Declining Phanerozoic background extinction rates: Effect of taxonomic structure?

A decline in the total and per-family extinction rate of marine families^1,2 during the Phanerozoic has been attributed to progressive improvements in the ecological properties of species. We believe this is not necessarily the case and suggest that the lower family extinction rates may reflect increases in the number of species per family, on the geological time scale, with higher species/family ratios being an inevitable consequence of the increase in species richness³ and the geometry of the branching evolutionary tree⁴. Because species-rich families are more likely to survive the stochastically constant background probabilities of species extinction, the biosphere has gradually accumulated species-rich clades and the total and per-family rates of extinction have consequently declined.