On 11 Apr 2002 22:20:18 -0700, jim.pamplin at relaypoint.net (Jim
Pamplin) wrote:
>I am curious to know if there is any known mechanism by which a
>microbial line
>of descent can eliminate sublethal mutations once they occur.
It's called natural selection. Bacteria with too many of the
deleterious mutations won't grow as well.
>In
>sexually
>reproducing popultations, independent segregation of chromosome
>segments allows some genes to not be passed on. Also, those
>recombinants containing the largest numbers of deffective genes are at
>higher risk of genetic extinction than those with few. These
>mechanisms allows sublethal mutations to be slowly decremented from
>the population.
>>But, what happens in asexually reproducing organisms? My limited
>reading on the
>subject of conjugation (the microbial version of sex) does not suggest
>a
>comparable mechanism. If I understand correctly, when one cell inserts
>some of
>its chromosomal DNA into another, it retains its own copy.
No. Only one copy is retained, either the original or the new one.
(the inserted copy cannot replicate unless it recombines with the host
genome, an event which would normally lead to replacement of the
original.)
But I am not sure that this plays a big role in general.
>So there is
>no loss by indepenndent segregation.
>>As I currently see it, each microbe stands at the end of an unbroken
>line of
>descent tracing back to its first ancestor. Along the way, multiple
>donars may
>have inserted additional DNA. All of its genes are said to have
>arisen by a long series of mutations, filtered by naturally selective
>processes. But beneficial mutations are considerably less frequent
>than
>deleterious ones. So, either A) Every living microbe comes from an
>incredibly
>LUCKY line of descent, receiving far more beneficial mutations than
>detrimental
>ones, or B) Sublethal mutations are flushed from lines of descent by
>some
>mechanism, or C) ...?
>>Thanks in advance for any insights or clarification you might offer.
Look up "periodic selection". Remember that microbial populations are
large, thus contain much diversity.
bob
