Tuesday, April 13, 2010

Measuring Time From DNA

"As we develop better molecular methods, people would like it if the molecular dates reconciled with the fossil record. Then everybody would be happy, but instead the gap is getting wider, and in the end, that might actually be interesting." ~ Michael Donoghue

Michael Donoghue endorsed a paper titled "An uncorrelated relaxed-clock analysis suggests an earlier origin for flowering plants" about the origins of angiosperms (flowering plants) that was recently published in the Proceedings of the National Academy of Sciences by a team of researchers from Yale University and The National Evolutionary Synthesis Center. In this paper the researchers talk a lot about a technique known as "molecular clock analysis." This technique is a way that evolutionary biologists attempt to understand the relationships between organisms and the time frames where the organisms (presuming an evolutionary model) diverged from each other on the evolutionary track. Evolutionary biologists will compare DNA sequences from organism that exist today, use that information to attempt to build evolutionary trees, and then use molecular clocks to determine when the organisms diverged from each other on the evolutionary tree.

Let me attempt to explain how molecular clocks work. DNA has genetic "letters," which are abbreviated A, G, C, and T. The linear sequence of these letters contains genetic information (much like a word in language contains information because of its definition). The letters build genetic "words," which harbor the genetic information of the DNA molecule. Mutations can alter the sequence of genetic "letters" (much like a misspelling in a word) and molecular clock analysis attempts to measure the rate of these "misspellings" from mutations. If you compare two different DNA sequences which come from organisms that (presuming an evolutionary model) have a common ancestor and you know the rate of change/mutation of the DNA then you can extrapolate the time frame in which the organisms diverged from each other.

The problem with molecular clock analysis is that you have to make assumptions about the rate of change/mutation of the genetic "letters" in the DNA. Evolutionary biologists will attempt to calibrate the rate of change in the clock by going to the fossil record, looking at when certain organisms appeared in the record (organisms that are assumed to be evolutionary relatives), and then correlating the lapses in time with the changes in DNA to create a rate. They then take this calibrated rate and apply it to the entire evolutionary tree they have built to attempt to determine when organisms may have diverged from each other. The problems with this are: 1) it is notoriously difficult to do and 2) the results achieved by the analysis most often do not agree with what is seen in the fossil record. These researchers (from the teams mentioned above) attempt to get around these problems by varying the rate of mutation over time. They "relaxed" the clock. They vary the rate (often times by a lot and with rates too rapid for an evolutionary model) in order to get molecular clock analysis to match up with the fossil record.

Often times molecular clocks are used as a very important tools in molecular biology and cited as evidence for evolution. I like Donoghue's quote above because he seems to recognize the inconsistency in using molecular clocks to validate evolution. The fact that the "gap is getting wider" is "interesting" because it then either invalidates naturalistic evolution or invalidates the molecular clock method itself. This study shows us how flawed this technique actually is. It shows that there is generally no agreement between molecular clocks and the fossil record and that in order to get agreement researchers are rigging their inputs (relaxing the clock) until they get something that agrees with the fossil record. I am sorry, but this is not the way to do science. You can say that the rates may change over time (as rates often do) but you cannot just assume whatever rate(s) you want in order to get the results you are looking for. You have to have criteria that justifies 1) differing rates over time and 2) the specific rates you are using. What justifies using these different rates? For a scientist to do this they must have objective reasons for doing it. Playing with the data until you get the results you were looking for is not a reason. This hurts the case for molecular clocks being support for evolution. If you play with the data until you get the results you want (for the naturalistic evolution model) you cannot then turn around and say it is evidence for the evolutionary paradigm and this study highlights that particular problem. This paper shows that either molecular clock analysis (as it stands right now) is very flawed, the naturalistic evolution models are flawed, or both.

By His Grace,

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