OPPOSING LEVELS OF SELECTION IN CAMERATE CRINOIDS

Species selection via differential origination rates opposes a trend increasing the simplicity of camerate crinoid cups. This simplification is mainly driven by a tendency for descendant species to be simpler than their ancestors, as indicated by a subclade test. Therefore, this macroevolutionary trend is a result of interactions of evolutionary processes occurring simultaneously at two levels of selection. If species selection were the sole driver of the this trend, camerates with complex cups would have the lowest net rate of diversification (origination minus extinction rate), while the simplest camerates would have the highest net rate. I found that complex camerates have the higher origination rates than simple ones, with no differences in extinction rates, thus yielding high net diversification in complex camerates and low net diversification in simple camerates. The trend goes in the opposite direction as expected from the rate differential. The fact that species selection opposes the main direction of the this trend has implications for measuring the relative importance of this process in other groups of organisms. Species selection must be studied directly and not be implicated simply by the exclusion of other possible mechanisms.