The fossil record shows long periods of stasis, but evolution predicts constant and gradual change.
Darwin seems to have expected constant and gradual change, which he believed failed to materialize because of the spottiness of the fossil record, but his expectations in this particular respect have been generally (though not universally) disconfirmed as more fossil evidence has come in. What does this mean for evolutionary biology? It does not mean that evolution has not occurred or that evolution does not proceed by the mechanisms Darwin thought. Rather, it shows that Darwin did not completely understand the operation of natural selection in the wild. This is not a very harsh criticism against him, because even though natural selection is a very simple principle, the system in which it operates in the wild is extraordinarily complex.
In any case, the operation of natural selection is consistent with long periods of stasis in the fossil record. Although natural selection acts constantly, and populations therefore change constantly, net stasis in the environment can mean that natural selection drives a population first one way and then the other, so that the average makeup of the population over a long period of time remains approximately constant. It is usually when significant long-term changes in the environment occur, and when populations go through a number of bottlenecks, that we see change, and many times this occurs in a much shorter period of time than the time of stasis.
Last updated: 21 Mar 2008
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