The Paradox of Stasis?

The word "stasis" entered our consciousness when Gould and Eldredge promoted the concept of punctuated equilibria (see Darwin on Gradualism ). Their observations of the fossil record showed that the morphology of many species remains unchanged for millions of years. Change occurs during speciation by cladogenesis when the new daughter species fixes morphological change, thereby allowing us to recognize it as a different species.

Stasis is an important observation. Here's how Gould describes it in The Structure of Evolutionary Theory (p. 759).

Abrupt appearance may record an absence of information but stasis is data. Eldredge and I became so frustrated by the failure of may colleagues to grasp this evident point—though a quarter century of subsequent debate has finally propelled our claim to general acceptance (while much about punctuated equilibrium remains controversial)—that we ureged the incorporation of this little phrase as a mantra or motto. Say it ten times before breakfast every day for a week, and the argument will surely seep in by osmosis: "stasis is data; stasis is data ..."

Okay, we get it.

What causes stasis? There are many possible explanations. One of the most common is stabilizing selection or the idea that a species is so well-adapted to its environment that any change will be detrimental. According to this view, evolution only occurs when the environment changes.

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