evolution and speciation

Eric Cabot ecec at quads.uchicago.edu
Thu Oct 24 11:19:29 EST 1991

In article <stormo.688250251 at beagle> stormo at boulder.Colorado.EDU (Gary Stormo) writes:
>Some time ago there was a discussion about the definition of evolution.

   You raise a couple of interesting points. Chromosomal rearrangement
could likely lead to speciation. The most striking examples are probably
those of autopolyploidy resulting in new species. There are 
good examplse in Drosophila too; e.g. D. miranda probably diverged from the
D. pseudoobscura-persimilis lineage as a direct consequence of an 
autosome-Y translocation (although I can't actually *prove* this). 

>If this makes sense, and it seems possible at least in theory, then one
>must conclude that speciation and allelic changes are not causally
>connected.  Certainly allelic changes can occur without speciation and
>significant allelic change can accompany speciation, but it can also
>occur in the absence of significant allelic change. 

But to be fair, you must admit that it is possible for speciation to
be the result of so-called allelic changes some cases. (I'm not sure 
that the phrase "allelic changes" is an adequate alternative to chromosomal
rearrangement. How would we classify the creation of neo-morphic genes?)

We've been studying male sterility genes in D. simulans, D. mauritiana, 
and D. sechellia. At least at the polytene level, the chromosomes of members
of this species triad are almost completely homosequential, yet they 
are bona fide species.  What we see by replacing individual 
chromosomal segments with those of a foreign but closely related species
is male sterilty "caused" by genes that work just fine in their own 
genetic background.  Clearly this is evidence for genetic interactions
at what you might refer to as the allelic level.

>speciation which occurred in the absence of allelic change would then
>lead to rapid diveristy in alleles between the two species, simply because
>there is no homogenization occuring and the allele frequencies will change
>independently.  This means that after a relatively short time it would
>be difficult to determine whether the speciation was the result of or the
>cause of the allelic changes.  

A good point. In the work that I mention above, we -- and others -- have
uncovered several independent "factors" that cause the male sterility.
Which, if any of these, caused speciation in the first place? (Granting
agencies note: further study is required :-)

I might also mention that two of the three species that we are studying
are island species. Thus geographical separation may have provided
sufficient pre-mating isolation that would serve the same end 
as the chromosomal rearrangments that you are envoking. 
>Just some thoughts that I would like feedback on.
>Gary Stormo        |  "People will occasionally stumble over the truth, 
>MCD Biology        |   but most of the time they will pick themselves up
>Univ. of Colorado  |   and continue on."    -  Winston Churchill
>Boulder, CO 80309  |

Eric Cabot                              |    ecec at midway.uchicago.edu
      "Non Nobis Nati Solum"            |    NOTE: New address!

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