Data from: Standard sister clade comparison fails when testing derived character states

Comparing species richness in sister clades that differ in a character is
one of the ways to study factors influencing diversification. While most
of its applications focused on traits that increase diversification, some
were used to study the association of a trait with less species richness,
e.g. the occurrence of dioecy in flowering plants. We show here, using
simulations and an analytical model, that the null expectation of equal
species richness that is generally used in sister clade comparisons is
wrong in the case of a derived trait occurring independently from
speciation: one should expect less species in the clade with the derived
character when there is no difference in diversification rates. This is
due to the waiting time for the derived state to appear, which causes it
to occur more often on longer branches. This has the important implication
that the probability for a clade to possess the derived state depends on
the tree geometry, and thus on species richness: less species-rich clades
are more likely to possess the derived state. A test for differences in
species richness should be based upon observed tree imbalance. We present
a method to do this, and find that we cannot confirm earlier work that
concluded that dioecy decreases diversification; on the contrary, it seems
to be associated to higher species richness than expected.