"Dohpaz"  wrote in message
> Thanks, that helps a lot.
> However, if it is true that
> 1) Animals with different numbers of chromosomes cannot interbreed
> and
> 2) All animals are decended from a common anscestor
> and
> 3) Not all animals have the same number of chromosomes.
>
> Then there must have been at least one incident in which an animal (or
> animals) were born with a different number of chromosomes than the
> rest of their species. Would these animals be forced to mate with their
> equivalently mutated brothers and sisters? Or was one of my premises
> wrong?
>
Let's look at Down's patients. Down's syndrome is caused by an extra copy of
chromosome 21. Generally this is because the mother is old, and her egg
isn't dividing properly. However there are some instances of Down's running
in families.
Downs patients have many phenotypic differences from normal humans. In some
situations these might be an advantage - for instance an evil empire might
prefer Downs patients as slaves, and kill off the normals.

Down's women are generally fertile and have a 50% chance of having a Down's
baby. Down's men are generally infertile, but there has been one reported
case of a pregnancy (Sheridan et al, 1989).

This gives us a clue. Imagine that the evil empire creates a situation in
which it is adaptively advantageous to have Down's. For a long time you
would have a large number of  Down's babies born to Down's women by normal
men. Then a fertile Down's male arises, breeding true for Downs, and the
genotype spreads rapidly through the population.

Eventually the slaves would be all Downs, whilst people not living under the
evil empire would be normals.

Finally, what would happen would be that the extra copy of chromosome 21
would specialise. There's not much point having three copies of one gene, so
the extra copy would be free to mutate and perform different functions.

Thus you would get a change in chromosome number, without the need for a
male and a female to simulataneously mutate, find each other, and have
offspring.

Sheridan, R., Lierena, J., Natkins,S. & Debenham, P. (1989). Fertility in a
male with trisomy 21.Journal of Medical Genetics, 26, 294-298.
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