Re: How Can Evolution Change the Number of Chromosomes?


kruskal@xxxxxxxxx wrote:
It is my understanding that similar animals with different number of
chromosomes have a really hard time breeding fertile offspring.

This is, in general, incorrect for a *single* chromosomal
rearrangement. That is, for example, a case where two chromosomes have
fused to become one or one chromosome has undergone fission to produce
two. In this case, an individual heterozygous for the situation where
one chromosome has broken in two but all the information is present
will produce gametes with 1) both fragments of the fissioned
chromosome, 2) the unbroken original chromosome, 3) the unbroken
original plus one of the two fragments, and 4) the unbroken original
plus the other fragment. If s/he fertilizes an individual with a
normal gamete, both cases 1 and 2 would be 'normal', with case 1 being
another heterozygous individual, transmitting the broken set to the
next generation. Only cases 3 and 4 would have the extra fragment
(partial triploidy) likely to lead to serious phenotypic deficit
(although, for some organisms and some small chromosomes, this may be
less deleterious than you think). Theoretically, of course, the ratios
should be about 25% for each possibility. In reality, there is often
more of 1) and 2) and the abberant cases 3) and 4) often die early
enough in development so as to have little reproductive impact besides
a slight lowering of fertility.

If the frequency of inbreeding in the local population is sufficiently
high and you know that individuals can have more than one offspring, it
would not be unusual or impossible to generate homozygotes for a
fission set in subsequent generations. Such homozygotes would be
perfectly normal in meiosis, but would have a different chromosome
number.

Even multiple rearrangements can be viable. In fact the evening
primrose has two translocation sets that form a ring structure at
meiosis, with gametes either getting one set of (I believe) 11
chromosomes or the other set. The two sets have different recessive
lethal alleles which prevents formation of viable seed *except* for the
heterozygotes. That is, in this plant it is 'normal' to have 50%
lethalityin one's fertilized seeds.

In general, however, the greater the number of such chromosomal
rearrangement differences (including inversions) two independently
evolving groups of organisms have, the lower the possibility of their
producing fertile hybrids. Since the deleteriousness of single events
is overblown, it is quite possible for two isolated populations to
accumulate such single differences almost in a selectively neutral
fashion over time. The longer two populations are isolated from each
other, the more differences in chromosome arrangement will accumulate.
Then when the two populations re-meet, they will be reproductively
isolated. This may be an important mechanism of 'speciation' in the
biological definition of 'species'. It is quite clear that organisms
can differ dramatically phenotypically and still be able to produce
viable hybrids (mules, ligars, zebra/horse, dogs). The reproductive
isolation is somewhat of a separate event from phenotypic difference,
although the latter can provide (via hybrid dysgenesis) a selective
reason to *favor* the accumulation of isolating events and encourage
speciation that way. Reproductive isolation can be by mechanisms
unrelated to the selective differences between two populations.

Given
that, a spontaneous mutation which added or deleted a chromosome would
have to confer a unbelievable advantage to overcome the breeding
difficulty disadvantage. This is because only non-mutated animals
would be available for breading initially.

As mentioned, a heterozygous individual for chromosome rearrangement
can easily produce heterozygous offspring (and more than one) even when
all the available mates do not have the rearrangement (at some cost in
fertility, which can be quite small). Depending on the degree of
inbreeding, local breeding, chance, etc., the odds favoring the
formation of individuals homozygous for these traits is not all that
much worse than the chance of accumulation of other relatively neutral
traits in a local population.

Perhaps there is some sort of fractional chromosome which would permit
a slow evolution to a change in number. Perhaps there is some other
answer which I can't dream up.

Fortunately, we are not limited to your ability to dream up answers.
We have evidence to support viable answers.

I've been trying to come up with some scenario of this evolution. Not
a fossil record -- just a scenario. Can someone help me in this?

Thanks -- Vincent

.

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