[1] One darwin is defined to be an e-fold (about 2.718) change in a trait over one million years.
An alternative form of this equation is: Since the difference between two natural logarithms is a dimensionless ratio, the trait may be measured in any unit.
Inexplicably, Haldane defined the millidarwin as 10−9 darwins, despite the fact that the prefix milli- usually denotes a factor of one thousandth (10−3).
[2] The measure is most useful in palaeontology, where macroevolutionary changes in the dimensions of fossils can be compared.
Several data points are required to overcome natural variation within a population.