Simpson distinguishes between different kinds of evolutionary rates, and one of the most important distinctions is between relative and absolute rates.
An absolute rate asks how much change occurred per unit of time. For example, how much did a tooth increase in height per million years? This sounds direct and satisfying. It connects biological change to geological time.
A relative rate, by contrast, compares the change in one trait, lineage, or group against another. For example, did skull length evolve faster than tooth height? Did one horse lineage change more rapidly than another? Did one group of animals show greater transformation than a related group during the same interval?
Both approaches are useful, but both have limitations.
Absolute rates depend heavily on accurate dating. If the time interval is uncertain, the rate becomes uncertain. They also depend on how change is measured. A millimetre of tooth height may not mean the same thing biologically as a millimetre of skull width.
Relative rates can help address some dating problems by comparing patterns within the same broad time frame. But they, too, require caution. Traits differ in developmental, functional, and ecological meaning. Comparing the rate of change in a tooth to that in a limb is like comparing the speed of a violin solo to the expansion of a thundercloud.
The chapter emphasises that evolutionary rates are not abstract mathematical ornaments. They are tools for asking biological questions.
Which traits are evolutionarily flexible? Which remain stable? Do related lineages change at similar speeds? Are bursts of change associated with new environments, new diets, or new ecological opportunities?
Relative and absolute rates give different windows into these questions. One ties evolution to time. The other compares the choreography of change across traits and groups.
Together, they help us see evolution not as a blur, but as a pattern with measurable texture.
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