Biodiversity today is huge, and it has a long history. Identifying rules for the heterogeneity of modern biodiversity-the high to low species richness of different clades-has been hard. There are measurable biodiversity differences between land and sea and between the tropics and temperate-polar regions. Some analyses suggest that the net age of a clade can determine its extinction risk, but this is equivocal. New work shows that, through geological time, clades pass through different diversification regimes, and those regimes constrain the balance of tree size and the nature of branching events.