Types of developmental change (2 of 2)

• Heterochrony
  Heterochrony is a change in the timing of developmental events. For example, a change in timing might slow down the development of the body, but not alter the maturation of the reproductive system. This change yields an adult organism with a form similar to the ancestral juvenile form.

Salamanders go through a larval stage in which they have feathery, external gills (left). Most salamanders lose these gills when they metamorphose into adults (center). Because of heterochrony, axolotls now retain the juvenile external gills as fully reproductive adults (right).
• Allometric growth
  Allometric growth is a change in the rate of growth of a dimension or feature relative to other features. For example, we can describe some of the evolutionary changes that produced bats in terms of allometry. Bat wings are basically paws with really long fingers and skin stretched between them. In order for these wings to evolve, the rate of growth of finger bones must have increased relative to the growth of the rest of the bat's body — or perhaps the rate of growth of the rest of the body decreased relative to the fingers. Either way, it is allometry.