Punctuated Equilibrium

Punctuated Equilibrium, theory in biology that evolutionary change is rapid during the origin of new species and that little or no evolutionary change occurs at other times (periods of equilibrium). The American paleontologists Niles Eldredge and Stephen Jay Gould first proposed the theory in 1972.

Eldredge and Gould argued that evolutionary change is mainly concentrated in rapid speciation events that punctuate the history of life. They suggested that once species have evolved they change little, a condition they called “stasis.” A new species is also more likely to arise in small, isolated populations than in the main population of an existing species.

Eldredge and Gould contrasted punctuated equilibrium with what they called “phyletic gradualism.” Phyletic gradualism describes the pattern in which evolution has a relatively constant rate both during speciation and at other times. A new species results from gradual accumulation of many small genetic changes over long periods of geologic time, not from a relatively rapid event.

Eldredge and Gould supported the theory of punctuated equilibrium with fossil evidence. The fossil record often shows species persisting for long periods with little change. It also often shows the sudden origin of new species, with few or no intermediate fossils between the new species and its ancestors. Eldredge and Gould maintained that the fossil record shows the real pattern of evolution. However, other researchers pointed out that the apparent pattern in fossils could reflect gaps in the fossil record. Also, the taxonomic procedures scientists use to assign particular fossils to named species may not reflect the true identity of the fossil animals.

The theory of punctuated equilibrium has stimulated much more rigorous research on the fossil evidence concerning speciation. Since 1972 many studies have been made about the rate of evolution. The pattern found for Caribbean bryozoans seemed to show punctuated equilibrium, while Welsh trilobites appeared to show phyletic gradualism. Other studies show a complex mix of gradual and punctuated changes in species over time.

The topic has been controversial because of claims, made more by Gould than by Eldredge, that the theory of punctuated equilibrium contradicts some of the ideas of Charles Darwin and many of Darwin’s followers. Gould also suggested that punctuated equilibrium implies that macroevolution (large-scale evolution) proceeds by different processes from microevolution (short-term evolution) within a species. Evolutionary theorists are divided about these theoretical issues, but this debate does not detract from the influence punctuated equilibrium has had on paleontological research.

Gould put his ideas about evolution into a book titled The Structure of Evolutionary Theory (2002). The section of the book on punctuated equilibrium was later extracted as the stand-alone book Punctuated Equilibrium (2007).