Testing between gradualism and punctuated equilibrium

The fossil record is incomplete and the punctuated pattern will therefore appear in most fossil samples whether the underlying evolutionary pattern is gradual or punctuated.

Two conditions must be met by any test of the ideas:

• The stratigraphic sequence should be relatively complete (i.e., sediments should have been laid down fairly continuously).

• Fossils should be measured biometrically. It is futile to reason with taxonomic evidence: if it is observed that species a persists for a certain amount of time and then a related species b suddenly appears, the underlying pattern of evolution could be either gradual or punctuated. Taxonomists include a range of forms within a single species, and the observation that a single species persists for a certain amount of time tells us nothing about whether its morphology is changing gradually, or is constant in form.

The theories represent two extreme theoretical points. The pattern of evolution itself in any one lineage, and the relative frequency of the patterns in different lineages, can occupy any point between these extremes. It is the task of paleontologists to find out what the frequency of the different patterns is.

We will examine two biometrical studies which illustrate the two patterns and the kinds of evidence that are available.