Plants have diversified at almost twice the rate of animals, and animals and plants have accumulated new species some ten times faster than prokaryotes such as bacteria.

Across the tree of life, some groups have many more species than others. To find out why, Joshua Scholl and John Wiens at the University of Arizona in Tucson collated published data on the number of species and their phylogenetic relationships in each group of living organisms. Contrary to some hypotheses, older groups did not have more species than young groups. Instead, the authors found that the balance of speciation and extinction over time, known as the diversification rate, determined most differences in species number between groups.

Ecological and evolutionary differences between the kingdoms of life could explain differences in diversification rates, the researchers say.

Proc. R. Soc. B 283, 20161334 (2016)