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Macroevolutionary Theories

A topic of debate in the field of macroevolution is the way in which evolution of species takes place over extremely long periods of time. Until recently, most biologists believed Darwin's theory, called phyletic gradualism, that evolution was a gradual and continuous process. However, the proposal of a new theory based on evidence from the fossil record, called punctuated equilibrium, has raised some doubts in the minds of scientists.

Phyletic Gradualism

Darwin felt that evolution took place gradually, in very small steps. This view is supported by the change we can observe in species on the scale of human history. Species seem to be slowly but constantly changing. Unfortunately, gaps in the fossil record make it impossible to support this view with direct fossil evidence.

Punctuated Equilibrium

Niles Eldredge and Stephen J. Gould developed the theory of punctuated evolution within over last thirty years. They propose that the change we can see is evolutionarily minor and that speciation occurs only in response to major crises or genetic events. The majority of time, they maintain, species exist in relative stasis, changing very little, until a crisis or major genetic event occurs and they undergo rapid change. It is important to understand that the term "rapid" is meant in the geological sense, meaning over thousands rather than hundreds of thousands or millions of years. Eldredge and Gould state that their theory is supported by the fossil record, which does show periods of sudden change separated by periods of apparent stasis. Unfortunately, the lack of completeness and detail of the fossil record makes it impossible to prove or disprove either of these theories.