Re: Basic evolutionary theory question.


"g" <gillawton@xxxxxxxxxxxxx> wrote in message news:...

"Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx> wrote in message
news:ectvtg$2de5$1@xxxxxxxxxxxxxxxxxxxxxx

(snip)

In the case of species which have diverged less, one
possibility is chromosome re-arrangements. Suppose there
are three possible ways of ordering the genes in a particular
chromosome - call these three orders A, B, and C. Also
assume that there is no problem for individuals with
two A chromosomes, two Bs, or two Cs. Also no problem
with AB or BC. But individuals with one A chromosome and
one C chromosome are inviable or infertile. Now assume
that the original population has everyone as a BB. Split
the population in two. In one of the two separated
populations, there is a mutation in one individual changing
a B to and A. So now this population will have a mix
of BB and AB individuals, and a few generations later
also some AAs. Over time, this population could drift to
100% AA. Meanwhile, over in the other population, a
mutation from B to C occurs, and that population drifts
to 100% CC. At this point the two populations can no
longer interbreed because any hybrid offspring would be
AC and that is infertile or inviable.

And -- however logically remote -- is it not within the realm
of the possible that two branches of a species could become
disparate genetically to the extent that only rarely would two
individuals -- one from each branch-off -- could reproduce...
and THEN, due to some selective event (or events) one or
both lines of descent would become decimated and somehow
the survivors of each (somewhat distinct from the other) line
would be selected...

THEN, it does not bend logic to the breaking point to assert
that it would be POSSIBLE for the two lines (thus rendered
a higher statistical chance of reproducing, and few prospective
partners to copulate with, would RECONVERGE.

Whyyyyyyyyy not?

(:>)

g

Perplexed,

I really muddled the above.

Let me try again:

Suppose that species A were, due some isolation factor (geographical
or other), become genetically distinct to the extent that only a few
would have "reproductively compatible" gene combinations. Let us
call the two branches A-1 and A-2. And let us say (for sake or example)
that only one out of 20 sexual parings between members of A-1 and A-2
could produce offspring, while the remaining 19 out of 20 would
result in non-fertilization or an aborted fetus (or an individual which
would be incapable of fertilizing or becoming fertilized).

Now, suppose that TWO circumstances change, one change being
the isolation factor becomes removed (a geographical barrier, such as
a mountain range perhaps, is breached by an earthquake's leaving a
passage between separated areas).

Now, also, let some other factor change, such as, perhaps, the rain
patterns become altered by the change in the mountain range, and
there is much less rain. And let us suppose that those individuals
who have certain genes (or gene combinations) happen to be the
individuals who are inter-branch compatible breeders.

Now we have a situation in which survivors of both branches are
once again able to mix and mingle, together with a circumstance in
which a much higher percentage of mating choices are reproductively
compatible.

Hey, I know this is a lot of if's.

However, I also know that a billion years or so is a long time, and
hence inclusive of opportunities for a lot of permutations of logical
opportunities... and that the enormous number of species on Earth,
throughout such periods of time... enable a lot of opportunities for
varieties of mixings and minglings, and separations and reunions...

I have a feeling that out of all those opportunities... even if it had
one chance in a thousand of occurring... probably has occurred
somewhere.

I mean, this is statistically comparable to saying that if getting an
Ace-high spade straight flush is enormously statistically unlikely --
given enough "runs" it becomes unlikely, also, that it would
NEVER occur.

There... maybe that is more clear.

(:>)

g


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