Darwin (unit)

The equation for calculating evolutionary change in darwins (${\displaystyle r}$) is:

${\displaystyle r={\frac {{\rm {ln}}X_{2}-{\rm {ln}}X_{1}}{\Delta t}}}$

where ${\displaystyle X_{1}}$ and ${\displaystyle X_{2}}$ are the initial and final values of the trait and ${\displaystyle \Delta t}$ is the change in time in millions of years. An alternative form of this equation is:

${\displaystyle r={\frac {{\rm {ln}}{\frac {X_{2}}{X_{1}}}}{\Delta t}}}$

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]