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Ferns have a high polyploid index (41.7%), meaning that nearly half of
recent changes in haploid chromosome number have occurred via polyploidization.
To calibrate this measure per speciation event, we estimated the fraction of
speciation events in ferns associated with a change in chromosome number, by
totaling the minimum number of chromosome changes found within each genus,
i.e. Sgenera(unique n values per genus minus one), and dividing this by the total
number of speciation events represented, i.e.
Sgenera(# species per genus minus
one). They ignored genera with a chromosome count from only one species. This
procedure indicated that ~16% of recent speciation events in ferns involved a
change in chromosome number. Because chromosome numbers are known from
only ~15% of fern species (L¨ove A, L¨ove D, Pichi Sermolli REG.
1977. Cytotaxonomical Atlas of the Pteridophyta.
Vaduz, Germany: Cramer), this estimate is necessarily coarse. Furthermore,
there will generally be more chromosome changes and speciation events in the
history of these taxa than estimated above. Keeping in mind researchers' uncertainty, they
estimate that ~7% (=41.7%×16%) of speciation events involve polyploidization
in ferns. |