Biological evolution is not gradual, but episodic, with long periods of stasis interrupted by bursts of rapid activity. ~ Stephan Jay Gould
In 1972, American paleontologists Niles Eldredge and Stephen Jay Gould proposed punctuated equilibria: that evolution occurs in spurts, with long stretches of stagnation. Substituting a stutter-step for a vector, punctuated equilibria is nonetheless of similar ilk of sophistically simplifying evolution with storytelling.
The idea of punctuated equilibria was an extension of mid-1950s speculations by German-born American evolutionary biologist Ernst Mayr. Mayr is generally credited with the modern definition of species. Rejecting reductionism in evolutionary biology, Mayr argued that evolutionary pressures acted on the whole organism, not genes.
American paleontologist Elisabeth Vrba’s turnover pulse hypothesis built upon the punctuated proposal. It suggested that evolution is nominally conservative, and that speciation occurs only when forced by environmental change.
An extinction event would be at the extreme end of an environmental pulse creating species’ turnover. Most environmental changes are much more modest.
The turnover pulse hypothesis ignores adaptations that are environmentally unforced: opportunistic evolution. Plants pioneered the land from coastal regions to take advantage of the sun and the air: rich inorganic sources of energy. Insects evolved on land to feast on available plant matter. Microbes evolve all the time for a variety of reasons, only some of which we comprehend, but all aimed at staying alive, which is the epitome of opportunity.
Primordial metazoa quickly radiated into 30–40 phyla. Archaic designs went extinct while new ones emerged. Animal evolution proceeded in waves and continues similarly.
Fundamental evolutionary change was not limited to an early burst of evolutionary experimentation. Animal designs have continued to evolve to the present day – not gradually as Darwin predicted – but in fits and starts, episodically through their evolutionary history. ~ English paleontologist Philip Donoghue