Punctuated equilibrium is a hypothesis of the mechanism of evolutionary change, put forward in 1971 by Niles Eldridge and Stephen Jay Gould then of Columbia University, New York, which disputes the standard Darwinian idea (see theory of evolution) that major adaptations and the appearance of new species come about through the slow accumulation of small random changes. Punctuated equilibrium posits that species are relatively stable over long periods of time, but undergo rapid bursts of change, leading to the sudden appearance of new species, when genetically distinct subgroups become geographically isolated.
In 1999, US paleontologist Jeffrey Schwartz, of the University of Pittsburgh, strengthened the case for punctuation with his suggestion1 that new species arise abruptly because of mutations in the genes that control the development of embryos. If such mutations occur in several individuals which then inter-breed, they may lead to a new taxonomically distinct organism. Schwartz cites the case of the origin of the backbone as a notochord in the free-swimming larva of sea squirts. The growth of a notochord is triggered by a specific gene, called Manx, that regulates embryonic development. If this gene is deactivated, the larva remains as a sessile, notochord-less animal. According to Schwartz, a minor random variation in the gene that regulates Manx could have led to a subgroup of sea squirts with primitive backbones and so to the evolution of all vertebrates (including ourselves).
1. Schwartz, J. "Homeobox Genes, Fossils, and the Origin of the Species," The Anatomical Record, 257, 15 (1999).