Stephen Dryden's note on "directed" evolution

DRBH at DB1.CC.ROCHESTER.EDU DRBH at DB1.CC.ROCHESTER.EDU
Fri Mar 12 13:00:09 EST 1993


Dryden's note on Directed mutations is likely to produce the usual
storm that is associataed with this topic, so I thought I'd set a few
things straight from the beginning.
(1) Selection-induced (directed) mutations really have nothing to do
with Lamarkian theory at all.
(2)  The observation in both bacteria and yeast is that during prolonged
periods of fairly intense selection, which probably produces considerable
cellular stress, mutations occur that are useful because they relieve
that particular stress (usually either amino acid or carbon source
starvation).
(3)	Those mutations can occur in non-dividing cells, and are time-dependent,not replication dependent.
(4)	The mutaions are specific to the environmental challenge being imposed
by the selective conditions in that (a) they only occur when they are
useful to the cell, and (b) they only appear to occur in the genes that
are under selection; i.e. the selectiove conditions are not GENERALLY mutagenic,they are only mutagenic with respect to the locus under selection.
(5)	These selection-induced mutations can be the result of missense
mutations, positive and negative frameshift mutations, or excision of mobile
genetic elements (when such excisions restore the function of a gene
that is under selection).  
(6)	Selection-induced mutations have been shown to occur in at least 8
loci in bacteria, and three loci in yeast.
(7)	The spectrum (relative frequencies of various missense and frameshift
mutations at the same site within a gene) of selection-induced mutations is 
different from the spectrum of mutations at those same sites by random
mutations in growing cells.
(8)	At least three models have been advanced to explain the specificity
of selection-induced mutations on the basis of underlying random events.
Data inconsistent with each of those models has been obtained.
(9)	As far as I am aware, no one has proposed and tested any additional
molecular models at the present
	Summary: the phenomenon is real, but we don't know how it works yet.

(10)	Cairns is not, and never was as far as I know, on the faculty at
Princeton.  Until recently he was at Harvard, and is now retired and living
comfortably in England.  Pat Foster, of Boston U. Medical School, continues
the studies that she ahd been conducting in collaboration with Cairns.

(11)	The existene of selection-induced mutations is not universally accepted.	
	The above is unlikely (I hope) to stem any discussion of the issues
raised by Mr. Dryden, but I hope that they will allow discussion to focus
on the real issues, rather than focusing on the political issue of
Lamarck vs Darwin.  I would point out that the existence of selection-induced
mutations in no way contradicts Darwinian theory.  It simply adds another way
for variation to be introduced into populations; a way that will have to
be taken into account as we try to understand the process of adaptive evolution.
Barry G. Hall
Biology Dept.
University of Rochester



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