Stem and crown group dynamics

For my master’s thesis I simulated evolutionary trees using birth-death models with variable speciation, extinction and diversification rates in order to examine their putative effects on the timing of clades’ crown group emergence.

The crown group of a clade is defined as a its most recent common ancestor together with all of its descendents. Stem groups are the extinct branches of the clade’s lineage that separate it from other clades and crown groups.

Given that the early origin of animal phyla is held as significant in research on the Cambrian explosion and early animal evolution, it is useful to investigate under which conditions we might expect early or late crown group emergence.

My work showed that absolute values of neither speciation nor extinction affect the timing of crown group emergence in a clade, providing the diversification rate, i.e. the difference between speciation and extinction rates, remains zero. In such instances the crown group emerges at the center of the lineage, on average.

However, with increasing rates of diversification, i.e. increasing rates of speciation relative extinction, the crown group emergence point moves further back in time along its lineage. These results are significant as they call into question the significance of the early arrival of the crown group of extant phyla and suggest that this may simply be an inevitable artifact of evolution, which, after all, requires speciation rates to be higher than extinction rates in order for extant clades to be extant.

The results of this study are published here [pdf].