Trilobyte try this:
"Foraminifera are ideal subjects for testing various aspects of evolutionary theory, because large populations of individuals, whose characteristics can be measured and treated statistically, can be obtained from closely spaced rock samples at carefully selected localities to provide an evolutionary time series. It is then possible to show how distribution of a particular characteristic changes over time within successive populations."
And this:
"During the late 1930s and early 1940s Finlay and J. Marwick, a macropaleontologist, collaborated to produce the scheme of fossil zones comprising New Zealand's Cenozoic Series and Stages. Their scheme is still used today, albeit with considerable refinement, for the biostratigraphic classification of New Zealand strata."
If there was no variation within a particular column there would be no way they could correlate across columns.
This;
"High resolution biostratigraphy, which involves identification of closely spaced bioevents, often in conjunction with various quantitative techniques, is a major research direction"
I'm sure that's enough for you to digest for now.
Anyway, back to these "types".
