Evolution, in general, means change over time: The end is different from the beginning. Change is not inevitable, but it seems to be common. And some change can almost always be observed, given enough time. When do we see change? How much time does it take? To study evolution we have to measure change and time, and to understand evolution we have to know how the two are related.

Evolution in biology is often described as a process, but it is really a collection of processes – including the processes of mutation, dispersion, drift, and selection – that together produce whatever change we see. These processes may each be complicated individually, and their interactions produce additional complexity. Evolution is widely acknowledged as a fundamental concept in biology, and yet we have a surprisingly anecdotal and casual knowledge of how evolutionary change takes place. One way to improve this understanding is to quantify change over time in terms of rate. Evolutionary rates are quantitative expressions relating change to time.

Evolution takes place on many scales of time. Field and laboratory experiments can be designed to study change on short time scales, and fossils provide the most direct and best information about evolution on long time scales. The principal problem with the fossil record is that the time scales involved, typically millions of years, are so long that they are difficult to relate to the time scales of our lifetimes (and those of other organisms). Biologists as a group have a surprisingly poor understanding of evolution on a geological scale of time, and paleontologists as a group have a surprisingly poor understanding of evolution on a biological scale of time.