curtadams{at}aol.com (CurtAdams) wrote in
news:cgj69n$2cj7$1{at}darwin.ediacara.org: 

> news{at}sisyphus.news.be.easynet.net writes:
 
>>I have a question about how through evolution the number of
>>chromosomes can change. When the number of chromosomes changes by
>>accident, how is this transmitted to the next generation and so
>>creating a new species ? 

> That said, it's still a mystery how many chromosomal changes happen. 
> Although heterozygotes are normally quite fertile, they do experience
> a fertility reduction
> and it doesn't take much to prevent such a mutant from ever getting
> fixed. Some changes, mainly translocations, always involve a big hit. 
> The old idea was
> that chromosome changes squeeze through in small populations (Wright's
> shifting balance) but the numbers don't work.  Ideas include
> hitchhiking (changes accidently include rare beneficial genes)
> position effects (changes beneficially alter nearby gene expression)
> and meiotic drive (altered chromosomes
> make it into gametes more often) but there's not much evidence for any
> of these.

Perhaps you could give a broader explanation of why "the numbers don't 
work" for chromosome changes via squeezing through in small populations. I 
ask because I think that some of the discussion of the shifting balance has 
assumed a steady state environment, ignoring the effect of events that are 
severe but (from an evolution standpoint) frequent.


Yours,

Bill Morse
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