Charles Darwin understood that evolution was a slow and gradual process. By gradual, Darwin did not mean "perfectly smooth," but rather, "stepwise," with a species evolving and accumulating small variations over long periods of time until a new species was born. He did not assume that the pace of change was constant, however, and recognized that many species retained the same form for long periods.
Still, if evolution is gradual, there should be a fossilized record of small, incremental changes on the way to a new species. But in many cases, scientists have been unable to find most of these intermediate forms. Darwin himself was shaken by their absence. His conclusion was that the fossil record was lacked these transitional stages, because it was so incomplete.
That is certainly true in many cases, because the chances of each of those critical changing forms having been preserved as fossils are small. But in 1972, evolutionary scientists Stephen Jay Gould and Niles Eldredge proposed another explanation, which they called "punctuated equilibrium." That is, species are generally stable, changing little for millions of years. This leisurely pace is "punctuated" by a rapid burst of change that results in a new species and that leaves few fossils behind.
According to this idea, the changes leading to a new species don't usually occur in the mainstream population of an organism, where changes wouldn't endure because of so much interbreeding among like creatures. Rather, speciation is more likely at the edge of a population, where a small group can easily become separated geographically from the main body and undergo changes that can create a survival advantage and thus produce a new, non-interbreeding species.
This hypothesis predicts that the fossil record at any one site is unlikely to record the process of speciation. If a site records that the ancestral species lived there, the new species would probably be evolving somewhere else. The small size of the isolated population which is evolving into a new species reduces the odds that any of its members will be fossilized. The new species will only leave fossils at the same site as the old one if it becomes successful enough to move back into its ancestral range or different enough to exist alongside its relatives.
Scientists have scrutinized the fossil records of many organisms looking for evidence of punctuated evolution. One group of coral-like sea organisms in particular, called bryozoan, shows this kind of pattern. The well-preserved fossil record of bryozoans shows that one species first appeared about 140 million years ago and remained unchanged for its first 40 million years. Then there was an explosion of diversification, followed by another period of stability for vast amounts of time.
Although the patterns predicted by punctuated equilibrium have been observed in at least some cases, debate continues over how frequently this model of evolutionary change occurs -- is it the norm, or only an exception? Punctuated equilibrium also generates interesting questions for further research. What, for example, are the processes that produce rapid evolution? Population genetic studies show us that small changes can accrue quickly in small populations. And evolutionary developmental biology is revealing new mechanisms that regulate the expression of small genetic changes in ways that can have a large effect on phenotype. Which evolutionary factors are primarily responsible for the periods of stasis -- in which lineages persist without change -- that can be observed in the fossil record? In seeking the answers to these questions, researchers will continue to advance our understanding of the evolutionary processes that produced the remarkable variety of life on Earth.
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