The diagram shows phenotypic changes in a population over a number of generations - HSC - SSCE Biology - Question 14 - 2009 - Paper 1

The diagram illustrates a series of phenotypic changes over multiple generations, resulting in distinct forms of a population. This gradual change in the population's traits corresponds to the concept of punctuated equilibrium.

Punctuated equilibrium, proposed by Stephen Jay Gould and Niles Eldredge, suggests that species exhibit relatively stable periods interrupted by brief episodes of significant evolutionary change, often in response to environmental shifts. The presence of distinct forms by the 12th generation suggests such a model, contrasting with gradualism where changes accumulate steadily.

Thus, the answer to the question is (D) Punctuated equilibrium.