Re: Article] Complex genomes evolved by chance

Tim Tyler <[EMAIL PROTECTED]> wrote in
news:[EMAIL PROTECTED]: 

> Robert Karl Stonjek <[EMAIL PROTECTED]> quoted:
> 
> [ Re: http://www.biomedcentral.com/news/20031124/03 ]
> 
>> Genome complexity
>> Complex genomes evolved by chance
>> By Cathy Holding
>> 
>> The question of whether the evolution of large and complex genomes in
>> complex multicellular organisms is due to natural selection or simply
>> a function of chance has been the subject of considerable debate.
>> [...] 
> 
>> Laurence Hurst, professor of evolutionary genetics at the University
>> of Bath explained, "If we ask the question why might a new mutation
>> (a point mutation, an insertion, deletion, duplication, whatever) go
>> from rare (which at first it must be) to common (aka, fixation),
>> then, in principle, there are two answers: either selection favored
>> it or it got there by chance (drift)," he told The Scientist by
>> E-mail. "If a population is huge, it will take ages and many chance
>> steps for a given new weakly deleterious mutation to get to fixation.
>> In a small population, it takes just a few lucky steps." 
> 
> If we ask the question why might a new mutation go from rare to common
> then, in principle, there are at least three answers:
> 
> 1. selection favored it;
> 2. it got there by chance;
> 3. It was in linkage disequilibrium with something selection *did*
> favour. 
> 
> This isn't just a minor nitpick - the effect of the third point can be
> a large one.
> 
> Consider the fact that much constructive evolution takes place by
> duplication and then variation of one of the copies.  If a variation
> in one of the copies proves beneficial, then the rest of the
> duplicated section can be dragged along for the ride - through virtue
> of being linked to it.
> 
> Linkage is at its strongest when we are talking about whole
> chromosomes - where there is no easy way for unlinking to happen - but
> can be a substantial factor elsewhere.

Actually No. 3 could be argued to be the same as No.2, since the linkage 
disequilibrium is still due to "chance". This _is_ a nitpick - drift can 
only be significant in small populations while linkage can affect much 
larger populations. But I do not think linkage is totally immune from 
statistics - in large populations crossover will soon break the link. So 
potentially we have three different ranges of effect, in small, medium 
and large populations.  It would be nice to see if real populations show 
different rates and types of gene frequency change based on effective 
population size.  


Yours,
 Bill Morse