Evolution is often pictured as a slow, steady accumulation of small changes, a marathon run over millions of years. A revised model suggests something closer to a series of sprints: long periods of relative stability interrupted by sudden bursts in which many new lineages appear at once.
The pattern is visible in the fossil record, where groups sometimes proliferate rapidly after a major innovation or an ecological opening. It is also appearing in molecular data, as improved methods for dating evolutionary splits reveal clusters of branching events rather than evenly spaced ones. New lineages seem to arrive in pulses, then settle into longer stretches of incremental change.
Explaining the pulses is the harder part. Candidates include key innovations that unlock new ways of life, the collapse of competitors, and changes in climate that open and close habitats. Whatever the cause in a given case, the revised model makes a practical difference: it implies that the pace of evolution depends on context, and that a lineage's history may be dominated by a few decisive moments.
