news user  wrote or quoted:

> 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 ?
> 
> Man has 23 chromosome pairs (46 chromosomes) and a chimpansee has 24 pairs
> (48 chromosomes). Gametogensis and the fusion of sperm and oocyte is a
> delicate process for which the distribution of equal pairs of chromosomes
> is important. A change in the number of chromosomes in one individual will
> isolate it from sexual transmission of its genes to the next generation as
> far as I can see ?

``Some may raise the objection that if the fusion was a naturalistic 
  event, how could the first human ancestor with the fusion have 
  successfully reproduced? We have all heard that the horse and the donkey 
  produce an infertile mule in crossing because of a different number of 
  chromosomes in the two species. Well, apparently there is more to the 
  story than we are usually told, because variations in chromosome number 
  are known to occur in many different animal species, and although they 
  sometimes seem to lead to reduced fertility, this is often not the case. 
  Refs 5, 6, and 7 document both the existence of such chromosomal number 
  differences and the fact that differences do not always result in 
  reduced fertility. [...]''

 - http://www.gate.net/~rwms/hum_ape_chrom.html
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