punctuated equilibrium: a standalone construct

Niles Eldredge and Stephen Jay Gould introduced punctuated equilibrium in 1972, in their chapter Punctuated equilibria: An alternative to phyletic gradualism, as a reading of the fossil record in which most species change little across long spans of time, a condition they called stasis, and the visible change instead concentrates in brief, geologically rapid episodes tied to the splitting of one lineage into two (speciation). They set it against phyletic gradualism, the older expectation that lineages transform slowly and steadily, and Gould and Eldredge revisited and defended the idea two decades later in a 1993 review in Nature. "Rapid" here is meant on the geological clock, thousands to tens of thousands of years, an interval that can look instantaneous between successive rock layers even though it spans many generations. The hypothesis is distinctive in reframing gaps in the fossil sequence not only as missing data but as a pattern to be explained, and in linking the tempo of morphological change to speciation events, a claim that can be tested against measured rates of change through stratigraphic sections.[1][2]

Unit

A macroevolutionary hypothesis about the tempo and mode of morphological change across geological time

Recovery means

Not applicable

Measured by

Assessed from stratigraphic sequences by quantifying morphological change over time and testing whether change concentrates at speciation events rather than accumulating steadily within lineages