Chapter 5: Life's Pulses

1. Niles Eldredge, in the late 1960s, wanted to reconfirm Darwin's axiom that organic change happens when species, responding to shifting environmental pressures, gradually evolve into another species. But, by studying trilobites, an extinct ancestor of the horseshoe crab, he could find no evidence.

2. Darwin stressed that species were modified very slowly as tiny incremental changes accumulated from one generation to the next. Yet, neither paleontology nor genetics have been able to supply clear-cut evidence for gradualism.

3. The explanation for the stability-jump-stability pattern from Cuvier seemed the only one that made sense. But logically, the long series of cataclysms would have whittled nature down to a bare handful of creatures by modern times.

4. In 1942 Ernst Mayr presented a helpful alternative. He argued that a small group might become isolated from the main group. Mutated genes that cropped up in this small group would have a greater impact on their offspring than if the mutations were diluted in the gene pool of the main population.

5. In 1972, Eldredge joined with Stephen Jay Gould to describe the process of evolution as long periods of species stability intermittently disturbed by bursts of new species creation. Gould coined the phrase "punctuated equilibrium."

6. Essentially, punctuated equilibrium says that evolutionary changes happens neither overnight nor over millions of years, but rather in bursts that stretch a few hundred or a few thousand years.

7. Unlike Lamarck's theory of "acquired characteristics," Cuvier's catastrophism, or Darwin's gradualism, punctuated equilibrium does not need to ignore contrary facts.

8. Several competing theories have been proposed to explain the periodic global debacles. One suggests a huge comet circles the sun once every 23 million years, causing a shower of asteroids to collide with the earth each time it goes past.

9. It is as if extinctions temporarily suspend the stabilizing pressure of natural selection, permitting nature to experiment with new versions of life.

10. Once a potential niche is filled, natural selection strengthens, new speciation drops, and ecological stability is reestablished. The sudden blossoming of innovation is a natural response to opportunity.