidently challenge any molecular biologist professing expertise in vertebrate taxonomy to join me in field work and we shall see who is the superior in identification. But in my blissful ignorance, I do chuckle a little when I see "facts" emerge from bases like the concept of "DNA molecular clocks" that are at best theoretical postulates. I was under the impression that the rate of nucleotide substitution is presumed to be a constant across the entire genome of a species, and the degree of divergence in these nucleotide sequences between two different species enables an estimation of the time of separation (ie the point when they branch from their common ancestor). Exciting stuff to be sure, but how reliable is the basis for this postulate, when the significance of the divergence it is presumably based on the delta T50H? I mean to say, this seems pretty hairy stuff to me - that the genetic relationship between the nucleotide sequences of two or more species can be determined by the difference between the temperatures at which DNA homoduplexes and DNA heteroduplexes undergo that magic 50% dissociation. And what about the accuracy of identifying repetitive sequences for their removal by interpretation of the reassociation kinetic data? Also, what if there is no independant dating reference point from the fossil record, thus obstructing the conversion of the delta T50H value to an absolute time interval? Don't get me wrong - I'm not bagging the molecular clock hypothesis as erroneous - I just see fertile ground for unwarranted extrapolation. To me the concept is more a "work in progress", than a complete understanding, so I am a little uneasy at the "absolutist" interpretations that are made on the basis of this tool at times. So please forgive me for not immediately jumping up and down in fits of joyous commendation everytime a molecular theory is transformed into a molecular fact by the slick application of some sort of statistical gymnastics or another. My scepticism may be more a product of ignorance than understanding...but I often wonder how much the molecular maestros REALLY understand about the game they are playing as well?

Richard Wells

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