What are you looking at?

Each panel simulates a birth-death process starting from a single lineage. At any moment, each living lineage can speciate (split into two daughters, rate λ) or go extinct (rate μ). The two simulations use identical parameter values — but because speciation and extinction are stochastic events, the resulting trees are completely different every time.

Gold branches are lineages that survive to the present. Grey branches went extinct before the end time. Red dots mark extinction events.

This variability is why two clades evolving at the same rates can end up with very different numbers of species, and why detecting shifts in diversification rate requires careful statistical methods — a lot of the apparent signal can be noise. Try increasing μ toward λ (high turnover) and watch how often complete extinction occurs. Try low μ and high λ for explosive radiations.

The net diversification rate is r = λ − μ. When r is small or negative, trees frequently go extinct entirely — even with a positive expected trajectory, any single realization can die out by chance (the stochastic extinction problem).