Tree of Life Reveals Clock-Like Speciation and Diversification

TLDR

Genomic data are rapidly resolving a time‑calibrated tree of life that provides a framework for diverse research, while prior restricted timetrees have shown declining diversification rates attributed to ecological interactions. The authors synthesized a global timetree of life from 2,274 studies covering 50,632 species, examined diversification patterns and speciation timing, and mitigated biases such as low taxon sampling, small clade size, and stem branch inclusion. The study shows that species diversity has largely expanded with a mostly constant diversification rate in eukaryotes, that time‑to‑speciation is consistently about 2 million years across plants and animals, and that these clock‑like patterns imply speciation and diversification are driven by random events while adaptive change is separate.

Abstract

Genomic data are rapidly resolving the tree of living species calibrated to time, the timetree of life, which will provide a framework for research in diverse fields of science. Previous analyses of taxonomically restricted timetrees have found a decline in the rate of diversification in many groups of organisms, often attributed to ecological interactions among species. Here, we have synthesized a global timetree of life from 2,274 studies representing 50,632 species and examined the pattern and rate of diversification as well as the timing of speciation. We found that species diversity has been mostly expanding overall and in many smaller groups of species, and that the rate of diversification in eukaryotes has been mostly constant. We also identified, and avoided, potential biases that may have influenced previous analyses of diversification including low levels of taxon sampling, small clade size, and the inclusion of stem branches in clade analyses. We found consistency in time-to-speciation among plants and animals, ∼2 My, as measured by intervals of crown and stem species times. Together, this clock-like change at different levels suggests that speciation and diversification are processes dominated by random events and that adaptive change is largely a separate process.