A pair of papers:
«Kinetochore reproduction in animal evolution: Cell biological explanation of karyotypic fission theory» and «Kinetochore reproduction theory may explain rapid chromosome evolution».
Recall that a kinetochore is the structure that attaches to the centromere of chromosomes during (eukaryotic) cell division.
I found these papers when reading Quammen’s «The Song of the Dodo». He somewhat outrageously backs up Mayr and says that sympatric evolution never happens. My first instinct is to think that surely sympatric evolution happens whenever chromosome number changes? So I started to look for stuff about chromosome number change.
These papers discuss one mechanism by which chromosome number might change: extra kinetochores get added during cell division, a chromosome with two kinetochores splits into two. Specifically, the diploid number might double by having all the (metacentric) chromosomes fission. It turns out, much to my surprise, that there is good evidence to believe that two fission-product chromosomes can match up (synapse) with a copy of the original unsplit chromosome. Two one-armed chromosomes match up with a normal two-armed chromosome. Fissioned pairs can be heterozygous with unfissioned singletons in a non-selective way throughout a population.
So the thing that historians of science will like is that some guy, Todd, came up with a fissioning theory years ago, but everyone ignored him. What modern biology brings to the table is a deeper understanding of cell function and therefore a plausible mechanism by which this might happen. Tension sensitive dephosphorylation, this is very cool.
So, I learn some Cell biology, Todd gets his moment in the sun, but no, I haven’t yet shown that chromosome number change is an instant slapdown for “sympatric evolution never happens”.
When you say that Quammen says that Mayr says that “sympatric evolution never happens”, do you mean speciation rather than evolution?
Yes.
Wikipedia has some examples of sympatric speciation. If that article is a fair summary, then the answer seems to be, it’s rare, it’s tricky to establish, but it does happen.