Macroevolution: Pattern and Process

In Macroevolution, Steven Stanley addresses, from a paleobiologist's perspective, the question of whether punctuated equilibria or gradualism offers the best account of the history of life. Punctuated equilibria, a view popularized by Stephen Jay Gould among others, holds that species remain evolutionarily static for long periods of time and undergo substantial genetic changes and develop new, primarily adaptive, strategies during speciation. In contrast, gradualism views large-scale changes as the result of continual and successive small-scale changes. Coming down on the side of those who favor the model of punctuated equilibria, Stanley argues that only "quantum speciation" (speciation that is rapid and radically divergent) can explain the story of life revealed in the fossil record; macroevolutionary trends, he contends, can be explained by selection among species and, to a lesser extent, by phylogenetic drift and directed speciation.