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.


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.

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).

So intially the 47-chromosome population must have been comparatively
successful.

It is interesting then to think that this successful population would die out to
separate the 46- and 48-chromosome population from each other; but I guess this
can be explained by the acquisition of other mutations, some of which may have
been beneficial for one sub-population but slightly unfavorable for the other;
this would paved the way for other mutations causing the bearer to consider one
sub-population more attractive than the other, or looking more attractive to one
sub-population than to the other, which would start reducing genetic interchange
between the two populations; this in turn would make it more acceptable for a
mutation to be unfavorable for one group, as it would automatically be less
common in that one; again, this would increase the benefit of mutations to
reduce interbreeding. Ultimately, mutations prevalent in each group might be so
severe to the respective other, that it would be more beneficial - for each of
the groups - to render any 47-chromosome offspring infertile, and a mutation to
this effect would stick, finalizing the genetic barrier between the two groups.
Later mutations would round off this process by making it impossible for a
female to conceive from a member of the other group, so as not to waste any
energy at all on raising infertile offspring, and by making the groups
unnattractive to each other (maybe even particularly ugly) so as not to waste
any energy on even *trying* to interbreed in the first place.

The 47-chromosome population, being the most unstable one by having only 50%
offspring carrying the same trait, would in all likelyhood be unable to sustain
itself, declining in numbers as the main groups cease to interbreed. To develop
a separate population they would have had to accumulate mutations that would
separate their gene pool from that of both 46-chromosome and 48-chromosome
bearers - but this would have included half of their own offspring in each
generation, and therefore wasn't an option.

Post a reply to this message