Why Some Traits Evolve Faster Than Others

Not all traits evolve at the same speed. This is one of the most important messages emerging from Simpson’s discussion of rates.

A lineage is not a block of clay reshaped uniformly. It is a living system made of parts with different functions, developmental constraints, genetic architectures, and ecological roles. Teeth, limbs, skulls, body size, ornamentation, and internal anatomy may each follow different evolutionary rhythms.

Why might one trait evolve faster than another?

First, selection may act more strongly on some traits. Teeth may respond quickly to dietary change. Limb proportions may shift with habitat use. Body size may change in response to climate, predation, or resource pressures.

Second, some traits may be developmentally constrained. A structure deeply integrated with many other body systems may be less free to vary without harmful side effects.

Third, genetic variation may differ among traits. Some traits may have abundant variation available for selection, while others may be more canalised.

Fourth, the fossil record itself may bias our view. Hard parts, such as teeth and bones, preserve better than soft tissues, behaviour, or physiology. We may think teeth evolve dramatically, partly because teeth are what we can measure most easily.

Simpson’s treatment of rate reminds us that evolution is mosaic. One part of an organism may change while another remains stable. This mosaic evolution is crucial for interpreting fossils. A species may look “primitive” in one trait and “advanced” in another. Evolution does not renovate the whole house at once. Sometimes it remodels the kitchen, reinforces the roof, and leaves the attic full of ancestral furniture.

For students, this is a liberating idea. Evolution is not a straight path from old to new. It is a patchwork of changes, constraints, experiments, and histories.

Different traits carry different clocks.