Evolutionary changes are often developmental

Morphological structures are produced by growth, and their form emerges from the process of development. Thus evolutionary changes in the form of an organ are frequently developmental: they are produced by changes in the rate or timing of developmental events. An organ may evolve to be larger if its growth speeds up, and it may change shape if the growth rate of one of its parts speeds up relative to other parts.

We shall see in this section that there are several types of evolutionary change in development; but we can begin by looking at one particular type: recapitulation.