Yesterday's post discussed the method of dating by genetics. I think it is important to note that the 'molecular clock' is not foolproof, and is based on assumptions. The molecular clock was derived by estimating when two lineages split in the fossil record (via radiometric dating), and comparing the differences in genetics to come up with a rate of genetic change. Click here for Berkeley University's explanation. But this should not be looked at as precise, because it is based on the following assumptions:
Changing generation times (A mutation generally becomes fixed only from one generation to another. The shorter this timespan is, the more mutations can become fixed)
Population size (Apart from effects of small population size, genetic diversity will "bottom out" as populations become larger as the fitness advantage of any one mutation becomes smaller)
Species-specific differences (due to differing metabolism, ecology, evolutionary history,...)
Evolving functions of the encoded protein (can be ameliorated by utilizing non-coding DNA sequences or emphasizing silent mutations)
Changes in the intensity of natural selection
I thought this was worth mentioning.
Sharon Moalem discusses mutation rates in his book "Survival of the Sickest" and brought up an interesting phenomenon (paraphrased here):
ReplyDeleteBasically, when a population is facing life-threatening situations, such as corn in a drought season, the organisms will begin a period of "hypermutation" where the rate of transposon (so-called "Jumping Genes") activity will increase dramatically, thereby increasing the rate of overall mutation.
The result, obviously, is that an increased number of random mutations will eventually result in a variety that is better suited to the changed environment, which would then be naturally selected.
In your post you mentioned that the mutation rate is not statically set, and I thought it would be germane to point out one of the reasons why. :)
Nice Job Aaron!
ReplyDeleteI neglected to mention that scientists are beginning to see the molecular clock as something that can tick too fast for short periods of time, and then go back to a point of equilibrium.