Tree of life reveals clock-like speciation and diversification

TL;DR

This study synthesizes a comprehensive global timetree of life, revealing mostly constant diversification rates across species and suggesting that speciation is driven by random events rather than adaptive processes.

Contribution

It provides the first large-scale, bias-aware analysis of diversification patterns across the entire tree of life, showing mostly constant rates and clock-like speciation intervals.

Findings

Species diversity has been mostly expanding overall.

Diversification rate in eukaryotes has been mostly constant.

Time-to-speciation is approximately two million years across plants and animals.

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, approximately two million years, 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.