No subject

Brett Lindenbach Brett_Lindenbach at QMS1.LIFE.UIUC.EDU
Mon Jun 22 14:38:58 EST 1992

        Reply to:   RE>No subject
regarding the
The Molecular Clock Hypothesis


  As I understand it, the molecular clock hypothesis asserts that the
  amount of interspecific variation in molecular sequences (eg., amino
  acid sequences in haemoglobin, cytochromes etc.) is proportional to
  the time which has elapsed since the separation of the evolutionary
  lineages leading to the two species being compared from their common
  ancestor.  In other words, the mutations effecting these molecular
  sequences take place at very nearly a constant rate per unit time, in
  widely different species.  If this were not the case, the observed
  pattern of interspecific sequence variation would be quite different.

  However, different species reproduce at vastly different rates, e.g.,
  elephants, field mice, bacteria, bristlecone pines...  Can we assume
  that the rate of cell division in the cellular lineage leading up
  to reproduction is exactly the same in a bristlecone pine as it is
  in a field mouse?  I doubt it, but I have no information on the
  rates of cell division in different species.  I would imagine that
  they vary considerably, perhaps by several orders of magnitude,
  making the rate of cell-division copying errors far greater in the
  more rapidly multiplying species.

Suppose that, in any particular species, in any one year:

  1). cosmic radiation causes X mutations

  2). there are Z cell divisions in the cellular lineage leading up to
      reproducution, and at each cell division, (on average) Y copying
      errors take place.

  3). Between cell divisions, there are P replications of the genetic
      material of the cell, and at each replication (on average) Q
      copying errors take place.

Then the total number T of mutations per year is given by:

                        T = X + PQ + YZ

I want to know:

  1). the relative magnitudes of  P, Q, Y and Z

  2). whether or not the terms P, Q, Y and Z are the same in different
      species (especially, in species which reproduce at different
      rates).  Examples of different values would be useful.

  Clearly, T can only be the same for two different species if the
  terms on the right hand side of the equation are all constant, or
  if the term which varies is very small, relative to the other terms.

  If any term is large and varies considerably, then T must also vary
  indication - a serious flaw in the molecular clock hypothesis.


 You have asked some very astute questions for a non-biologist. I would
like to comment on a few points. First, your mathematical
simplification of mutation rate may or may not be valid. However, be
careful not to extrapolate into the phenotype of whole organisms (esp.
eukaryotes) the results of your function, which seems to require that
genetic material be an ever-changing, random string of information. Do
not forget that nature acts as an "editor" of mutations, constantly
testing new mutations for fitness. This happens on many levels,
including the molecular as well as organismal layer. For instance, the
16s rRNA makes an excellent molecular chronometer (Andrew, if you gain
library access, check out the works of Carl Woese, and Pauling &
Zuckerkandl, to name a few) because some regions of the RNA are very
slow to change. They may mutate just as quickly, but because these
regions are so important to the cell's ribosomal machinery, most
mutations are selected against and lost. Thus, different regions of
this one molecule are conserved at different rates. This brings up the
point that a lost mutation is impossible to see. Therefore, change in
an organism may be hard to see if that organism's evolutionary niche
has stringent selective pressures. (Geez- I'm starting to sound like
some of my profs...|8-)  
Good luck- I hope this helps

Brett Lindenbach
Dept. Microbiology
University of Illinois
(brett_lindenbach at qms1.life.uiuc.edu)

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