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Stasis and character change

When organisms leave a particularly dense fossil record through time — that is, a record with few gaps of substantial length — scientists can examine the rate of evolutionary change directly. Foraminiferans are common, single-celled, water-dwelling organisms that build shells — a set of characteristics that has allowed them to leave a dense fossil record.

Foraminiferan, Globorotalia albeari A foraminiferan from the Paleocene

Foraminiferan shell size over time The graph you see here shows the evolution of shell shape in a foraminiferan lineage over the past 10 million years. It shows that most of the time, shell shape exhibits relative stasis: it changes a little but does not seem to be changing in any particular direction most of the time. However, two periods of relative stasis are divided by a brief period of rapid morphological change about 5.5 million years ago. Evidence like this is critical in evaluating hypotheses about the rate of evolutionary change.

Foraminiferan micrograph courtesy of the National Collection of Foraminifera © Smithsonian Institution; Foraminiferan graph after Malmgren, Berggren, and Lohmann (1983)