🚀 go-pugleaf

In article <2052@arthur.cs.purdue.edu> (Colin Jenkins) writes:
>> I imagine speciation in the following way (here I am way out of my line):
>> Some event occurs (eg. a chromosome re-arrangement) such that an offspring can
>> not mate productively, or, at least not have fertile offspring, with any
>> members of the group except its parent or perhaps its siblings.
>> Offsping of these matings are also constrained in there mating.  
>
>On this point (and I am just as far out of my line at this point) I have 
>trouble because such a breeding incompatibility would probably only come about
>through radical genetic changes, rather than small ones.  I don't think that
>evolution supports an idea of radical change, rather gradual changes selected
>by the environmental factors to be passed on through reproduction.

Evolution "supports" whatever mechanisms are needed.  I know of no evolutionist
that requires ALL changes to be small and gradual.  In particular, the
speciation "event" most likely is quite radical (environmental if not genetic).

>
>The other problem with this thought is that it seems to presuppose that the
>mutant will be somehow aware of its reproductive incompatabilities and make
>the fortunate choice of breeding with its relatives.  More complicated is the
>fact that the none of the other relatives may have received the mutations, so
>they should have just as much difficulty mating with the mutant as any other
>normal organism.
>
Here comes my yearly description of the comparison of chromosomal arrangements
among the great apes (which includes man).  If you have heard this before,
there is no need to read the rest, you can just send your flames automatically.

I have before me a diagram (SCIENCE 1982, 215:1525-1530) which shows gives a
comparison of the banding patterns of humans, chimpanzee, gorilla, and
orangutan.  There are several remarkable things about the diagram.

1)	The chromosomes match up very nicely with the exception of human
chromosome #2, which matches up with TWO chromosomes of each of the other
great apes.  This suggests that a our #2 is a fusion of two chromosomes of
our (the great apes) common ancestor or that the great apes chromosomes arose
from a fission of on chromosome from our common ancestor (and humans retained
the single chromosome).
2)	If you look at each chromosome, the banding patterns are quite similar.
However, for all but the smallest chromosomes (19-Y) there are obvious
inversions.  Now recalling your elementary genetics class, the important
features to remember about inversions are:  Inversions have no detectable
effect on the individual with an inversion (with the possible exception of
regulatory groups, etc); Individuals who are heterozygous for the inversion
(that their two chromosomes have one "normal" and one inverted) have a much
lower reproductive fitness (crossing over within an inversion lead to large
chunks of chromosomes either deleted or duplicated).

What this means is the populations with small effective size can quickly
become effectively reproductively isolated from their neighbors -- one of
the conditions for speciation to occur.  (Obviously speciation can occur
in other ways -- this describes just one way).
-- 
Ted H. Emigh, Dept. Genetics and Statistics, NCSU, Raleigh, NC
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