Further comments on single gene vs. chromosomal changes in speciation:
What seems to be interesting about this issue is that there are different
lineages where karyotypic changes seem to be common and play a major role
in isolation, whereas other lineages have all sorts of rearragements with
little evidence for isolation (see above comments by Steve Schaeffer and Eric
Cabot). It is important here to distinguish between inversions and
translocations of whole chromosome arms. The latter should have more effect.
M. J. D. White studied orthopterans (Grasshoppers, crickets etc.) and was
the strongest proponent of "chromosomal" speciation. Robert Baker from Texas
has also written about the important role of chromosomal changes in
mammalian speciation. I may be wrong, but most of these well documented
cases involve transolcations rather than inversions.
It would be interesting to some Coyne-Orr-Wu et al studies in Orthopterans
to see what the relative contributions of single-gene, gene interatcions
and chromosomal changes play in the isolation of species. Obviously the
genetics of most grasshoppers and crickets are sufficiently primitive that
it is almost impossible to do this.
So, in short, while Eric Cabot and other Wu lab-mates were disturbed to
find that the answer to the GRE question about species isolation is
"chromosomal changes", the issue really depends on what the other possible
answers were (e.g., "unhappy childhood?" or "bad attitiude?" or "point
mutation?", or "none of the above?")
For some groups, chromosomal changes IS the best answer
David M. Rand
Department of Ecology and Evolutionary Biology
Brown University
David_Rand at brown.edu
