(none)

MBY134 at sysh.surrey.ac.uk MBY134 at sysh.surrey.ac.uk
Mon Sep 9 09:14:47 EST 1991


RE: MOLECULAR CLOCK QUESTION

Tom Holroyd's question about rates of evolution discusses the
possibility that evolution rates may be erratic and rapid
bursts of change may be responsible for speciation events -
essentially the punctuated equilibrium theory of evolution.

Most comments have pointed out that the difference between
molecular evolution which may largely be neutral and procede
at a steady rate and the phenotypic evolution we see which
could well be episodic due to varying selective pressure.

However - isn't there evidence that molecular evolution may
also vary.

Carl Woose's execellent review on bacterial evolution (Woose,
C. 1987. Microbiological Reviews, _51_: 221-271) as determined
by rRNA sequencing points out that rRNA is a rate-sensitive
compound chronometer. The more variable positions (low
mutation rate chronometer) changes at a steady rate whereas
other more conserved positions appear to be invarient until a
period of high mutation rate switches them on (high mutation
rate chronometer). Changes at these more conserved positions
act as sequence-signatures for different genera suggesting
that they represent molecular "scars" recording the lineage's
bout of rapid molecular evolution at the ancestral speciation
event. The speciation event is therefore associated with
periods of high (molecular) mutation rate (which changes the
sequence at the high level rRNA chronometer) but following
speciation the mutation rate returns to normal and the high
level chronometer switches off leaving a fosilized signature
sequence for the descendent group.

Johnjoe McFadden
University of Surrey
Guildford, UK.
E-mail: mby134 at UK.AC.SURREY.SYSH



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