Why meiosis?

Andrew J. Roger roger at evol5.mbl.edu
Wed Jun 18 05:44:57 EST 1997


MSP1234 wrote:
> 
> MSP1234 at AOL.COM
> 
> I know that this is probably a very basic question, 

You are right..it is a basic question. However, it is also
a question to which there is no widely accepted answer-- thus
it is an excellent question.


>but can anyone tell me
> why sexual reproduction came about in the first place during evolution?
> To me it doesn't make sense as in simple organisms, mitosis is more
> beneficial in the short term because your genes are going to be replicated
> faster and this outnumbering is more 'beneficial' to a parent's genome
> than short-term diverity.

I know only a little about this subject. But it is my understanding
that there several interrelated questions.

1) why meiosis?
2) why syngamy/karyogamy?

Together one might consider these as:

3) why sex?

Since asexual cell division is almost certainly the ancestral state
for eukaryotes, it seems likely that mitosis evolved prior to meiosis.
With this in mind, one can ask the question: why would meioisis evolve
when mitosis already existed?  There are many similarities in the
mitotic
and meiotic processes and it has been shown that homologous genes are
involved
in both processes. Thus meiosis probably evolved FROM mitosis.

My favourite rationale for this is in reference to ploidy cycles.  If 
an organism goes through many cyoles of DNA synthesis without mitosis
then it becomes polyploid. It is thought that this is not a particularly
good state for an organism to be in (the concept of mutational load
discussed by Kondrachov). Thus the organism needs a way of reducing
its ploidy. Meiosis could have evolved to serve this end.  

Later when syngamy and karyogamy evolved (possibly polyphyletically as a 
result of selfish genetic elements trying to further their own
reproductive
potential), the organism had a build in mechanism to reduce the ploidy
caused by syngamy/karyogamy-- MEIOSIS.  Recombination might have also
evolved as a result of selfish genetic elements.

Like I said, I don't know very much about this.  I point you to
the papers of Kondrachov, Hickey and Rose and Graham Bell (these contain
much
more detailed and probably more accurate accounts of the ideas I
mention.

Cheers
Andrew J. Roger



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