Re: How Can Evolution Change the Number of Chromosomes?


kruskal@xxxxxxxxx wrote:
hersheyhv wrote:
Some sequences have two chromosome structures they can
equally well arrange themselves into.

So you can have a gene sequence which can only arrange itself into
chromosomes one way.

This seems to be confused. Chromosomes, at the level of its DNA, is
basically an unbroken double helix having genes present in a specified
order. Genes ordinarily do not re-arrange themselves (certain
transposable sequences excluded), but chromosomal DNA strands can break
and reform so that the order of the genes becomes changed.

It can evolve into one which can arrange itself
in two ways. So for a while, you have a population with two chromosome
numbers all interbreeding with each other. As more evolution happens,
you can again have a gene sequence which can arrange itself only one
way -- with a different number than in the beginning.

This occurs in a stepwise fashion, not all in a single massive amount
of rearranging. No single rearrangement prevents interbreeding
completely, but the accumulation of a number of events can. Remember
that if you have a population that has one set of chromosome numbers
and an isolated population with a different set, and they now meet,
hybrids between them will have a *reduced* fertility. That is, there
is hybrid dysgenesis (hybrids are less reproductively fit than either
parent population). The result is either extinction of one of the
populations or more complete reproductive isolation by selection *for*
any changes that increase reproductive isolation. IOW, partial
reproductive isolation, even if a chance event, can lead to further
reproductive isolation for selective reasons. If the two populations
have accumulated other differences that allow them to exploit slightly
different niches, it is possible that both can survive. Otherwise, one
will probably eventually be driven to extinction, even if by chance
alone.

OK, let me try again to net out what I am hearing as the answer to my
original question -- as you requested.

A simple spontaneous change in number of chromosomes -- a break or
merger which does not involve a change in the genes or their order --
will not effect fertility.

It generally *will* affect fertility negatively, but will not
absolutely *prevent* transmission of that changed chromosomal structure
to the next generation. What is possible is a balanced transmission.

And in some cases, will be passed on to
offspring even when mating with nonmutants.

Yes. As long as you can form a gamete that has all the genes and no
major duplications/deletions of genes, that gamete can fuse with a
nonmutant gamete and produce a phenotypically normal (or near normal,
since some small duplications/deletions are survivable and not
particularly harmful) offspring which would also be heterozygous.

Such a mutation will not, by itself, confer a survival advantage but it
might make future genetic mutations more likely which might.

No. The occurrence of future mutations are unaffected by past
mutations. Selection because of heterozygote dysgenesis would *favor*
the acquiring by selection of other (spontaneously occurring) mutations
that further reduce waste of sperm/egg on unfit hybrids, but would not
cause them.

Thus a
population can exist for a long time with different number of
chromosomes waiting for some future mutation which will split them into
two species or wipe out the original form.

Usually the two populations are initially isolated by some other
barrier (time or space). This allows the two populations to drift
apart or become better adapted to their local conditions. When the two
populations then meet again, they may have already acquired traits that
make for hybrid dysgenesis. Chromosomal rearrangements are one such
mechanism.

Now do I have it?

.

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