Dear all,

I am revising a manuscript for PLoS ONE; it's a modeling study concerning effects of mass extinction on phylogenetic tree balance.  It's very similar to Heard & Mooers (2002), but background extinction and diversity-dependence have been added to the model so that the modeled tree recovers to an equilibrium size and then undergoes turnover while evolution continues.

In the discussion, I wanted to be very clear about the limitations of the results, and the conditions under which they would apply, so I raised the following points that the effects of mass extinction on tree balance would not be enduring IF:

i) how traits and rates evolve remain unchanged after the mass extinction

ii) the tree cannot continue to expand indefinitely after either reaching its pre-extinction size, or converging on a new equilibrium size

iii) trait values, and the rates that depend on them, are subject to hard limits that result in eventual erosion of trait/rate variance

iv) there is no mechanism for isolating slowly-diversifying clades from very rapidly-diversifying ones.

And the reviewer's comment was:

"Which raises the obvious question of whether these conditions prevail for real clades! This seems like a real gap in the MS."

Stick foot in mouth time. :(

It seems to me like what they want is a point-by-point justification of each of these, with citations.  I think I have (or can easily come up with) references to support points II) and III), but I'm having trouble with point I) and especially point IV.  Basically, what the last point is saying is that slowly-diversifying clades can continue to persist if they're isolated in some way (spatial/ecological, temporal etc.) from more rapidly-diversifying related clades.  In the model as I have it, no such isolating mechanism exists and so clades with the slowest diversification rates inevitably disappear as the simulation progresses.  (NOTE:  for point I), the original study provided no biological justification as to why they maintained constant parameters after their mass extinction event.)

Can anybody help me out here?  Can you suggest any study that demonstrates for a real clade that more slowly-diversifying members within that clade are somehow isolated from more rapidly-diversifying ones?

Thanks in advance,

Gabe