On 19-7-2009 19:09, clipka wrote:
> andrel <a_l### [at] hotmailcom> wrote:
>> You are not able to interbreed with chimps and bonobos. Not only because
>> you find them (probably) unattractive but also because our genes do not
>> match. Somewhere along the line two chromosomes fused to one bigger one,
>> so that we now have 23 pairs of them and the chimp still 24. There are
>> also some other large reshuffelings of genes that will prevent mixed
>> offspring. How the 46/48 chromosome hybrids ever were compatible is a
>> thing that amazes me everytime, but that is something for another
>> discussion.
> 
> But I guess this is exactly the point creationists will make.
> 
> In order for evolution to work with sexual reproduction to produce new species,
> there *must* always be a phase where the two subspecies still *can* interpreed
> (genetically), but are less *likely* to do so successfully, because either...
> 
> (a) they're genetically fully "compatible" with one another but separated into
> different populations, or
> 
> (b) they're *slightly* less "compatible" with one another (e.g. having a higher
> rate of birth defects, or invariably having offspring that have a minor genetic
> disadvantage).
> 
> In case of a basic change of the genetic structure, like a different number of
> chromosomes, this appears quite amazing, because obviously it cannot evolve
> over time so (a) cannot play a role, and (b) seems unlikely due to the dramatic
> proportions of the change.
As Darren pointed out, a different number of chromosomes may have severe 
consequences when that is because one chromosome did not make it to the 
next generation or if there is a duplication. (if there is down there 
must also have been fertilized eggs with one chromosome 21 missing. I 
think we can safely assume that is not compatible with life). Luckily in 
our case it was a fusion of two chromosomes into one.
http://www.don-lindsay-archive.org/creation/translocation.html

> 
> BTW, the compatibility of the 46- and 48-chromosome population is very unlikely
> to have been a problem: They would invariably have carried 23+24 chromosome
> (i.e. 47 chromosome). They *must* have been compatible to some degree, because
> the first mutation will in all likelyhood have affected only *one* chromosome
> set, producing such a 47-chromosome individual.
> 
> This 47-chromosome individual must have proven successful enough to spread this
> trait (with only 48-chromosome individuals available as partners, half of its
> offspring would have had the regular set of 48 chromosomes, the other half 47
> chromosomes, though there's reason to expect a higher percentage of
> miscarriages or other complications in the 47-chromosome offspring), and its
> offspring likewise, so that the 47-chromosome population could become large
> enough to mate among themselves.
As incest is not common among chimps, we need at least two successful 
generations of 47 chromosomes.


> Only thus was it possible to produce the first 46-chromosome offspring (47/47
> parents would have 25% 48-chromosome, 50% 47-chromosome and 25% 46-chromosome
> offspring; again, the percentage of offspring actually born is likely to have
> been different).
yes, they were probably different.
> 
> So intially the 47-chromosome population must have been comparatively
> successful.
not necessarily. I also like to speculate on what happened, the problem 
is a bit that we are reasoning from the fact that we know that in the 
end humanity arose. Take also into account that the bottleneck between 
chimps and humans is estimated at at least 1000 individuals, then it is 
clear that interbreeding must have been possible for some time (and that 
that bottle neck may not have been at this point in time).

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