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.
A foraminiferan from the Paleocene
||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.