evolution and speciation

Gary Stormo stormo at boulder.Colorado.EDU
Wed Oct 23 15:37:31 EST 1991

Some time ago there was a discussion about the definition of evolution.
While a consensus did not emerge there was considerable support for any
change in allelic frequencies.  There have also been discussions of the
mechanisms of speciation and how that fits within evolution.  A common
conception is to equate speciation with macro-evolution, arising when
"sufficient" micro-evolutionary events (allelic changes) have occured 
that two organisms can no longer mate productively.  While this may be
the normal situation, it occured to me that it need not be that way.
While daydreaming during a recent meeting the following experiment
occured to me (perhaps this has been done, anyone know?).  
Given the large number of chromosome translocations and inversions
that exist in Drosphilia, one ought to be able to construct some flies
whose offspring are competent to mate with each other (or at least a 
subset of their sibs) but would have very low fertility in matings to
any other strains, even the parental.  If so, these would represent a
new species, by the usual definition, in which there has been no change
in allelic frequencies (at the gene level, I'm not counting a rearranged
chromosome as a new "allele").  

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.  I would expect that
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.  

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  |

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